EPA/600/R-12/540
June 2012
Version 1.5
Technology
Assessment Report
Aqueous Sludge Gasification
Technologies
Prepared by:
Greenhouse Gas Technology Center
SEPA
Operated by
Southern Research Institute
Under a Cooperative Agreement With
U.S. Environmental Protection Agency
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EPA REVIEW NOTICE
This report has been peer and administratively reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
The data used in development of this report was derived from secondary sources. No
attempt has been made by the U.S. EPA or Southern Research Institute to confirm
source data quality.
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ACKNOWLEDGMENTS
Southern Research Institute and The Greenhouse Gas Technology Center wish to thank the
original sponsors of this project, Jason Turgeon, EPA Region 1 and Maggie Theroux, EPA Office
of Research and Development, National Risk Management Research Laboratory, for their support
throughout the development and completion of the project. We also thank David Burns and Mike
Parker of the Maine DEP for their support and input in guiding the project. Finally, the input and
support provided by various stakeholders participating in this project, as well as the technology
vendors, the state and EPA regulators, and others, were critical to allowing this assessment to
move forward.
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TABLE OF CONTENTS
Summary 1
1. Project Description and Objectives 1
1.1 Roles and Responsibilities 2
1.2 Approach 4
2. Conventional Sludge Disposal Practices 5
2.1 Overview 5
2.2 Issues with Current Practices 7
3. Gasification Technology 8
3.1 Overview 8
3.2 Types of Gasifiers 9
3.3 Historical and Current Applications 12
4. Sludge Gasification 13
4.1 Sludge Characteristics 13
4.1.1 Pulp and Paper Mill Sludge 13
4.1.2 Sewage Sludge 14
4.2 Unique Aspects of Sludge Gasification 16
4.3 Environmental Consequences 18
4.3.1 Criteria Air Pollutants 18
4.3.2 Hazardous Air Pollutants 20
4.3.3 Greenhouse Gases 22
4.3.4 Wastewater 23
4.3.5 Direct Environmental Advantages of Gasification 23
4.3.6 Social Sustainability 24
4.4 Brief Technology Comparison 25
5. Commercial Status of Sludge Gasification 26
5.1 Industry Assessment Results 26
5.2 Selected Technology Profiles 28
5.2.1 Maxwest Environmental Systems24' 28>72 28
5.2.2 M2 Renewables (M2R) & Pyromex AG20< 30<75 30
5.2.3 Kopf29'35 32
5.2.4 Nexterra & City of Stamford WPCA19< 31< 32< 33<34 34
5.2.5 Tokyo Bureau of Sewerage60'69 36
in
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5.3 Economic Assessment 38
6. Cogasification 39
7. Regulatory Requirements 40
8. Conclusion and Recommendations 42
8.1 Summary of Key Findings 42
8.2 Conclusion 42
8.3 Recommendations 43
References 44
IV
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LIST OF FIGURES
Figure 1 - Project Participants 4
Figure 2 - Summary of Waste Water Solids Management in the U.S.1 5
Figure 3 - Conventional paper mill sludge disposal practices in the U.S.13 6
Figure 4 - Example Gasification System 9
Figure 5 - Diagram of the zones in an updraftand downdraft gasifier 10
Figure 6 - Two types of fluidized-bed processes; Bubbling bed (left] and circulating bed
(right] gasifiers 12
Figure 7 - Cumulative worldwide gasification capacity 13
Figure 8 - Process flow diagram of Maxwest system after modifications 29
Figure 9 - Pryomex Ultra High Temperature gasifier in Munich, Germany20 31
Figure 10 - Kopf PFD29<35 33
Figure 11 - Feasibility design for Nexterra, a possible PFD for electricity generation19 35
Figure 12 - Tokyo Bureau of Sewerage PFD60 37
v
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LIST OF TABLES
Table 1 - Summary of Gasifier Types 11
Table 2 - Ultimate analysis of different types of paper mill sludge 14
Table 3 - Ultimate and proximate analysis of different sewage sludge samples 15
Table 4 - Emissions data submitted by Maxwestto the Florida environmental agency 19
Table 5 - Metal concentrations in the different stages of the UoS study 20
Table 6 - Estimated amounts of metals processed during sludge gasification 21
Table 7 - Concentrations of metals analyzed by ICL 21
Table 8 - GHG emissions in kg of CC>2 equivalents per ton of dry sludge processed 22
Table 9 - GHG emissions using BEAM 23
Table 10 - Brief Technology Comparison Incineration vs. Gasification 25
Table 11 - Technology readiness level (TRL) parameters 26
Table 12 - Original list of vendors and the result of their investigation 27
Table 13 - Maxwest and M2 Renewables overview 31
Table 14 - Kopf and Nexterra overview 35
Table 15 - Tokyo Bureau of Sewerage overview 37
Table 16 - Summary of sludge gasification economics under varying local conditions 39
Table 17 - Emission limits for existing and new sewage sludge incinerator units 40
Table 18 - Emission limits for EU and German waste combustion units in 2000 41
Table 19 - Data Source Qualification 49
VI
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ACRONYMS AND ABBREVIATIONS
APTB
BEAM
Bbl
Btu
C
CAP
CCCSD
cfm
CH4
CO2
CO
COS
cs
°c
dscfm
EPA
ETV
Fe
FeO
FT
GHG
GWP
H
H2
H2O
H2S
HAP
HC1
HCN
HF
Hg
HHV
ICE
ICL
kg
kW
kWth
Ib
LHV
MEDEP
MMBtu
MMm3
MWth
N
Air Pollution Technology Branch
Biosolids Emissions Assessment Model
Barrel
British thermal unit
Elemental Carbon
Criteria Air Pollutant
Central Contra Costa Sanitary District
cubic foot per minute
Methane
Carbon dioxide
Carbon monoxide
Carbonyl sulfide
Carbon monosulfide
Degree Celsius
Dry standard cubic foot per minute
Environmental Protection Agency
Environmental Technology Verification
Iron
Iron oxide
Fischer-Tropsch
Greenhouse gas
Global warming potential
Elemental hydrogen
Hydrogen
Water
Hydrogen sulfide
Hazardous air pollutant
Hydrogen chloride
Hydrogen cyanide
Hydrogen fluoride
Mercury
Higher heating value
Internal combustion engine
Imperial college of London
Kilogram
kilowatt
Kilowatt thermal
pound
Lower heating value
Maine Department of Environmental Protection
Million British thermal units
Million cubic meters
Megawatt thermal
Elemental nitrogen
vn
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N2
N2O
NH3
NOX
NRC
NRMRL
NYSERDA
O
02
PAH
PCB
PCDD/F
PM
POTW
ppm
PV
QA
R&D
RAG
S
SO
Southern
SOX
SWG
SWPCA
Syngas
TNSSS
TPD
TPY
TRL
UoS
us$
voc
WWT
ZnO
Nitrogen
Nitrous oxide
Ammonia
Oxides of nitrogen
National Research Council
National Risk Management Research Laboratory
New York State Energy Research and Development Authority
Elemental oxygen
Oxygen
Polyaromatic hydrocarbon
Polychlorinated biphenyl
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans
Particulate matter
Publicly owned treatment works
Parts per million
Photovoltaic
Quality assurance
Research and Development
Rheinbraun AG
Elemental Sulfur
Sulfur monoxide
Southern Research Institute
Oxides of sulfur
Supercritical water gasification
Stamford water pollution control agency
Synthesis gas
Targeted national sewage sludge survey
Tons per day
Tons per year
Technology readiness level
University of Seoul
U.S. dollars
Volatile organic compound
Wastewater treatment
Zinc oxide
Vlll
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TECHNOLOGY ASSESSMENT REPORT:
AQUEOUS SLUDGE GASIFICATION
Summary
Southern Research Institute (Southern), under a cooperative agreement with the U.S.
Environmental Protection Agency's Environmental Technology Verification (ETV) Program, has
conducted a study to evaluate existing sludge gasification technologies, their impacts, and stages
of development. Critical components and impacts of these technologies were identified and
quantified to assess their development status, potential benefits and drawbacks, and
environmental and economic impacts.
The study reveals that sludge gasification is a potentially suitable alternative to conventional
sludge handling and disposal methods. However, very few commercial operations are in
existence. The limited pilot, demonstration or commercial application of gasification technology
to sludges, and the unique characteristics of each different technology and site make it difficult to
provide an overall assessment of the impacts and economics of the entire technology category at
this time. Gasification of sludges and biosolids should be considered an early commercial
technology with limited data, and should be evaluated on a site and technology specific basis for
potential commercial applications. However, the scarcity of commercial plants should not
discourage the consideration of sludge gasification as a beneficial alternative. Given the potential
benefits, continued research, development, and deployment of sludge gasification technologies
should be followed as the technology progresses into full commercial availability.
1. Project Description and Objectives
Sludge production in the United States is increasing with an increase in population. It is estimated
that 7.2 million dry tons of treated and tested sewage sludge and 5.5 million tons of paper mill
sludge are generated in the U.S. annualy.58'74 Consequently, there is an increased need for efficient
and environmentally sound management practices for sewage sludge from publicly owned
treatment works (POTWs) and sludge from pulp and paper mills. Traditional methods to dispose
of the sludge can require significant energy inputs, utilize otherwise useful real estate, and
sometimes result in negative environmental aspects, including impacts on air, land, water, and
greenhouse gas (GHG) emissions. Municipalities and companies are under increasing pressure to
become more energy and cost efficient in their sludge disposal methods, and to reduce their GHG
emissions and carbon footprints. In addition, as land use, water quality, air emissions, public
health and social pressures increase, the public and regulatory acceptance of traditional sludge
disposal methods is rapidly diminishing. In response to these various pressures, other options for
disposal or utilization of wastewater treatment and paper industry sludges need to be investigated.
Sludge gasification is a potentially viable solution to these issues. There is a general consensus
among gasification experts that this technology, when properly configured, is capable of
delivering net energy gains while reducing environmental impacts when compared to
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conventional management practices. Through independent, academic and government funded
demonstrations and studies, the process of gasification has been successfully shown to convert
numerous types of carbon based feedstocks into a synthesis gas (syngas) which can be directly
combusted for heat and energy production, or further processed into a variety of liquid fuels and
other chemicals. By significantly reducing the volume of the residual biosolids, gasification also
reduces the costs associated with transportation and disposal in a landfill.
The growth and implementation of sludge gasification systems has been very limited, in part as a
result of the lack of independent data, demonstration, and evaluation of technology impacts,
economics, and capabilities. This lack of information results in contradicting claims from those
who favor and those who oppose the technology, in terms of the environmental benefits, costs,
and effectiveness. Vendors and technology developers claim that gasification is the optimal
solution to sludge disposal, while some environmental organizations argue that gasification is no
better than incineration.
This project sought to independently evaluate sludge gasification technologies based on a review
of currently available data. The pros and cons of sludge gasification and its environmental
impacts, sustainability, costs, and efficiency in converting sludge into usable fuels were studied.
This independent assessment of gasification technologies provides unbiased information from a
variety of sources to aid in decision-making and evaluation for purchase, implementation,
regulation, and public acceptance of these systems.
1.1 Roles and Responsibilities
A primary objective of this project was to address some of the technical issues regarding the use
of gasification technology to process water-laden wastes such as are found in paper
manufacturing and sewage treatment processes. Gasification could recover some energy from the
organic material in these sludges and reduce the burden of landfill waste, which would support
EPA objectives of reducing solid waste and energy consumption. Even though the information in
this report could be used by policymakers in EPA and other regulatory agencies, it should be used
for technical guidance only and does not reflect EPA policy.
This project was funded by the EPA Office of Solid Waste and Emergency Response (OSWER),
based on a proposal by EPA Region 1, as supported by the State of Maine, and the EPA National
Risk Management Research Laboratory (NRMRL). The State of Maine Department of
Environmental Protection (ME DEP) also provided funding to support the completion of the
project. The project was completed by Southern and the Greenhouse Gas Technology
Verification Center, which it operates under a cooperative agreement with NRMRL. Southern is
an expert in gasification and is currently conducting research at bench and pilot scale for various
clients at their facility in Durham, NC. However, Southern does not manufacture, sell or license
internally developed gasification systems. In addition to funding support, the following
participants provided input and support of the program as described below.
The ME DEP served in an advisory capacity, provided technical support, and served as a liaison
with the Maine paper industry. Ultimately, the ME DEP will consider the results as the State re-
evaluates regulations regarding the use of non-incinerator technologies in waste to energy
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applications, including those utilizing aqueous sludges. Specifically, the current Maine legislature
passed a resolve requiring ME DEP to review whether facilities using emerging waste-to-energy
technologies that provide environmental and energy benefits (e.g., gasification) should be
excluded from Maine's statutory ban on the establishment of new commercial solid waste
disposal facilities. It has been determined that, presently, the ban does apply, and that the overall
regulatory structure under which a new incinerator proposal would be evaluated would apply to a
new gasification proposal.
The EPA NRMRL ETV GHG Center is a public/private partnership between NRMRL and
Southern. The GHG Center verifies the performance of technologies that produce, mitigate,
monitor, or sequester greenhouse gas emissions, including technologies for advanced energy
production, waste-to-energy conversion, oil and gas production and transmission, and other
energy efficiency technologies. The ETV Program has a cooperative agreement with Southern.
As a result, NRMRL contributed staff support for communications, reviews, and management.
This support included the participation of the current ETV Project Officer and ETV Quality
Assurance (QA) Manager, who provided reviews and approval of this final report.
As the operating partner in the GHG Technology Center, Southern Research managed the
technology assessment project. In addition to synthesizing information available through literature
review and interviews with developers, vendors and other stakeholders, the assessment aimed to
determine whether verification testing under the ETV Program is needed to better characterize
performance capabilities of gasification technologies.
As is typical with the ETV program, a multi-interest and balanced stakeholder group was
established to help guide the assessment and verification efforts. Stakeholders were selected to
represent a broad community of those with interests in sludge gasification technologies, and
included regulators, researchers, and industry associations. Stakeholders reviewed and provided
input on project plans and this final report. In addition, stakeholders provided technical input,
contacts with technology vendors, supporting technical and regulatory data, and other information
important to the assessment. The project hierarchy is listed in Figure 1, including stakeholder
identification.
NRMRL's Air Pollution Technology Branch (APTB), with expertise and experience in
conducting assessments of combustion-based processes, created a parallel report about the use of
incineration technology in sewage and pulp and paper mill sludges.52 The report focused on an
evaluation of performance, emissions and cost for multiple-hearth and fluidized bed incinerators.
EPA Region 1 prepared the initial proposal for funding of the technology assessment project
based on their needs to better understand the potential impacts of the emerging sludge gasification
technologies, as more end users began looking at options for sludge disposal beyond traditional
practices. The Region 1 staff participated in the development of the planned scope of work for
the project, provided input regarding technology vendors and end users within Region 1 that had
expressed interest in potential projects in the Region, reviewed project plans, and reviewed this
final report.
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Tim Hansen
GHG Center
Eric Ringler
GHG Center
QA Manager
David Bums
Richard Bain
Robert Bastian
JudyJamefield
Howard Levenson
VatMo&rt fl'frwi-nrv*
jSjiLuiccii \j L uiiiiur
Chan Park
Mike Parker
Michael Razanousky
Ben Thorpe
Jason Turgeon
Project Officer I
MEDEP
National Bioenergy Center at NREL
EPA Office of Wastewater Management
NYSERDA
CalRecycle
MVCPPFIA
IN I oXlCvU/V
University of California at Riverside
MEDEP
NYSERDA
Biorefinery Deployment Collaborative
EPA Region 1
)irector QAMana
Wes Kowalczuk
flTTf"; (""firttnr
Project Manager
Figure 1 - Project Participants
1.2 Approach
The Gasification Technology Assessment Report aimed to summarize the anticipated benefits and
limitations of commercial or near commercial sludge gasification systems, screen out systems
with limited promise, and identify significant information gaps necessary to properly evaluate the
gasification systems.
Data for the technology assessment was collected through literature searches; contacting
knowledgeable industry stakeholders; and direct inquiries of manufacturers, project developers,
and facility owner/operators. Literature search activities were facilitated through use of
bibliographic databases and indices.
Data was obtained from the following source types, listed from highest quality to lowest:
1. peer reviewed journals or government reports - results based on independently measured
validated data;
2. non peer reviewed government reports, conference presentations (non-marketing), peer-
reviewed journal articles not based on independently obtained data;
3. direct contact with technology vendors or commercial project development team; and
4. non-reviewed articles, websites, marketing presentations, advertisements, press, etc.
An evaluation of each data source by type can be found in Table 19.
The list of candidate technologies that was developed includes gasifiers that claim or appear to be
capable of handling the subject sludge streams. Those candidate technologies that appeared to be
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promising, in that they are demonstrated at a commercial or pilot scale, received a more detailed
examination and analysis.
2. Conventional Sludge Disposal Practices
2.1 Overview
The most utilized methods of pulp and paper and sewage sludge management have been land
disposal, land application and incineration. In 2006, the EPA released a report on biosolids
management in the United States which details the current practices, quantities, and distribution
of sludge disposal methods.1 Figure 2 displays the breakdown of conventional municipal sewage
sludge disposal practices.
Landfilled, 17
Incinerated , 22
Other Beneficial
.Other Disposal, 1
Land Application,
41
Advanced
Treatment, 12
Figure 2 - Summary of Waste Water Solids Management in the U.S.1
In 2007, the Northeast Biosolids and Residuals Association (NEBRA) released a report that
included data on sewage sludge management practices in the U.S. In short, the study found that
55% of the 7.2 million tons of biosolids were used in agronomic, silviculture, or land restoration
of derelict land and/or stored for this use. The remaining 45% was disposed of in municipal solid
waste (MSW) landfills, surface disposal units, and/or incineration facilities.58
Paper mill sludge disposal practices differ from sewage sludge, in that most (65%) is landfilled.
Figure 3 gives a breakdown of the disposal methods as of 1995. The total is greater than 100% as
some mills use multiple processes.
Landfills have been the primary approach for sludge disposal in the US. However, this option is
becoming limited due to increasing disposal costs, diminishing landfill sites, and possible long
term environmental impacts.49 In a landfill operation, sludge, which has been mechanically
dewatered to approximately 20% solids, is transported off-site to a landfill, where a tipping fee for
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disposal is paid. Some paper mills operate their own landfills, thereby negating the tipping fee,
however there are costs associated with operation of a landfill. Due to the high moisture content
of the sludge, a significant amount of resources are directed to the handling of water. Waste
deposited in a landfill generally undergoes three steps. The first stage is degradation under aerobic
conditions, where the aerobic micro-organisms consume the available oxygen in the waste. Once
a majority of the oxygen is consumed, acetogenic and fermentative bacteria decompose the easily
degradable portion of the waste, resulting in a lower pH , thereby increasing the solubility of
inorganic substances such as heavy metals. In the last stage, methanogenic bacteria propagate
rapidly to produce methane. The pH value increases, and the organic content of the leachate
decreases.47
Other Methods, 8
Land spreading, 8
Figure 3 - Conventional paper mill sludge disposal practices in the U.S.
The sludge incineration process involves heating, under excess oxygen, in order to completely
oxidize the organic portion of a feedstock. Completely dewatering the sludge being fed is not
necessary, but it will reduce the need for supplemental fuel.52 The heat created during combustion
is typically recovered and used to remove the moisture in the feedstock. In most cases, the heat
recovery will be counterbalanced by the heat used for reducing the water content of sludge. The
outputs of incineration are heat, ashes, flue gases and wastewater.47 Most sludge combustion units
fall into two types, multiple hearth (MH) incinerators and fluidized bed (FB) incinerators.52 MH
units are comprised of multiple zones for heating and burning sludge, whereas FB units have only
one zone. In the U.S., 218 sewage sludge incineration units are owned by 97 entities. Of the 218
units, 55 are FB units and 163 are MH units. Additionally, there are 57 pulp and paper sludge
combustion units.3 Eastern Research Group, Inc (ERG) assessed the technological performance of
the two types of incinerators, with the following conclusions.52
The moisture content of sewage sludge for FB incinerators may be variable, however MH
units are less amenable to variable sludge moisture levels due to MH units consisting of
multiple zones. MH units generally require more supplemental fuel than FB incinerators for
the same reason.
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Average capital costs and operating costs on a per-ton of dry sludge basis for the FB unit were
found to be substantially less than MH units.
The most common emission control devices used in sewage sludge incinerators are Venturi or
impingement tray scrubbers.
The most popular emission controls among pulp and paper sludge incineration units are
cyclone separators, electrostatic precipitators, and Venturi scrubbers.
Incineration has particularly strong benefits for the treatment of certain waste types such as
clinical wastes and certain hazardous wastes where pathogens and toxins can be destroyed by
high temperatures. Examples include chemical multi-product plants with diverse toxic or very
toxic wastewater streams, which cannot be routed to a conventional wastewater treatment plant.
Land application takes advantage of recycling the compounds of agricultural value present in
sludge. The major benefit being the use of nitrogen, phosphorus, potassium, and calcium present
in the sludge. Prior to recycling, the raw sewage sludge is aerobically or anaerobically digested,
lime stabilized, heat dried/pelletized, or often composted with other organic materials.
2.2 Issues with Current Practices
Although many measures are instituted in order to properly seal landfills, there will potentially be
leaks from breaches in the landfill liner system due to the high water content resulting from
sludge disposal. Toxins contained in sludge that is disposed of in landfills may be collected in the
leachate and enter ground water through the breaches in the liner system. If sewage sludge is
placed in a landfill in which gas is vented to the atmosphere, a significant increase in GHG
emission results, although in some cases this is avoided by recovery of the biogas and its use as a
biofuel. In addition, landfill capacities are limited, and land use restrictions and social acceptance
are limiting the development of new landfills, further decreasing the available options for
landfilling of sludges.
Incineration is a combustion reaction. Outputs are flue gases, ashes, and wastewater, as well as
the production of energy. During the incineration process polyaromatic hydrocarbons (PAH),
heavy metals, poly chlorinated biphenyls (PCB's), poly chlorinated dibenzo-p-dioxins and
polychlorinated dibenzofurans (PCDD/Fs), and acidic gases: SOx, HC1, HF, NOx, etc. are
liberated or created and must be captured before sending the flue gas to the atmosphere.
Incineration processes also produce fly ash and bottom ash, which must be treated and disposed
of as a solid or hazardous waste, depending on the composition. Technological advances have
been made regarding many of these issues; however, many of these advances have decreased
overall system efficiency, thereby increasing investment and operating costs by nearly two
thirds.15'16
Wastewater sludge can be dewatered, dried or composted and then spread on agricultural and
non-agricultural land to replace the use of conventional fertilizers. When land applying, POTWs
are required to adhere to 40 CFR Part 503, which limits the amount of metals and organic
pollutants contained in the sludge. The regulation is based on the cumulative loading of each
pollutant, thereby eliminating a piece of land's availability for land application once the ceiling is
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reached. Individual states may also have regulations which are often more strict than federal
regulations.58'59 In addition, social perspectives on sludge land application of sludges are
becoming less favorable, with many environmental activists and communities increasing
resistance to application as an acceptable disposal means. As suitable space for land application
and disposal becomes less available, POTWs will be forced to seek alternate disposal methods.
3. Gasification Technology
3.1 Overview
The basic principle of gasification is to convert a carbon based material into H2 and CO with the
addition of heat and a combination of steam, oxygen and/or nitrogen in a reaction vessel. Other
than H2 and CO, the remainder of the syngas includes N2, traces of CH4 and other hydrocarbons,
tar, particulates, and CO2. Once produced, the syngas can be cleaned through the use of a variety
of cleanup devices, including ash-capturing cyclones, solvent based tar scrubbers, and water, acid
or caustic scrubbers for capturing nitrogen, chlorine, sulfur and various heavy metals. Once
cleaned, the syngas can be converted to a liquid fuel using a catalytic Fischer-Tropsch (FT)
process, fed into an internal combustion engine-generator for electricity production, combusted
for heat recovery, used in fuel cell applications, or used for the production of a variety of
chemicals. In theory, any form of biomass may undergo gasification. Limitations on the
efficiency of the gasifier's operation include high moisture content of the feed, ash fusion
temperatures, design of the feeding system, and the mixing and separation of the feedstock.14
The gasification process begins by preparing the feedstock by drying to the appropriate moisture
content, typically between 10 - 20%. Once dried, the feedstock is then transferred to a feeding
system, which can vary in design based upon the pressure in the gasifier and the physical
properties of the feedstock. After entering the gasifier, the feedstock is then converted into syngas
of varying compositions, depending on the gasifier type and feedstock composition. A cyclone
installed downstream of the gasifier will capture additional ash/PM that is not captured in the
gasifier. Heat can be recovered in the form of steam with the use of a heat recovery steam
generator and fed into the dryer to supplement the drying system. The cool raw syngas is then
treated through a liquid and/or dry cleaning system. Finally, the cleaned syngas is then fed into
the conversion system to create electricity, heat, fuels, and/or chemicals. If using a combustion
generator, additional thermal energy can be removed from the exhaust to further supplement the
drying system. An example generic system is shown in Figure 4.
Generally speaking, in addition to the value of the end products, the availability of the feedstock,
pretreatment requirements, gasification system efficiency, syngas conversion process and site
specific energy costs all have a significant effect on whether or not a system will make it to
commercial status.
8
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VVfetFeedstorx
Condensed Steam
1
Heat Recovery
Steam Generator
Acid/Caustic/
Water Scrubber
Tar Cracking/
Recovery
Fuel/
Chemicals
Figure 4 - Example Gasification System
3.2 Types of Gasifiers
There are a variety of gasifier designs currently being used for commercial applications in the
production of fuels and chemicals. However, this report only covers the major types which could
be applicable to sludges. These include fixed bed, fluidized bed and those grouped as
plasma/other. A summary of some of the key aspects of the main gasifier types can be found in
Table 1.
Historically, the fixed-bed process is the oldest form of gasification. Fixed-bed updraft gasifiers
are characterized by a bed in which the feedstock moves slowly downward under gravity as it is
gasified by a gasification medium that is in a counter-current flow to the feedstock. In such a
counter-current arrangement, the hot syngas from the gasification zone is used to preheat and
pyrolyze the downward flowing feedstock.7 In a fixed-bed downdraft gasifier, the feedstock flows
concurrently with the gasification medium. Figure 5 displays the updraft (left) and downdraft
(right) process.
In a fluidized bed gasifier, the gasification medium and feedstock must pass through a bed of inert
particles (e.g., alumina oxide). There are two types of fluidized bed gasifiers: bubbling and
circulating. Bubbling fluidized bed gasifiers are typically appropriate for medium size projects of
25 MWth or less, while circulating fluidized bed gasifiers can range from a few MWth up to very
large units.11 Fluidized bed gasifiers offer extremely good mixing between feedstock and oxidant,
which promotes both heat and mass transfer. This ensures an even distribution of material in the
bed, but a certain amount of partially reacted fuel is inevitable and will be removed with the ash.
This places a limitation on the carbon conversion of fluid-bed processes. The operation of
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fluidized bed gasifiers is generally restricted to temperatures below the softening point of the ash,
since ash slagging/agglomeration will disturb the fluidization of the bed.7 Figure 6 displays the
bubbling and circulating bed processes.
Plasma gasification is a relatively new process with respect to traditional gasification. The
primary heat source in a plasma gasifier is the plasma torch, where gas is passed through an
electric arc and dissociated into ions and electrons creating extremely high temperatures (>5,000
°C). The high temperatures enable very large carbon conversion percentages and good control of
the hazardous materials captured in the slag; however, plasma gasifiers are relatively costly and
have relatively higher parasitic energy consumption when compared to traditional gasifiers.
Liquid metal gasification is a field which is being studied and even practiced at pilot scale by a
handful of companies. Feedstock is introduced into a crucible filled with molten metal, usually
iron, at around 1300 °C. Water in the feedstock is split into H2 and Q^ In theory, the iron is then
oxidized to FeO, then reduced back to iron after the O2 reacts with carbon in the feedstock to
make CO gas. The H2 and CO gas are the main two components in the syngas. In order to
favorably shift the equilibrium, oxygen gas can be introduced. The iron also helps to capture
unwanted waste like chlorine and sulfur into a glass like material (slag).
Supercritical Water Gasification (SWG) is a process which utilizes super critical water (pressure
> 320 psi, temperature > 600 °F) to convert organics into a hydrogen rich syngas. SWG requires
feedstocks with moisture contents ranging from 70 to 95%. The reforming of biomass and
biological residues in supercritical water is a rather novel process. Significant R&D work will be
required prior to implementation and commercialization.10
-Feed
Air
Air-
Combustion
Zone
Figure 5 - Diagram of the zones in an updraft and downdraft gasifier
10
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Table 1 - Summary of Gasifier Types'
Gasifier Type
Downdraft Fixed
Bed
Updraft Fixed Bed
Bubbling Fluidized
Bed
Circulating
Fluidized Bed
Plasma
Liquid Metal
Supercritical Water
Scale
5 kWfl, to 2 MWft
<10MWth
<25MWth
A few MWfl,
up to 100 MWfl,
<30MW
<7MW
UNK
Fuel Requirements
Moisture
<20%
up to
50% - 55%
<15%
<15%
any
<5%
70 - 95%
Flexibility
Less tolerant of fuel
switching
Requires uniform particle
size
large particles
More tolerant of fuel
switching than downdraft
Very fuel flexible
Can tolerate high ash
feedstocks
Requires small particle size
Very fuel flexible
Can tolerate high ash
feedstocks
Requires small particle size
Greater feed flexibility
without the need for extensive
pretreatment
solid waste capability
Generally requires low
moisture due to the possibility
of steam explosion
Suitable for the conversion
of wet organic materials
Efficiency
Very Good
Excellent
Good
Very Good
Very Good
Very Good
Good
Gas Characteristics
Very low tar
Moderate
Particulates
Very high tar
(10% to 20%)
Low particulates
High methane
Moderate tar
Very high in
particulates
Low tar
Very high in
particulates
Lowest in trace
contaminants; no tar, char,
residual carbon, only
producing a glassy slag
.Low trace contaminants;
virtually no tar, char,
residual carbon
Suppressed formation of
tar and char
Other Notes
Small scale
Easy to control
Produces biochar at low temperatures
Low throughput
Higher maintenance costs
Small and medium scale
Easy to control
Can handle high moisture content
Low throughput
Medium scale
Higher throughput
Reduced char
Ash does not melt
Simpler than circulating bed
Medium to large scale
Higher throughput
Reduced char
Ash does not melt
Excellent fuel flexibility
Smaller size than bubbling fluidized bed
Large scale
Easy to control
Process is costly
High temperature (5000°-7000°F)
High syngas quality
Short reaction time
High energy conversion efficiency by
avoiding the process of drying step
Selectivity of syngas with temperature
control and catalysts
11
-------
Biomass
Feed
Preheated Gas
Biomass Feed
Preheated Gas
Figure 6 - Two types of fluidized-bed processes; Bubbling bed (left) and circulating bed (right) gasifiers
3.3 Historical and Current Applications
Towards the end of the 18th century, gas was produced from coal by gasification on a large
scale. With the foundation in 1812 of the London Gas, Light and Coke Company, gasification
finally became a commercial process. Ever since, gasification has played a major role in
industrial development. The most important gaseous fuel used in the first century of industrial
development was town gas, which was produced via coal gasification and used for lighting and
heat.7
By far, the most abundantly applied form of gasification today is the gasification of coal. The last
10 to!5 years have seen the start of a renaissance of gasification technology, as is clear from
Figure 7. There are several reasons for this, but first and foremost is the dramatic increase in
energy costs. For the 20 years prior to 2003, oil prices were between $20 and $30 per barrel.
Prices since 2005 have mostly been in the $55 to $120 per barrel range. Similarly, with natural
gas, the U.S. commercial price from 1983 to 2003 was mostly between $5 and $6 per MMBtu,
rising slightly toward the end of the period; between 2005 and 2009 it remained consistently over
$10 per MMBtu, peaking at $15 at the end of 2005.7
12
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180 -,
1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014
Figure 7 - Cumulative worldwide gasification capacity
Around the world, more than 100 biomass gasifier projects are operating or ordered.11 The
prospect of woody biomass gasification is being explored due to the abundance of timber in the
United States and other areas, renewable energy standards and goals and greenhouse gas
emission regulations and reduction goals. Woody biomass gasification is capable of delivering
thermal and electrical energy, as well as liquid fuels. As these processes become more common,
proven, and accepted, other similar systems processing more complex feedstocks will be
demonstrated and commercialized, including those for sludges and municipal solid waste.
4. Sludge Gasification
4.1 Sludge Characteristics
Sewage sludge is the solid, semisolid, or liquid organic material that results from the treatment of
domestic wastewater by municipal wastewater treatment plants (WWTPs), also known as
publicly owned treatment works (POTWs). Paper mill sludge is representative of materials
discharged from virgin pulp and recycle mills. The characteristics of these sludges vary widely,
depending on the location and facility. Due to the varying compositions of sludge, it is difficult
to show precise quantitative data that represents sludge as a whole across the United States.
Typical and example compositions are provided in the sections to follow.
4.1.1 Pulp and Paper Mill Sludge
Generally, pulp and paper mill sludge is the solid residue recovered from the wastewater stream
of the pulping and papermaking process. Sludge is produced at two steps in the process of
treating the effluent. Primary sludge is recovered by the first stage of the processing at the
primary clarifier. Primary clarification is usually carried out by sedimentation, but can also be
performed by dissolved air flotation. In sedimentation, the wastewater to be treated is pumped
into large settling tanks, with the solids being removed from the tank bottom.13
13
-------
Secondary treatment is usually through aerobic digestion. The resulting solids are then removed
through clarification as in the primary treatment. The resulting sludge is then mixed with the
primary sludge prior to dewatering and disposal. In general, primary sludges are easier to
dewater than the sludges resulting from the second stage.
The amount of sludge produced per bale of raw material received varies by plant type. Kraft,
sulfite and deinking mills typically produce approximately 58, 102 and 234 kg of sludge per ton
respectively.13 These sludges contain approximately 40-50% water by weight and a heating value
of about 3600 Btu/lb (dry).55 The low heating value is a result of high levels of clay, calcium
carbonate and titanium oxide. The ash content in paper mill sludge can be as high as 50%.
Not only do mills produce varying amounts of sludge, the sludges they produce are distinctly
different in composition. High ash sludges have a significantly lower heating value than low ash
sludges, which affect its suitability for certain disposal methods (e.g., incineration and
gasification13).
Table 2 - Ultimate analysis of different types of paper mill sludge13
Sludge Type
Bleached Pulp Mill
Pulp mill
Kraft mill 1
Kraft Mill 2
Deinking Mill 1
Deinking Mill 2
Recycle Mill
Analysis (%)
Solids
33
42
37
40
42
42
45
4
0
6
0
0
0
0
Ash
1.9
4.9
7.1
8.0
20.2
14.0
3.0
C
48
51
55
48
28
31
48
7
6
2
0
8
1
4
H
6.6
5.7
6.4
5.7
3.5
4.4
6.6
S
0.2
0.9
1.0
0.8
0.2
0.2
0.2
42
O
.4
29.3
26
.0
36.3
18
30
41
.8
.1
.3
0
0
4
1
0
0
0
N
2
9
4
2
5
9
5
4.1.2 Sewage Sludge
As defined by the EPA, final sewage sludge is the liquid, solid, or semi-solid residue generated
during the treatment of domestic sewage, receiving secondary treatment or better, in a treatment
works, which may include sewage sludge processed to meet land application standards.12
Wastewater entering a POTW is typically screened, then placed into a grit chamber, where sand,
grit, cinders, and small stones settle to the bottom. In most cases, the wastewater then flows into
a primary sedimentation tank where gravity, flotation, and chemical coagulation or filtration
order to remove another portion of the solids (as primary sludge). Finer solids, organic matter
and some dissolved materials are removed during secondary treatment, typically by attached or
suspended growth biological processes. After these biological processes are completed, the
effluent typically passes through a secondary clarifier, where additional sludge is removed (as
secondary sludge).17 Biological secondary sludge contains micro-organisms that retain water
within tough cell walls, making mechanical dewatering difficult. It should be noted that during
digestion, some carbon in volatile organic matter is converted into CC>2 and CH4, thereby
lowering the heating value.
14
-------
Typical waste water entering a POTW contains between 0.01 and 0.04% suspended solids.
Therefore, a plant processing 1 million gallons of water per day will produce between 800 and
3000 Ibs of dry sludge per day.39'57 After treatment, sewage sludge contains 79-99% water by
weight, while the remaining part is solid, such as organic matter, metals and microorganisms.
Table 3 gives the results of ultimate and proximate analysis performed on dried biosolids by five
companies/institutions which have researched biosolids gasification. As with paper mill sludge,
the composition of sewage sludge will determine if it is a suitable candidate for gasification.
High ash contents yield low heating values, while high sulfur and chlorine content may
foreshadow high costs in the removal of these chemicals in the syngas prior to combustion or
from emissions after combustion.
Table 3 provides data on sewage sludge composition as reported by the University of California
Riverside (UCR), Nexterra, M2 Renewables (M2R), the University of Seoul (UoS) and the
Tokyo Institute of Technology (TIT) in separate reports relating to sewage sludge.
Table 3 - Ultimate and proximate analysis of different sewage sludge samples.
Analysis
UCR18
Ultimate Analysis Mass %, dry
Ash
Carbon, C
Hydrogen, H
Nitrogen, N
Sulfur, S
Chloride, Cl
Oxygen, O (difference)
Total
Proximate Analysis
Volatile Matter
Fixed Carbon
Ash
Moisture
Total
HHV(Btu/lb)
20.85
41.62
6.03
7.82
0.95
0.00
22.73
100.00
NA
NA
NA
NA
NA
Nexterra19
Mass %, dry
17.30
44.40
6.31
5.10
0.00
0.07
26.82
100.00
Mass %, dry
72.41
10.29
17.30
0.00
100.00
8490.00
M2R20
Mass %, dry
6.00
45.00
6.00
3.00
1.00
0.00
39.00
100.00
Mass %, dry
80.00
15.00
5.00
0.00
100.00
8000.00
UofS21
Mass %, daf
0.00
55.50
8.20
7.40
1.10
0.00
27.80
100.00
Mass%
66.80
0.80
26.80
5.60
100.00
7380.00
TIT22
Mass %, daf
0.00
69.20
4.60
2.20
1.70
0.00
22.30
100.00
Mass%
39.30
19.40
30.10
11.20
100.00
NA
* Dry ash free
In 2009, the EPA released the Targeted National Sewage Sludge Survey (TNSSS) where 84
sludge samples from 74 WWT facilities in the U.S. were collected and analyzed for 145 different
analytes.
The TNSSS was designed to: (1) obtain updated occurrence information on nine analytes of
potential concern, and (2) obtain occurrence information on a number of contaminants of
emerging interest identified by EPA and the National Research Council (NRC) that may be
present in sewage sludge generated by POTWs.12
15
-------
Briefly, the survey found:
Nitrite/nitrate, fluoride and water-extractable phosphorus was found in every sample.
27 metals were found in virtually every sample, with one metal (antimony) found in 72
samples.
Of the six semivolatile organics and poly cyclic aromatic hydrocarbons, four were found
in at least 72 samples, one was found in 63 samples, and one was found in 39 samples.
Of the 72 pharmaceuticals, three (i.e., cyprofloxacin, diphenhydramine, and triclocarban)
were found in all 84 samples and nine were found in at least 80 of the samples. However,
15 pharmaceuticals were not found in any sample and 29 were found in fewer than three
samples.
Of the 25 steroids and hormones, three steroids (i.e., campesterol, cholestanol, and
coprostanol) were found in all 84 samples and six steroids were found in at least 80 of the
samples. One hormone (i.e., 17a-ethynyl estradiol) was not found in any sample and five
hormones were found in fewer than six samples.
All of the flame retardants except one (BDE-138) were found in every sample.
The reason for the inclusion of the results of this report are to inform the reader that sewage
sludge composition is much more complex than what can be found in a typical ultimate analysis.
The effect of gasifying many of the analytes found in the survey has yet to be determined.
4.2 Unique Aspects of Sludge Gasification
During gasification, sludge undergoes a physical and chemical change, similar to other biomass
feedstocks. Due to the high moisture content, (the mass of untreated sludge being 79-99% water)
it is necessary for the sludge feed to be dried or dewatered in some way before entering the
reactor (this is true for most reactors, although some technologies that operate at much higher
temperatures, such as plasma gasification, have the ability to handle sludge without pre-
treatment). The gasification process itself will not be influenced by the high moisture content,
but because of the high energy demand required by the system to vaporize the moisture, the
capacity and economics are affected.14
Perhaps the largest obstacle related to sludge gasification is reducing the water content to a level
suitable for gasification. Mechanical processes are more desirable than thermal processes from
an energy standpoint, but secondary (e.g., activated) sludge can only be mechanically dewatered
to about 40% solids.65 The difficulty in removing water from secondary sludge lies in the water
trapped inside the sturdy cell walls of the organisms used to consume the biological oxygen
demand of the wastewater that remains after primary settling. To remove this moisture, the cell
walls must be broken.
Paper mill sludges are capable of being reduced to 50% moisture using a belt press followed by a
screw press.56 To finish the preparation process, thermal energy is required to remove the
remaining moisture.
Biosolids are the nutrient-rich organic materials resulting from the treatment of sewage sludge
(the name for the solid, semisolid or liquid untreated residue generated during the treatment of
16
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domestic sewage in a treatment facility). Sewage sludge becomes biosolids when treated and
processed to achieve the pathogen and/or pollutant limits set forth by the EPA's Part 503
Biosolids Rule. The limits can vary based on the biosolids use or disposal and classification.73
Composting and lime stabilization are techniques that would be eliminated in a gasification
scenario. Aerobic digestion would also likely be eliminated. Anaerobic digestion, although
compatible with gasification, is not preferred as a pre-treatment step since much of the chemical
energy of the biosolids is removed and thus unavailable for recovery in the syngas.
After the treated sludge is dewatered then dried to a low enough water content to be properly
gasified (80-90% solids), the dried sludge enters the gasifier chamber, and undergoes the first
step of gasification in a typical fixed bed downdraft gasifier, drying.14
In the drying zone, as with other feedstocks, the sludge descends into the gasifier and moisture is
evaporated using the heat generated in the zones below. The rate of drying depends on the
surface area of the fuel, the recirculation velocity, the relative humidity of these gases and
temperature differences between the feed and hot gases, as well as internal diffusivity of
moisture within the fuel. Sludge with less than 15% moisture loses all moisture in this zone.14
In the pyrolysis zone, the irreversible thermal degradation of dried sludge descending from the
drying zone takes place using the thermal energy released by the partial oxidation of the
pyrolysis products. The release of volatiles from sludge begins at about 250 °C, and 60-70% of
sludge is converted to a complex liquid fraction comprising water, tars, oils, a gaseous phase
including CO2, CO, H2, and a variety of other hydrocarbons, and un-reacted char and ash. As
with other feedstocks, it is expected that pyrolysis of sludge in a reactor typically occurs at
temperatures between 350 and 500 °C.14
In the throat zone (often referred to as the oxidation zone), the volatile products from the
pyrolysis process are partially oxidized in highly exothermic reactions, resulting in a rapid rise in
temperature (up to 1100 °C). The heat generated is used to drive the drying and pyrolysis of
sludge and the gasification reactions. The oxidation reactions of the volatiles are very rapid and
the oxygen is consumed before diffusing to the surface of the char. No combustion of the solid
char can, therefore, take place. Oxidation of the condensable organic fraction to form lower
molecular weight products is important in reducing the amount of tar produced. During sludge
gasification, it is expected that oxidation zone temperatures would be between 1000 and 1100
°C. The products, including CO2, CO, H2, H2O, high chain hydrocarbon gases, residual tars and
char, then pass on into the gasification zone.14
In the reduction zone (often referred to as the gasification zone), the char is converted into gas by
reaction with the hot gases from the upper zones. The gases are reduced to form a greater
proportion of H2 and CO. Temperatures of the gases entering this zone are about 1000-1100 °C
and exit around 700 °C.14
17
-------
Example: Pulp and Paper Sludge Energy Balance
Typical pulp and paper mill sludge energy content (HHV) is around 3600 Btu/lb, dry. If sludge at
10% moisture is fed into a fixed bed, air blown gasifier, the syngas energy content would be
approximately 130 Btu/scf. If syngas coming out of a 5 ton/day gasifier is sent to an electrical
generator with a 40% electrical efficiency, 108 kW of gross electricity could be produced. Taking
into account a parasitic load from the gasifier, dryer, and cleaning system of approximately 75 kW, a
positive net output is possible, but extremely difficult to achieve.
Example: Sewage Sludge Energy Balance
Typical sewage sludge energy content (HHV) is around 8000 Btu/lb, dry. If sludge at 10% moisture
is fed into a fixed bed, air blown gasifier, the syngas energy content would be approximately 190
Btu/scf. If syngas coming out of a 5 ton/day gasifier is sent to an electrical generator with a 40%
electrical efficiency, 240kW of gross electricity could be produced. Taking into account the parasitic
load from the gasifier, dryer and cleaning system, a net output of about 165 kW can possibly be
achieved. If this system is creating a syngas composition of 15% H2j 15% CO, 3% CH4 17% CO2and
50% N2, the emissions will be approximately 200 ppm NOxand 2000 ppm CO at 5% O227
4.3 Environmental Consequences
Most of the research conducted in the area of sludge gasification has been strictly on gasifier
performance, with the syngas created immediately being combusted after exiting the gasifier.
Although this is one option, most research efforts have been focused on coupling multiple
processes. To accurately determine the effects on the environment, a gasifier would need to be
coupled with a syngas cleanup system, syngas combustion or conversion system and thermal
oxidizer (or whatever the final setup would be at commercial scale). Quantitative data relative to
all waste streams of a gasification system is limited due to the lack of fully integrated processes.
However, data relating to individual processes of different systems has been obtained and is
presented in the sections to follow.
Despite the dependency on feedstock composition and process, there are some waste products
which are produced in nearly all gasification processes which must be treated to prevent release
into the atmosphere. The pathways and forms of the wastes are dependent on many conditions
within the system, but should be accounted for when viewing the system as a whole. Criteria air
pollutants (CAPs), hazardous air pollutants (HAPs), GHGs and waste water streams apply to
nearly all gasification systems and need to be considered when determining the environmental
impacts of a process.
4.3.1 Criteria Air Pollutants
The amount of SOx, CO, NOx and particulate matter (PM) created during the sludge gasification
process will vary according to gasifier type, syngas clean up system, end use (combustion system
and/or thermal oxidizer), and composition of feedstock.
Sulfur in the feedstock, when gasified, will produce H2S, COS and low concentrations of
mercaptans and CS2. The sulfur content in the feedstock as well as the gasifier temperature will
influence the distribution of sulfur in the product gas. Most of these sulfur compounds, unless
18
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removed through a cleaning system, will become 862 when combusted.23 Using an alkali
absorption solution or a dry sorbent (ZnO), most of the sulfur containing compounds can be
captured. A study conducted by UC Riverside found that only 10% by mass of the sulfur in the
feedstock was recovered in the ash. Based on UC Riverside's ultimate analysis, sulfur comprises
1-2% of the original dry feedstock mass.18 Therefore, a 5 TPD processing plant would need to
capture approximately 90 kg/day of sulfur in a cleanup system downstream of the gasifier for
sulfur free emissions.
Carbon monoxide concentration in syngas is highly dependent on the gasifier and can range
widely. CO produced during the gasification process is combusted in an engine, turbine or
oxidizer, depending on the system. Therefore, the remaining portion of CO in the gas stream
after combustion is dependent on the efficiency of the energy conversion system. Table 4
provides emissions values coming out of the exhaust of a commercial scale gasifier processing
sewage sludge which is coupled with a thermal oxidizer and bag house in Sanford, FL. The heat
produced in the thermal oxidizer and gasifier in this system is used to dry the feedstock entering
the system.
Table 4 - Emissions data submitted by Maxwest to the Florida environmental agency.
Pollutant
Cadmium (Cd)
Carbon Monoxide (CO)
Dioxin/Furan TEQ
Hydrogen Chloride (HC1)
Lead (Pb)
Mercury (Hg)
Oxides of Nitrogen (NOX)
Paniculate Matter (PM)
Sulfur Dioxide (SO2)
MaxWest
7.23E-05
7.87
0.0285
1.8
8.19E-04
7.98E-03
432.17
9.6
4.17
Allowable
0.095
3800
0.32
1.2
0.3
0.28
220
80
26
Unit (7% O2)
mg/dscm
ppmvd
ng dscm
ppmvd
mg/dscm
mg/dscm
ppmvd
mg/dscm
ppmvd
HC1 and NOX values are greater than allowed, but MaxWest has three years to meet the HC1
allowable limit and preliminary tests show that current NOX technology on the MaxWest system
are capable of meeting limits.
Due to the high nitrogen content in most of the sewage sludge samples, NFL? and HCN,
precursors to NOx, will be produced during combustion, if the syngas is used in a combustion
process. Nitrogen containing compounds can be removed from the syngas stream by means of
liquid scrubbing or through the use of dry sorbents. Removal of these molecules prior to
combustion is ideal, thereby reducing NOx emissions after combustion, which can be costly.
Very little, if any, data is available on PM levels in a sewage sludge gasification plant. PM is
typically captured using cyclones, water scrubbers and bag houses in the syngas cleaning
process. Once again, the levels are dependent on the efficiency of the system being used. For the
commercial scale Maxwest system mentioned earlier, 11.96 tons per year (TPY) of PM
emissions are predicted by Maxwest without a baghouse and 0.12 TPY with a baghouse.24
19
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4.3.2 Hazardous Air Pollutants
41
There are currently 187 HAPs regulated by the EPA. As with CAPs, HAP levels are highly
dependent on the process and can only be determined through empirical data analysis.
The amount of HC1 and dioxins (chlorinated organics) created during gasification is dependent
on temperature and feedstock composition. In the presence of oxygen, chlorine, organic
compounds and temperatures above 300 °C, dioxins are formed. A substantial amount (50 to 90
wt %) of the chlorine is typically bound in the ash. The removal of HC1 in the syngas can be
achieved through liquid scrubbing systems or a dry system in which the syngas is passed through
an absorbent such as sodium carbonate or calcium oxide. The production of dioxins decreases
with an increased temperature (>850 °C), increased oxygen content, low Cl content in the
feedstock and small (<2s) residence time.23 Dioxin formation as a result ofde novo synthesis
occurs from the presence of unreacted carbon in fly ash or flue gas, C^, C>2 and a metal
catalyst66, but research on the formation of dioxins during combustion of syngas could not be
found in the references used for this report. It should be noted that very little carbon and C12 are
present in clean syngas that is fed into a combustion process.
In a study conducted by the University of Seoul (UoS), a number of metals were identified in the
char produced during gasification of sewage sludge. The UoS used a laboratory scale two-stage
gasifier in which the first stage was a fluidized bed and the second stage, a reactor filled with
activated carbon for tar cracking. The activated carbon, as well as the char captured in the
cyclone was analyzed for metals. The concentration of metals in the char is larger because the
mass of the sludge is reduced while maintaining nearly the same mass of total metals. Table 5
shows how the metals entering the system are accounted for after gasification in the ash and
activated carbon filter.21 The UoS study did not provide information on the amount of sludge fed
during the run, but the feed rate was 18g/min.
Table 5 - Metal concentrations in the different stages of the UoS study21
Element
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn
Concentration (ppm) of metal in
Dried Sludge Activated Carbon
below 20
2
603
633
22
63
91
1377
1
3
9
8
1
6
0
below 20
below 1
12.67
13.3
below 20
21.39
3.415
41.7
Char
below 20
4.8
1278.0
1456.0
27.0
139.9
205.2
2678.0
Based on the concentrations shown above, the amount of these metals processed in a 5 TPD
plant is shown in Table 6. The metals can either be collected in the dry solids stream (ash or
char), liquid stream from gas cleanup or in the flue gas stream after combustion.
20
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Table 6 - 1
.stimated amounts of metals processed during sludge gasification.
Element
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn
Weight ( kg/day),
processed in 5 tpd plant
<0.1
0.011
3.017
3.170
0.114
0.315
0.458
6.8850
In a separate study by the Imperial College of London (ICL), dried sewage sludge samples were
collected from five European water companies. A trace and minor element analysis was
conducted on the samples prior to being gasified in a laboratory scale fluidized bed gasifier.
Table 7 shows the results of the element analysis.
Table 7 - The range of concentrations of metals in the five different samples analyzed by ICL25
Sample
1
2
3
4
5
Composition of minor elements (ppm, wt)
Na Mg K Al Ti
2745
1058
2402
2393
2117
2740
4990
5120
6700
3687
4874
4684
3363
7140
3523
9421 1305
22426 1665
19563 1776
25606 1507
41259 1291
Ba
235
406
302
438
542
Composition of trace
Cr Hg Mn
33
94
75
224
304
1.1
2.1
1.0
2.1
1.0
273
203
236
188
121
elements
Ni
20
31
20
55
151
(ppm, wt)
Pb
57
250
112
215
79
Zn
761
789
492
966
1095
The ICL then analyzed the composition of the trace elements in the bed material and fines
collected in the gasifier. Their research concluded that virtually none of the mercury in the feed
was retained in the char. Additionally, the research concluded that lead, zinc and barium can be
released into the syngas stream, depending on gasifier temperature. This being the case, exhaust
emission quality will be highly dependent on the gas cleaning system. Wet scrubbing systems are
effective in removing all of the elements in the syngas except possibly mercury. An activated
carbon system may be necessary for removing the mercury. Leachability of elements captured in
the ash will need to be quantified to determine the effect on disposal to landfills, as little to no
data are available.
25
In general, the ICL study suggests that many processes in the gasifier bed contribute to the
release of trace elements from the feed. Attrition, entrainment, and volatilization of trace
elements in a gasifier can all have an effect on the distribution of the elements. The relative
importance of these processes is influenced by both the mode of the occurrence and the chemical
speciation of the trace elements. Trace elements that enter the gas phase may, in turn, be
transformed back to a condensed phase solid or liquid aerosol as the gases cool after leaving the
gasifier, by a combination of chemical reaction, homogeneous and heterogeneous condensation,
and absorption mechanisms.25
21
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4.3.3 Greenhouse Gases
CO2, N2O, CH4 and fluorinated gases are all GHGs, which trap heat in the atmosphere. GHGs are
often represented in terms of CO2 equivalents (CO2e), where the mass of the gas being
considered is multiplied by a global warming potential (GWP) to give the CO2e. N2O has a GWP
of 310 and CH4 has a GWP of 21, while fluorinated gases can range from 140 to 23,90026. These
potentially large numbers emphasize the importance of controlling these gases if identified in a
process.
Table 8 displays GHG emissions of several sewage sludge treatment processes represented in
CO2e. The numbers presented in the table were extracted from five articles and represent a
variety of processes. Each source used different boundaries and conditions, resulting in a large
variance from one source to another. Varying energy production methods, assumed system
efficiencies, distances traveled, feedstock composition, virgin fertilizer emissions and inclusion
or exclusion of biogenic CO2 may also be some contributing factors to the large ranges of values.
Based on the information extracted from the articles, no clear conclusions can be made.
Generally speaking, focusing only on the carbon pathway in the sludge, one ton of dry sludge at
40% carbon, if fully oxidized, will produce 1466 kg of CO2e. This does not include energy
consumed or created in the overall process. To provide an accurate representation of GHG
emissions from multiple sludge disposal processes, a life cycle analysis catered to a specific set
of conditions will need to be performed. This, however, is outside of the scope of this report.
Table 8 - GHG emissions in kg of CO2 equivalents per ton of dry sludge processed.
Source
Hospido 2005
Hong 2009
Lundin 2004
Poulsen 2002
Hong 2009
Poulsen 2002
Hospido 2005
Hong 2009
Hwang 2000
Poulsen 2002
Hospido 2005
Hospido 2005
Hong 2009
Process
Anaerobic digestion + Belt dewatering + Land application
Anaerobic digestion + Belt dewatering + Land application
Pasteurization + Land application
Anaerobic digestion + Dewatering + Land application
Anaerobic digestion + Belt dewatering + Landfilling
Anaerobic digestion + Dewatering + Landfilling
Centrifuge dewatering + Incineration
Belt dewatering + Incineration
Thickening + Dewatering + Incineration
Anaerobic digestion + Dewatering + Incineration
Press dewatering + Thermal drying + Pyrolysis (using only syngas)
Press dewatering + Thermal drying + Pyrolysis (using syngas, char and tar)
Belt dewatering + Thermal drying + Gasification
CO2e
1550
251
-77
72
728
-179
1800
334
83
222
1250
1650
1019
Using a GHG calculator tool (Biosolids Emissions Assessment Model, BEAM)68, GHG
emissions were estimated for composting, landfill disposal, fluidized bed combustion and land
application of one dry ton of undigested, lime stabilized sewage sludge at 25% solids. Default
values of 5% nitrogen, 1.9% phosphorus, 70% total volatile solids and 39.2% organic carbon
were used in the model. For the landfill scenario, it was assumed that no electricity was
22
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generated from the collected methane, the combustion scenario assumed that no electricity was
generated and 75% of the heat was recovered and the compost scenario assumed a windrow
operation. The GHGs accounted for are strictly for the individual processes and not what occurs
up and down stream. The results are displayed in Table 9.
In the composting and land application scenarios, the use of the sludge as a replacement for
commercial fertilizer and the sequestration of carbon into the soil, significantly reduces the
CO2e. With the landfill option, CO2e is large due to methane and nitrous oxide creation, as well
as the CO2 emissions from flaring the biogas. Similar to the landfill scenario, combustion values
are large mainly as a result of nitrous oxide creation and the liberation of carbon in the
combustion process.
Table 9 - GHG emissions using BEAM68
GHG
kg CO2e per dry ton
Compost
10
Landfill
2880
Combustion
1780
Land Application
-300
4.3.4 Wastewater
Depending on the process, waste water production and disposal will need to be considered when
analyzing a system. Potential sources of wastewater from a gasification process are from the
drying process and gas cleaning process as represented in Figure 4. Once again, the composition
of these streams varies, depending on the process. The moisture coming from the dryer will be
feedstock and temperature dependent. When using high temperatures to dry a feedstock, some of
the volatiles in the sludge may become liberated and will reside in the water stream once
condensed. Data on the composition of the waste water stream from the dryer was not available
for this report.
In the cleaning system, post gasification, a variety of methods may be instituted to remove
pollutants. If utilizing a caustic, acid or water scrubber to remove contaminants, a wastewater
stream will inevitably be created which must be disposed or treated appropriately. A complete
syngas cleaning system was not identified for any of the systems researched for this report,
therefore no quantitative data is available.
4.3.5 Direct Environmental Advantages of Gasification
Gasification, when compared to incineration, potentially poses several desirable environmental
benefits by reducing and preventing many emissions. One reason for this is the ability to remove
compounds through simple cleaning and scrubbing which would later form pollutants during the
combustion process. In addition to many possible cleaning methods, gas flow coming from a
gasifier is significantly lower than gas flow from an incinerator of the same sludge processing
capacity. Typical fixed bed gasifiers require approximately 40% of the amount of stoichiometric
air required67 for complete oxidation, while incinerators typically require greater than 100% of
stoichiometric air required for complete oxidation. The addition of this air in an incineration
process increases the total gas flow through the system, requiring larger equipment and handling
more dilute gas streams.
23
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Many of the CAPs and HAPs mentioned in sections 4.3 and 4.3.2 are retained in the ash, which
is captured in cyclones, removed from the bottom of the gasifier or in bag house filters. This
allows better control of the pollutants which can be disposed of properly once consolidated.
Pollutants in the gasifier product gas can be captured with water, caustic, or acid scrubbers or dry
processes such as zinc oxide, sodium oxide, or calcium oxide beds. Removing these compounds
prior to combustion helps to reduce emissions created during the combustion process.
Gasification also has the capability of reducing GHG emissions via utilization of a renewable
waste feedstock to produce electricity or thermal energy production which may have been
otherwise produced by fossil fuels. The magnitude of GHG reductions is highly site, technology
and/or feedstock specific. The destruction of methane during the combustion section of the
system and the control of nitrogen during the cleaning process will also help to reduce GHG
emissions.
4.3.6 Social Sustainability
To be socially acceptable, new technology must often prove to be more beneficial in a variety of
aspects than traditional practices. Ideally, an overall reduction in GHG emissions, more
favorable HAP and CAP emissions as well as net energy gains, presented in a verified fashion,
would quell any social concern. It is also critical to educate applicable populations in a clear and
understandable manner, to eliminate the fear of the unknown.
Although this report provides some information, the lack of commercially deployed technology
and associated independent data inhibits the ability to fully evaluate sludge gasification and its
impacts on social acceptance.
The Blue Ridge Environmental Defense League (BREDL) has not published any reports
specifically related to sludge gasification, but a report on solid waste gasification was published
in 2009.43 In the report, the BREDL describes a gasification process which immediately
combusts the syngas upon exiting the gasifier without a cleaning process or energy conversion.
The report also presents emission values from a system in which the only pollution control
device is an electrostatic precipitator. More specifically, the BREDL claims that gasification
plants emit nitrogen oxides, sulfur dioxide, particulate matter, carbon monoxide, methane,
hydrogen chloride, hydrogen fluoride, ammonia, dioxins and furans, without presenting verified
data. If restricted to these variables, gasification seems to be no different from incineration,
however little to no gasification processes exist which follow this model. With proper syngas
cleaning, most of the pollutants can be captured and contained, all while staying within EPA
regulations.
The Sierra Club has created videos explaining the disadvantages of gasification, specifically of
medical waste. Arguments made by the Sierra Club are similar to those made by BREDL, in that
well engineered gasification systems are not considered and definitive data showing the alleged
pit falls of gasification are not presented.
Regardless of the benefits of emerging technologies designed to dispose of sludge, there are
coordinated programs that can help to alleviate the negative aspects of sludge disposal. As an
24
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example, Central Contra Costa Sanitary District (CCCSD) instituted a hazardous waste
collection program along with an amalgam separation program through local dental offices,
which helped to reduce mercury concentrations in the waste stream by 70% from 2004 to 2008.
In concurrence with the amalgam separator program, CCCSD also worked with local hospital
and schools to reduce Hg going into the sewer which also contributed to the decrease. The 70%
decrease in Hg translated into an approximate 40 - 55% decrease in Hg stack emissions coming
from their multiple hearth incinerator. To meet the proposed limits, discussed in section 6.0, it is
imperative that prevention programs be instituted. A reduction in heavy metals in the influent
greatly reduces the cost of capturing these metals after being incinerated.
Because of the wide variety of designs, including end uses and cleaning systems, processes may
be developed that have negative impacts and are not socially acceptable. However, the systems
that are designed that meet or exceed regulations will be favored. Users must be cautious in their
evaluations and conclusions and remember that not all gasification systems are the same and
must be evaluated on a case by case basis.
4.4 Brief Technology Comparison
Gasification and incineration are often compared as sludge management processes, in that they
both convert hydrocarbon-based materials in sludge into simple, nonhazardous byproducts.
However, the conversion mechanisms, chemical reactions, and the nature of the byproducts vary
considerably.16 The clear advantages of gasification are a more versatile product, as syngas may
be used in a variety of applications, and lower costs associated with gas cleaning, depending on
the ultimate goal, as syngas volume is significantly lower than flue gas volume from an
incinerator of a similar processing capacity. The National Energy Technology Laboratory
(NETL) revealed the key differences between gasification and incineration as summarized below
in Table 9.
Table 10 - Brief Technology Comparison Incineration vs. Gasification1'
Subsystem
Incineration
Gasification
Combustion/
Gasification
Designed to maximize the conversion of
feedstock to CO2 andffiO
Large quantities of excess air
Highly oxidizing environment
Operated at temperatures below the ash melting
point
Mineral matter converted to bottom ash and fly
ash
Designed to maximize the conversion of feedstock to
CO and m
Limited quantities of oxygen
Reducing environment
Operated at temperatures above the ash melting
point
Mineral matter converted to glassy slag or ash and
fine particulate matter (char)
Gas Cleanup
Flue gas cleanup at atmospheric pressure
Treated flue gas discharged to atmosphere
Fuel sulfur converted to SOx during
combustion and discharged with flue gas or
scrubbed in a flue gas treatment system.
Treated syngas used for chemical production and/or
power production (with subsequent flue gas
discharge)
Recovery of reduced sulfur species in the form of a
high purity elemental sulfur or sulfuric acid byproduct
25
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Subsystem
Residue and
Ash/Slag
Handling
Incineration
Bottom ash and fly ash collected, treated, and
disposed as hazardous wastes
Gasification
Slag from a high temperature gasifier is non-
leachable, non-hazardous and typically suitable for
use in construction materials.
Gasifier ash is handled similarly to Incinerator ash
Fine particulate matter recycled to gasifier or
processed for metals reclamation
5. Commercial Status of Sludge Gasification
To determine the commercial status of sludge gasification, a list of known gasification vendors
was first created. The list of known vendors was compiled from internet searches, marketing
materials, journal searches and referrals from stakeholders.
5.1 Industry Assessment Results
Several case studies and pilot/demonstration plant projects have been completed and a few
commercial facilities are in operation. Forty three vendors were originally identified for the
purpose of this study, as potentially capable of sludge gasification, based on marketing material,
journal searches, internet search engine searches, referrals, end user discussions, etc. Internet
searches were performed for each vendor to verify that sludge was mentioned as part of their
gasification capability. If the information found via the web based search did not provide enough
information on the vendor's process to determine their technology readiness level (TRL, see
Table 11), the vendor was contacted via phone, email or both to obtain additional information
not found in literature. From that list, vendors who were not contacted via phone initially were
then contacted to obtain data which was unavailable in literature searches. The vendors which
could be confirmed, based on the data available, as having a TRL rating of 4 or 5 were selected
for further analysis in this report.
Table 11 - Technology readiness level (TRL) parameters
TRL
0
1
2
3
4
5
Description
No data available or irrelevant technology.
Data contains basic principles that are observed and
reported.
Sensible applications and theories from basic principles
are devised. Basic principles are not only observed, but
applied with reasonable awareness. These theories are
still exploratory and may contain little or no commercial
evidence to verify assumptions.
Verification of a specific concept within a given
technology.
Data confirmed in a relevant environment.
Data confirmed in an operational environment.
Examples
Dewatering technology company or unsupported claim.
Projected values, engineering assessment/modeling, literature
studies of fundamental criteria
Analytical Studies; Modeling
Analytical Studies coupled with laboratory studies; bench
scale measurements
Pilot Plant
Demonstration scale/ commercial plant actual results;
independent results
26
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Of the 44 vendors (Table 12) originally identified as claiming to have the capability of sludge
gasification, only two were identified as running consistently at commercial levels, Kopf and
Maxwest. The quantity of data available for M2 Renewables and Nexterra, stemming from a
significant amount of research and testing being done in the field of sludge gasification, along
with a TRL rating of four, enabled a comparison with the Kopf and Maxwest systems. Based on
the volume of literature available and research completed at bench, pilot, demonstration and
commercial scale, it is clear that there is a significant amount of interest in sludge gasification.
Despite the abundance of literature and research, very few companies have produced data
necessary for a complete assessment. Necessary data are those from a pilot or commercial
process, where sludge is being used as a feedstock. It should be noted that many companies have
achieved TRLs for their gasifiers preferred feedstock, but the TRLs listed in this report only
relate to sludge gasification. There are many potential reasons why there are so few commercial
sludge gasification operations running at present, with the primary being economics, energy
prices, regulatory restrictions, social acceptance and lack of capital.
Table 12 - Original list of vendors and the result of their investigation
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Company
ACTI
Allied Syngas Corp
BioConverters LLC
Biomass Gas and Electric LLC
Bio-Petrol
Biosyn
Carbona
Coaltec Energy USA, Inc.
Community Power Corp
Ebara
Energy Products of Idaho
Enertech
Ensyn
Foret Plasma Labs
Genahol LLC
Grand Teton Enterprises
Green Planet Fuel and Energy
Inetec
Innovative Logistics Solution
Interstate Waste Technologies
Lurgi
Masada Resources Group
Maxwest
Nexterra
Omnifuel Technologies, Inc.
Technology
Fixed-bed Gasifiction
Fixed-bed Gasifiction
Biological Destruction
Fluid-bed Gasifiction
Pyrolysis
Fluid-bed Gasifiction
Fluid-bed Gasifiction
Fixed bed
Fixed bed
Fluid-bed Gasification
Fluidized bed
Gasification
Sludge Pyrolysis
Plasma Gasification
Biomass Gasification
Gasification
Gasification
Anaerobic Digestion
Gasification
Gasification
Catalytic process
Hydrolysis
Fixed bed
Fixed bed
RDF Gasification
Sludge
TRL
1
1
0
0
1
1
0
0
0
3
0
0
0
0
0
1
0
0
1
1
0
0
5
4
1
Notes
Not Gasification
Company dissolved
Co-gasification with MSW
Combustion
Fossil fuel gasification
Type unknown
Type unknown
Not Gasification
Merged with Pyromex
Type unknown
Not gasification
Not gasification
Selected for report
Selected for report
No sludge gasification
27
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No.
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
Company
Pinnacle Biotech
PRM Energy
Prime Energy
Princeton Environmental
Pyromex AG
Ren Waste
Skelde
Solena
Startech Environmental Corp
Taylor Biomass Energy LLC
TRI
US Centrifuge
Westinghouse
Wright Environmental
Ze-gen
City of Stamford WPCA
Bureau of Sewerage, Tokyo
Kopf
M2 Renewables
Technology
No info available
Fixed-bed Gasification
Fixed-bed Gasification
Plasma Gasification
Fixed-bed Gasification
Gasification
No info available
Plasma Gasification
Plasma Gasification
Fluid-bed Gasification
Fluid-bed Gasification
Dewatering technology
Plasma Gasification
No info available
Liquid Metal Gasification
Fixed bed
Fluidized bed
Fluidized bed
Dewatering technology
Sludge
TRL
0
1
3
1
4
1
0
1
1
1
1
0
0
0
0
4
5
5
4
Notes
Selected for report
Type unknown
Bankruptcy
No sludge gasification
Only develop torches
Selected for report
Selected for report
Selected for report
Selected for report
5.2 Selected Technology Profiles
In the sections to follow, a summary is given for four vendors which were identified for further
analysis. Among the seven identified sludge gasification companies in Table 12, Nexterra and
City of Stamford WPCA worked together on the project, M2R and Pyromex AG worked
together as well. The summaries provided below are a combination of available literature, phone
conversations and email correspondences with each vendor. Tables 13, 14 and 15 provide
technical data on each of the processes.
5.2.1 Maxwest Environmental Systems24'28'72
Maxwest and its strategic partner, CPH Engineers, Inc., permitted, constructed and
commissioned a commercial scale gasification system in September 2009 that focused on
reducing sludge disposal costs and requirements. According to Maxwest, the system is estimated
to save the city of Sanford, FL approximately thirteen million dollars on natural gas purchases
over the contract life (20 years).
The system utilizes a continuous feed dryer system manufactured by Therma-Flite, Inc., which
replaced the city's existing batch fed dryer, and feeds into a fixed bed updraft gasifier. The
syngas created during the gasification process is fed directly into a thermal oxidizer, while the
ash is removed from the bottom of the gasifier. Once in the thermal oxidizer, the syngas is
combusted (differentiating it from an incineration process) and the heat is recovered in an
28
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economizer, while the exhaust flows through a bag house and cooling tower/scrubber for PM and
pollutant control. The heat recovered in the economizer is transferred via a thermal oil system.
The hot oil is pumped to the continuous feed dryer to help dry the sludge being fed into the
gasifier. A process flow diagram of the system is displayed in Figure 8.
During startup, natural gas is required to heat the dryer, gasifier and oxidizer. Once heated, the
gasifier creates its own heat through the energy released in the exothermic reactions occurring in
the gasifier. At this point, no natural gas is needed to heat the gasifier. Once syngas is being
produced and fed into the oxidizer, enough heat will be created through the oxidizing of the
syngas to maintain the temperature of the oxidizer without the need for natural gas. The heat
collected in the economizer provides enough energy to dry the incoming sludge to the desired
moisture content, thereby eliminating the need for natural gas in the dryer. Once fully running,
the system is self-sustaining, achieving a net zero thermal energy demand, but electrical inputs
will still be required.
Maxwest submitted an applicability determination to the EPA's Office of Enforcement to justify
an exemption from 40 cfr §61.52, to differentiate the system from incineration. The regulation
states that emissions to the atmosphere from sludge incineration plants, sludge drying plants, or a
combination of these that process wastewater treatment plant sludges shall not exceed 3.2 kg (7.1
Ib) of mercury per 24-hour period. Maxwest argues that because their system is not considered a
combustor, the regulation does not apply. As of this moment, Maxwest has not received a
response to the applicability determination. Mercury emissions from the MaxWest system based
on the values in Table 4 are 0.00059 kg per 24-hour period.
Sanford Process Flow Diagram
Figure 8 - Process flow diagram of Maxwest system after modifications
29
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5.2.2 *M2 Renewables (M2R) & Pyromex AG20'30'7S
M2 Renewables (M2R), formerly Micro Media Filtration, specializes in designing solids
removal systems for POTWs using microscreen technology. Their systems are designed to create
favorable, low moisture, sludge conditions for gasification. M2R is currently working with
Powerhouse Energy, Inc. to develop a complete sewage sludge to energy system. The ultra-high
temperature gasifier technology comes from Pyromex Holding A.G., which recently released a
manufacturing and distribution license to Powerhouse. According to a M2R representative, the
gasifier was recently certified by the European Union for the treatment of biosolids. To complete
the system, M2R is in the process of selecting a post mechanical treatment dryer, syngas cleanup
system and power generation system. A picture of the Pyromex UHT gasifier is displayed in
Figure 9.
The gasifier operates at temperatures around 1,150°C in the absence of oxygen. A small amount
of nitrogen is used as the gasification medium. This is achieved by using silicon carbide electric
resistance heating elements to supply enough energy to complete the endothermic gasification
reactions. The main source of oxygen and hydrogen in the syngas comes from the moisture
contained in the feedstock. A representative from M2R states that a carbon to oxygen molar ratio
of 1:1 in the feedstock is ideal for the UHT gasifier. This translates into a moisture content of
approximately 20%, depending on the composition of the dry components in the biosolids.
After completion of a program to characterize its solids through a sampling and analysis
protocol, M2R proceeded to test various biosolids samples in al TPD Pyromex pilot gasifier in
Munich, Germany. A summary of the fresh solids analysis is shown in Table 3. In January and
June of 2010, numerous tests were run with varying feedstock composition and the syngas was
analyzed to determine its optimal use following gasification. Due to the high efficiency of the
mechanical drying system, less thermal drying is required during pretreatment. This fact, coupled
with the high energy density syngas produced in the air free gasification chamber, enables net
energy gains.
Based on the testing done on the pilot unit, scaling calculations were performed by M2R to
represent the energy balance of a system operating at a 20 MOD WWTP. The energy balance is
based on the use of M2R's screening technology and the Pyromex gasifier using typical solids
loading rates and biosolids composition. In summary, the WWTP would require a 1.8 MW ICE
(derated to account for high hydrogen content), producing approximately 1.4 MW of power. The
internal energy consumption is around 430 kW, with 63% being attributed to the resistance
heaters. An energy balance based on the tests performed in the pilot unit is displayed in Table 13.
* Following peer review, Southern was contacted by a representative of M2 R with additional technical
information. The information extracted from reference 75 is contained in the 2nd and 4th paragraphs of this
section has not been peer reviewed.
30
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Figure 9 - Pryomex Ultra High Temperature gasifier in Munich, Germany
Table 13 - Maxwest and M2 Renewables overview
Maxwest24'28'72
M2
Renewables/Powerhouse/Pyromex
20,
30
Location
Sanford, Fl
Emmerich, Germany
Technology
Fixed Bed Updraft, refractory-
lined steel gasifier
Ultra high temp electrically heated
gasifier
Readiness Level
Commercial
Demonstration
Feedstock Pretreatment
Post digestion sludge at 2-3%
solids is mechanically dried with
a belt filter press to 16-18%
solids. The sludge is then fed into
a continuous indirect heat
biosolids dryer to 80-90% solids.
M2 system: Fresh solids are dewatered
to 30% solids w/ intergral screw auger,
to 55% solids w/ hydraulic ram, then to
95% solids with indirect heat dryer. For
this test, used 17% moisture.
Gasification System
Performance
Max Capacity (dry)
Internal Energy Consumption
Energy Output
Gross Chemical (syngas)
Gross Thermal
Net Electrical
Net Thermal
1440 Ibs/hr
10 MM Btu/hr
NA
10 MM Btu/hr
NA
NA
831bs/hr(lTPD)
12 kW
0.1 MM Btu/hr
NA
12.6 kW
NA
31
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Syngas Composition
Gas Cleanup
By products/Waste Streams
Potential Emissions
Cadmium (Cd)
Carbon Monoxide (CO)
Dioxin/Furan TEQ
Hydrogen Chloride (HC1)
Lead (Pb)
Mercury (Hg)
Oxides of Nitrogen (NOx)
Paniculate Matter (PM)
Sulfur Dioxide (SO2)
Product/Byproduct end use
Economics
Maxwest24'28'72
Not Available
Baghouse
Water from dryer, ash from
gasifier, PM and ash from bag
house
7.23E-05 mg/dscm
7.87 ppmvd
0.0285 ng/dscm
1.8 ppmvd
8. 19E-04 mg/dscm
7.98E-03 mg/dscm
432. 17 ppmvd
9.6 mg/dscm
4.17 ppmvd
The syngas is fed directly into a
thermal oxidizer where an
economizer captures the heat to
run the dryer.
Not available
M2
Renewables/Powerhouse/Pyromex20'
30
63% H2, 29.9% CO, 2.6% CO2, 1.8%
CH4
Liquid system using caustic and /or
acidic solutions
Ash from gasifier, water from dryer and
liquids from scrubbers
Not available
Syngas will be fed into an internal
combustion engine or turbine for
electricity production.
Scale up to a 15 MW plant is estimated
at $60 million
5.2.3 Kopf29'
35
The main components of the Kopf gasification technology are: a solar drying unit, a fluidized-
bed gasification unit, a gas engine unit for energy recovery and a post combustion chamber for
burning excess syngas. Some of the specifics of the system can be found in Table 14 with a
schematic of the system displayed in Figure 10. A unique feature of the process is the Thermo
System solar drying unit, which dries the wet digested sludge to a solid content of between 70
and 85% over a period of 2 to 8 weeks, depending on the weather conditions. Since this thermal
energy is 'free', the energy and operating cost requirements compared to other processes using
fossil fuel for drying are substantially lower, with a reduced carbon footprint. With 36 sludge
dryers operating in Europe, solar drying appears to be completely adaptable to the European
climate.
In the gasifier, which operates at 900 °C, pre-heated air is used to ensure the fluidization of the
bed. Inside the reactor, dried solids are converted into inert ash granules and combustible gas.
The gas is recovered and cooled to a temperature below 35 °C, dried and fed into an ICE. The
gas engine produces electricity, which is used to operate the gasification process and to offset the
energy demand of the sewage works. Recovered thermal energy is used to heat the digesters at
32
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the waste water plant. Natural gas is required for plant start-up, but after the start-up phase, no
external fuel is needed.
The Balingen Sewage Works treats an annual wastewater flow of 10 MM m3. To utilize the
energy content of the digested sludge, the local association for wastewater cleaning installed a
sludge gasification plant. In August 2004, a fluidized-bed gasification plant, manufactured by
Kopf was constructed at the WWTP for processing the digested biosolids and recovering energy.
The Balingen plant processes about 230 kg of sewage sludge per hour. Depending on the degree
of drying, this is the equivalent of 160 to 180 kg of dry sewage sludge. According to the
company, the ash produced amounts to about 85 kg/hour. The plant produces about 300 m3 of
exhaust per hour. Based on mass and energy balance data, 0.5 kWh of electricity is produced per
kg of total solids (TS) treated. Only 0.1 kWh per kg of TS treated is used for the gasification
installation and the remaining 0.4 kWh is used by the sewage plant.
Electricity H*at
Figure 10-KopfPFD
33
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5.2.4 Nexterra & City of Stamford WPCA19'31'32'33'34
The Stamford Water Pollution Control Agency (SWPCA) in Stamford, CT began research on the
prospect of sewage sludge gasification less than five years ago to develop a process for
managing the cities' sewage sludge disposal. After research and preliminary design, testing was
done on a bench scale by a local contractor and then scaled up to pilot scale by the same
contractor once the technology was proven.
Undigested class A biosolids, which were dried and pelletized to 6.7% moisture using an Andritz
DDS 40 rotary dryer, were fed at a rate of 20 kg/hr into a trailer mounted pilot scale fixed bed
updraft gasifier. The syngas gas produced in the gasifier was sent through a vortex particle
separator followed by a dry filtering system to remove tars, paniculate and other pollutants. Once
cleaned, the syngas produced by the pilot plant was either sent directly to a flare or run through
an internal combustion engine. The results of these trials enabled SWPCA to verify a biosolids
gasification proof of concept using biosolds created in Stamford county.
While testing was being performed on the pilot scale gasifier in Stamford by the contractor
responsible for building the trailer mounted unit, SWPCA sent sludge samples to three different
demonstration/full scale gasifiers to be tested. The three facilities selected for the tests were not
specified, but based on a presentation given by SWPCA34, it can be deduced that the three
gasifiers were Kopf, Nexterra and Prime Energy. SWPCA chose the Nexterra gasifier for its'
commercial facility design based on the results of testing completed at Nexterra's facility in
Canada.
The Nexterra trials were completed in 2009 at a research facility located in Kamloops, British
Columbia. The facility is built around a fixed bed updraft gasifier which is capable of operation
at a maximum capacity of approximately 8 MMBtu/hr. Some of the specifics of the system can
be found in Table 13 and a schematic of the system is displayed in Figure 11.
According to Nexterra literature,19 fuel, with a maximum dimension of 3 inches, is bottom-fed
into the centre of the dome-shaped, refractory lined gasifier. Gasification air is introduced into
the base of the fuel pile. Partial oxidation, pyrolysis and gasification occur at 1,500 to 1,800 °F,
and the fuel is converted into syngas and non-combustible ash. The ash migrates to the base of
the gasifier and is removed intermittently through an automated in-floor ash grate. In this
process, the ash will typically contain only a small fraction by weight of carbon, indicating a
high conversion efficiency of fuel into syngas. The syngas can then be directed through energy
recovery equipment or fired directly into boilers, dryers and kilns to produce hot water, steam
and/or electricity. The temperature of the syngas exiting the gasifier is a function of the fuel
moisture content. Likewise, the overall system efficiency is directly related to the fuel moisture
content, with dryer fuels resulting in higher system efficiencies.
34
-------
D«y BlosolkJs
Figure 11 - Feasibility design forNexterra, a possible PFD for electricity generation
Table 14 - Kopf and Nexterra overview
Location
Technology
Readiness Level
Feedstock Pretreatment
Gasification System
Performance
Max Capacity (dry)
Internal Energy Consumption
Energy Output
Gross Electrical
Gross Thermal
Net Electrical
Net Thermal
Kopf29'35
Balingen, Germany
Fluidized bed
Commercial-Constructed in 2004
Solar drying digested sludge to 70-
85% solids in 2 to 8 weeks
depending on weather. Any
electricity needs for the drying
system are supplied by PV panels.
375 Ibs/hr
17 kW
85 kW
0.52 MM Btu/hr
69 kW
NA
Nexterra/Stamford
WPCA/Jenbacher/Andritz19' 31' 32' 33'
34
Kamloops, BC
Fixed Bed Updraft
Pilot
Mechanically dewater to 22% solids,
then use an Andritz DDS 40 rotary
dryer reduce moisture 93% solids. @
4000kg/hr using 12MM Btu/hr.
1354 Ibs/hr
NA
Assuming 70% efficiency, syngas
produced is 8 MM Btu/hr31
NA
NA
NA
NA
35
-------
Syngas Composition
Gas Cleanup
By products/Waste Streams
Potential Emissions
Exhaust flow rate
Product/Byproduct end use
Kopf29'35
8% H2, 8% CO, 4% CH4, balance
N2 and CO2
Not available
177 cfm exhaust gas (composition
unavailable). 187 Ibs/hr of mineral
granulate produced. 80 gallons of
condensate/ton of sludge is
collected and sent back into the
plant.
177 cfm
Composition not available
80% of energy produced during
gasification is used to operate
treatment plant. Mineral granulate
is used for asphalt, phosphorus
recovery and construction
materials.
Nexterra/Stamford
WPCA/Jenbacher/Andritz19' 31' 32' 33'
34
Not available
Syngas conditioning system, cyclone
and particle filter
"A few thousand ppm, HCN" from N
components in sludge. Cleaned out in
Scrubber. The concentrations of
metals found in ash samples fell
within the guidelines for disposal at
landfill locations. Siloxanes present in
sludge produce silicon oxide
compounds when combusted. NOx,
SOx and PM are present in flue gas.
Silicon Oxide compounds were found
downstream of the thermal oxidizer.
Not available
In this particular trial, the goal was to
use the syngas to displace the natural
gas needs of the dryer. Long term
goals would be to create gas for an
internal combustion generator.
5.2.5 Tokyo Bureau of Sewerage60'69
At the 2007 Water Environment Federation Technical Exhibition and Conference (WEFTEC),
the Tokyo Bureau of Sewerage presented a white paper on a 15 TPD fluidized bed gasification
system that was constructed in Kiyose, Japan for the treatment of sewage sludge. Starting in
2005, the plant began demonstration tests and completed 3400 hours of testing prior to the 2007
WEFTEC.
In the Tokyo Bureau of Sewerage (TBS) demonstration plant, sewage sludge is dried to a
moisture content of 20% in a drier and sent to the gasification chamber of an internally
circulating fluidized-bed gasifier. Once the feedstock enters the gasifier, it is pyrolyzed at a
temperature of 650 - 750 °C and reformed with air into syngas in a downstream gas reforming
furnace at 800 - 900 °C. Heat from the syngas leaving the gasifier is used to dry the feedstock
before being sent to a liquid gas scrubber. The syngas is then converted to motor power via an
aeration blower or electricity via an internal combustion generator. The solids, unused carbon,
and condensed water removed in the scrubber are fed into the combustion chamber of the
fluidized-bed gasifier for stabilization. Hot exhaust coming out of the combustion chamber, heats
the fluidization gas before going through a bag house and out to atmosphere. A process flow
diagram of the TBS demonstration plant is shown in Figure 12.
36
-------
According to TBS, scale up to a 100 TPD gasifier would reduce GHG emissions by 17,000 tons
CC>2 per year versus a conventional system and 4,600 tons of CC>2 per year versus an incineration
system.
Sewage
Sewage sludge treatment plant
Sewage
treatment
water
1
Sludge drier
Heating furnace
Gas engine drive
aeration blower
/ Gas reforming .p-^
*""**# **H ^er
9 / n^i
'
Gasification
' chamber
ibustion
chamber
4^
4^J
Gassc
T /
Solid/liquid separation
Heat exchanger \~j
Motor
power
Electric
power
Combustion exhaust gas Gas engine
generator
Scrubber sludge
Heat^xchaVige«-J'entilation ^
in the system
Exhaust gas
Figure 12 - Tokyo Bureau of Sewerage PFD60
In an email correspondence with the TBS, a 100 TPD gasifier was built and started processing
sludge in July 2010. Further details on the 100 TPD system can be found in Table 15.
Table 15 - Tokyo Bureau of Sewerage overview
Location
Technology
Readiness Level
Feedstock Pretreatment
Gasification System
Performance
Max Capacity (dry)
Internal Energy Consumption
Energy Output
Gross Electrical
Tokyo Bureau of Sewerage60'69
Kiyose, Japan
Circulating Fluidized Bed
Commercial - Constructed in 2010
Sewage sludge from the wastewater plant is fed into a high pressure screw
press to a moisture content of 70-80% then fed into a drier that decreases the
moisture content to 20%. The drier consumes 350 kW.
8000 Ib/hr
500 kW
NA
37
-------
Gross Thermal
Net Electrical
Net Thermal
Syngas Composition
Gas Cleanup
By products/Waste Streams
Potential Emissions
Product/Byproduct end use
Economics
Tokyo Bureau of Sewerage60'69
NA
150 kW
NA
8.5% H2, 1 1% CO, 1 1% CO2, 7.5% CH4, balance N2 small amounts of C2
and C3 hydrocarbons
Liquid gas scrubber and bag house
Wastewater from a de-moisturizing tower, ash from the bag
gas
house and flue
Not yet published
The syngas in combusted in an ICE generator and aeration blower for
electricity production
Estimated $100 million for operation of 20 years (includes construction,
manpower, maintenance and operating costs)
5.3 Economic Assessment
Cost is a decisive aspect in energy and resource recovery from sludge. Two primary types of
costs are associated with each technology: The capital cost and the operating and maintenance
(O&M) costs. If the present worth cost (capital and O&M) of a technology that looks
environmentally attractive is not affordable, the technology is unlikely to be adopted unless other
market drivers come into effect.
29
Determination of the economic feasibility of energy and resource recovery from sludge is a
complex issue. For each technology, this depends on several factors. In general, the more
complex the technologies are, the more costly they are. Capital and O&M costs depend on the
type of technology, the size of the installation, the type and number of input materials for the
operation of the installation, plus local conditions such as land and labor costs. Economic
feasibility will also depend on the type of resource that is to be recovered, such as, electricity,
heat, phosphorus, methane from digestion. The cost may also depend on the efficiency goals,
product quality, or regulatory limits that must be met. Higher efficiency or quality typically
requires higher capital and O&M costs.29
For the purpose of this report, a waste heat recovery and electricity generation gasification
system was used as an example for a basic economic analysis. The model system consists of a
dryer, gasifier, syngas cleanup, and internal combustion engine for electricity and heat
production. Capital costs were estimated based on the average cost of a biomass gasification
system.64 O&M costs and profit from electricity generation were estimated by SRI using the U.S.
Department of Energy's proforma and compared under varying local conditions. In Table 16,
capital costs, energy produced and annual operating cost are presented in USD per one dry ton
(DT) per day of processing capacity. Four different scenarios were analyzed by SRI using
different energy values, resulting in varying times for estimated payback. Table 16 gives the
results of the analysis.
38
-------
Table 16 - Summary of sludge gasification economics under varying local conditions
Case
Price of elec. ($/kWh)
Tipping Fee ($/DT)
Annual Operating Revenue,
Electricity+Tipping Fees ($)
Annual Operating Cost ($)
Capital Costs ($)
CAPEXperkW($/kW)
Payback Years
National Average
Wholesale Electricity
Rate
$0.0420
$70.00
$41,624.00
($36,995.00)
($269,815.00)
$4,651.98
21
New England Average
Wholesale Electricity
Rate
$0.0495
$70.00
$44,583.00
($37,665.00)
($269,815.00)
$4,651.98
21
National Average
Industrial Electricity Rate
+ $0.0435/kWhRE
Tarriff*
$0.0855
$70.00
$58,783.00
($40,881.00)
($269,815.00)
$4,651.98
11
New England Average
Industrial Electricity Rate +
$0.0435/kWh RE Tarriff*
$0.0930
$70.00
$61,742.00
($41,551.00)
($269,815.00)
$4,651.98
7
*RE Tariff = renewable energy tariff, which may be applied for electricity produced from renewable resources, for which biosolids gasification would apply
A representative of M2 Renewables estimated that a scale-up to 15 MW of their current design
would cost approximately $60 million in capital.30 Using these values in proforma, at the
national average and New England average wholesale electricity cost resulted in pay back
periods of 21 and 17 years respectively. Due to the lack of information on the planned M2R
commercial system, accurate cost projections could not be calculated.
6. Cogasification
Cogasification is the process of combining sludge with feedstocks used in developed coal
gasification or biomass gasification technologies. A study conducted by the National Institute of
Engineering in Portugal found that the presence of sewage sludge has a positive effect on syngas
quality, as it allows an increased energy conversion during Cogasification with both coal and
straw. The increased concentration of hydrocarbons results in a higher calorific value in the
syngas.36
Activite de Promotion, D'Accompagnement et de Suivi (APAS), a clean coal technology R&D
program supported by the European Commission, was set up to research the gasification of
sludge, biomass and other wastes as co-feedstocks with coal. Included in the program was
Rheinbraun AG (RAG), which uses a high temperature fluidized bed (Winkler Process) to gasify
brown coal. Various tests were conducted by RAG in a 30 tonne/hr demonstration plant. In total,
504 tonnes of sewage sludge and 32 tonnes of loaded coke were co-gasified. Emissions were
well below German regulatory limits and conversion efficiencies and syngas yield for the sewage
sludge was adequate. RAG concluded that co-gasification of sewage sludge with dried brown
coal offered significant potential for disposing of sludge without impairing plant efficiency and
emissions/
36
In a complementary study, the British Coal Corp. examined the use of sewage sludge as a partial
feedstock with hard coal at its Coal Research Establishment. The tests involved adding up to 25
percent (dry) of sewage sludge to hard coal being fed into a fluidized bed gasifier. The study
39
-------
found that the addition of sludge did not adversely affect the gasifier operability or performance,
providing a fuel conversion efficiency of 78 percent.
36
Starting in February 2000, Ebara Corporation began the commissioning of a shredding residue
(mostly vehicles) gasifier in Aomori, Japan. The gasifier also has the ability to process
mechanically dewatered sewage sludge in the amount of up to 30% of the initial feedstock
weight. As of April 2004, the plant had processed 300,000 tons of shredding residues and 60,000
tons of sewage sludge. The gasifier also has the ability to process hospital waste and bone meal.
The thermal energy produced in the process is converted into electricity, which is used to operate
other plants of the same company; the excess is fed to the grid37.
Although co-gasification seems to be an appealing disposal method, one must consider the
availability of existing gasification plants, ability of that plant to handle sludge, and proximity to
the plant to sludge sources, which may limit its applicability. The dispersed nature and size of
sewage treatment operations seems to favor simple small-scale plants operated at atmospheric
pressure on sewage sludge alone, without the cost and infrastructure complexities of adding coal
or transporting sludge long distances to large coal gasifiers25.
7. Regulatory Requirements
Currently, there are no EPA regulations that specifically relate to sludge gasification. The
applicable regulations will be determined on a case by case basis until regulations specific to
sludge gasification have been established.
Starting in January of 2009, the EPA's Office of Resource Conservation and Recovery began
proposing a new rule pertaining to the disposal of sewage sludge by incineration. After an open
comments period, the rule proposed that sewage sludge be classified as a solid waste as regulated
by section 129 of the Clean Air Act if it is processed for destruction rather than energy
production. This ruling does not redefine sewage sludge or biosolids that are not incinerated
(e.g., sludge that is composted, land applied, etc.) as solid waste, only sludge that is incinerated
for destruction. It has yet to be determined if gasifying sludge for heat recovery will be included
in this rule. If a gasification unit disposes of sludge as a "solid waste", the facility will be subject
to Section 129 of the Clean Air Act40. There are separate emission limits for existing units and
new (commissioned after October 14, 2010) units. Table 17 shows the emission limits for
multiple hearth (MH) and fluidized bed (FB) incinerators instituted by the EPA under the 2011
Standards of Performance for New Stationary Sources and Emission Guidelines for Existing
Sources: Sewage Sludge Incinerator Units.
Table 17 - Emission limits for existing and new sewage sludge incinerator units3
Pollutant
Cadmium (Cd)
Carbon Monoxide(CO)
Dioxin/Furan (D/F 1MB)
Dioxin/Furan (D/F TEQ)
Normalized Units (7% O2)
mg/dscm
ppmvd
ng/dscm
ng/dscm
Existing Facilities
MH
0.095
3800
5.0
0.32
FB
0.0016
64
1.2
0.1
New Facilities
MH
0.0024
52
0.045
0.0022
FB
0.0011
27
0.013
0.0044
40
-------
Pollutant
Hydrogen Chloride (HC1)
Lead (Pb)
Mercury (Hg)
Oxides of Nitrogen (NOx)
Particulate Matter (PM)
Sulfur Dioxide (SO2)
Normalized Units (7% O2)
ppmvd
mg/dscm
mg/dscm
ppmvd
mg/dscm
ppmvd
Existing Facilities
MH
1.2
0.3
0.28
220
80
26
FB
0.51
0.0074
0.037
150
18
15
New Facilities
MH
1.2
0.0035
0.15
210
60
26
FB
0.24
0.00062
0.001
30
9.6
5.3
A public hearing on the proposed rule was held on October 29, 2010 at the EPA campus in
Research Triangle Park, NC. Numerous POTW representatives made comments during the
hearing on the costs associated with the new standards along with the ability to technologically
achieve the standards. During the hearing, no mention was made to how the new standards
would affect gasification; this may have been due to the lack of sludge gasifiers in the United
States.
As was mentioned in section 4.2.1, 40 cfr §61.52, which relates to mercury emissions, may also
apply to sludge gasification. This regulation is not technology based, therefore a simple metric of
mercury emissions released per day, regardless of throughput, is the standard. Once again, the
applicability of sewage sludge gasifiers to 40 cfr §61.52 has yet to be determined.
As a reference to international standards, in 2000 the European Union issued the Directive
2000/76/EC for the incineration of waste. It was largely based on a German guideline, the 17th
Ordinance for the Implementation of the Federal Act on Emission Control 1990 (17th BlmSchV).
Due to deviations between both guidelines, the 17th BlmSchV was amended and completed in
August 2003. Table 18 presents the emission limits defined in 2002 for sewage sludge in the EU
regulation and the 17th BlmSch regulation.
Table 18 - Emission limits for EU and German waste combustion units in 200050
Pollutant
Cadmium (Cd)
Carbon Monoxide(CO)
Dioxin/Furan
Hydrogen Chloride (HC1)
Leab (Pb)
Mercury (Hg)
Oxides of Nitrogen (NOx)
Particulate Matter (PM)
Sulfur Dioxide (SO2)
Normalized Units
(7% 02)
mg/dscm
mg/m3
ng/dscm
ppmvd
mg/dscm
mg/dscm
ppmvd
mg/dscm
ppmvd
Existing Facilities
EU-Directive
2000/76/EC
0.03
32
0.1
4
0.5
0.03
68
-
12
17th BlmSch V* of
19/08/2003
0.03
32
0.1
4
0.5
0.02
68
-
12
41
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8. Conclusion and Recommendations
8.1 Summary of Key Findings
Gasification offers a potentially viable option compared to conventional methods for sludge
disposal. Gasification is capable of providing a clean and manageable process with the
possibility of net energy gains. The variability and lack of information on commercial scale
systems however, makes it difficult to ensure a complete analysis and concrete conclusions on
sludge gasification's viability.
Unlike incineration, there is potential for sludge gasification to deliver negative GHG emissions.
This is accomplished through energy production from biogenic sources and avoiding GHGs
which would have been created in a different process. The emergence of systems, like the
MaxWest system described in section 5.2.1, designed to process the sludge throughput of
individual plants will also help to reduce GHGs through the avoidance of burning fossil fuels
during transportation. As is mentioned in section 4.3.5, the magnitude of GHG reductions is
highly site, technology and/or feedstock specific. Therefore, a general statement cannot be made
that identifies gasification as having a lower carbon footprint than other management practices.
As can be seen in Table 16 in section 5.3, wholesale electricity prices will have a significant
influence on the economics of a sludge gasification plant. Only through individual analysis of
each system can an accurate cost projection be obtained. Once again, umbrella statements cannot
be made on the economic feasibility of gasification as a whole.
There are many companies that claim to be able to gasify sludge, but supporting independent
data on their processes is not available. In addition, many different system uses and designs are
available, even among the handful of early commercial systems. As a result, a complete technical
and economic analysis will only be feasible for this technology and industry when implemented
more broadly through a case by case basis analysis. More specifically, when a pretreatment
process, gasifier, clean up system and energy recovery process have been integrated and
commissioned, the system can be thoroughly evaluated through collected data.
It is also difficult to evaluate and summarize the performance of a system without empirical data,
because gasification's chemical and thermochemical processes are so diverse (e.g., hydrogen
concentrations ranging from 10 to 60% and carbon monoxide concentrations ranging from 8 to
35%). At this time, only through direct measurement at existing pilot and commercial scale
facilities, can we fully evaluate all of the impacts of the technology.
Once in place, EPA regulations may have a significant impact on the design, economics,
performance and feasibility of a gasification system, because emission limits may dictate gas
cleanup and gasifier technology requirements.
8.2 Conclusion
Based on the quantity of research data pertaining to sludge gasification, it is evident that there is
significant interest around the globe in developing this technology to commercial scale.
Although there are many options when it comes to novel methods of sludge disposal and
42
-------
utilization, gasification is currently receiving the most attention. Other novel technologies, such
as, super critical water oxidation, which are less mature may be suitable options, but not enough
research of these technologies has been performed. With a handful of gasification systems in the
final stages of development, only a leap from pilot scale to commercial scale is needed, with
more technologies following closely behind.
Although there is little detailed information on commercial sewage sludge gasification facilities,
there is even less information on pulp and paper mill sludge gasification. A review of available
literature and discussions with industry experts has revealed that pulp and paper mill sludge may
not be a suitable candidate for gasification with current technology. The high moisture and
mineral content in sludges result in low energy values, ultimately making full scale operation
uneconomical, at least until sludge waste disposal becomes more problematic and costly.
8.3 Recommendations
Future work should include an independent assessment of existing systems with the goal of
collecting and verifying performance and environmental data via direct measurement. The
multitude of component combinations available in gasification systems makes it difficult to
speak of the technology as a whole. Each system will need to be evaluated individually to
determine its overall appeal. Prior to data collection, a number of items should be considered to
determine if a system will meet a facility's objectives. This list includes:
It is critical to investigate a system's ability to adhere to Clean Air Act standards along
with any other applicable federal and state regulations. The design, economics and
performance of a system will be influenced by waste stream restrictions. Approach this
issue by taking full account of all elements entering and exiting the system (i.e., if there is
mercury in the feedstock, there will be mercury in a waste stream).
When considering performance, it is important to verify energy consumption of the entire
process, including mechanical pretreatment, drying, gasifier energy demand and gross
output. Keep in mind that if digested sludge is being used, there will be a loss of potential
energy from removal and release of carbon in the form of CH4 or CC>2 created during
digestion.
Capital costs, operating costs and maintenance costs should all be thoroughly
investigated. Many of the chemicals in the sewage sludge may corrode a system, leading
to unforeseen high maintenance costs.
Finally, continuous evaluation of emerging technologies should be conducted to ensure that
impacts of sludge gasification technologies, both positive and negative, are determined prior to
broad implementation. This diligence will help to ensure proper regulation, implementation and
social acceptance of the technologies.
43
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43Blue Ridge Environmental Defense League, 2009. Waste Gasification: Impacts on the
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44 Sun Grant Initiative and the University of Tennessee, 2007. Gasification of Biomass. [online]
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[Accessed
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46Recovered Energy, Discussion on Plasma Gasification, [online] Available at:
[Accessed September 2010]
47 European Commission, 2001. Disposal and recycling routes for sewage sludge PartS-Scientific
and technical report, [online] Available at:
[Accessed
October 2010]
48 European Commission, 2002. Disposal and Recycling Routes for Sewage Sludge, [online]
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[Accessed October 2010]
49 Kandaswamy, D.S., Warren, D.W., Mansour, M.N., 1991. Indirect steam gasification of paper
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55Frederick, J., et al., 1996. Energy and materials recovery from recycled paper sludge. Tappi
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59 Environmental Protection Agency, 1993. 40 CFR Part 257. Standards for the Use or Disposal
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48
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68Brown, S., Beecher, N., Carpenter, A., 2010. Calculator Tool for Determining Greenhouse Gas
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72 Cairney, P., pcairney@maxwestenergy.com, 2011. Emissions numbers and process description
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75Gikas, P., Noll, S.A., Stedman, K., 2011. Gasification of Primary Fine-Screened Solids for
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Table 19 - Data Source Qualification
Peer reviewed journals or government reports - results based on independently
measured validated data
1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 15, 16, 17, 18, 21, 22, 23, 25, 26, 36, 39, 41, 42, 47, 48, 49, 50, 54, 55,
56, 57, 58, 59, 61, 62, 63, 65, 66, 67, 68, 70, 71
Non peer reviewed government reports, conference presentations (non-marketing),
peer-reviewed journal articles not based on independently obtained data
13, 19, 20, 24, 29, 37, 44, 45, 52, 60, 64, 74
Direct contact with technology vendors or commercial project development team
27, 28, 30, 31, 32, 33, 34, 35, 38, 51, 53, 69, 72, 75
Non-reviewed articles, websites, marketing presentations, advertisements, press, etc.
5, 10, 14, 40, 43, 46
49
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