United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 2771
*
Research and Development
EPA/600/S7-86/051 Mar. 1987
Project Summary
In-Process Control of Nitrogen
and Sulfur in Entrained-Bed
Gasifiers
E. F. Aul, jr., R. C. Adams, R. A. McAllister, and S. V. Kulkarni
The objective was to evaluate the
theoretical aspects and engineering
considerations of in-process pollutant
control of the entrained-bed slagging
coal gasification process as applied to
combined cycle operation or to the
retrofit to existing boilers. The pollutants
of concern are the nitrogen and sulfur
oxides (NOX and SO,) which, without
controls, are products of combustion of
the gasifier product gas. A literature
search and theoretical evaluation were
conducted to identify the chemical/
physical conditions and flow charac-
teristics of entrained bed slagging
gasification as they relate to in-process
control of NO, and SO, precursors. A
tentative scheme was postulated to
maximize the conversion of fuel nitrogen
species to elemental nitrogen by op-
erating modifications and to remove
H2S and HCN with the slag by injecting
alkaline metal oxides into the gasifier
gas space. However, the degree of suc-
cess of the suggested in-process con-
trols could not be projected.
The applicability of potential in-
process control was examined. It was
concluded that in-process controls, if
feasible, are applicable for reducing
NO, and SO, precursors so that the
capacities of downstream control de-
vices can be reduced with subsequent
cost savings.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
This project evaluated the theoretical
aspects and engineering considerations
of in-process pollutant control of en-
trained-bed slagging coal gasification
processes as applied to combined cycle
operations or the retrofit to existing
boilers. The pollutants of concern are the
NOX and SOX which, without controls, are
products of combustion of gasifier product
gas. Therefore, the goal of the study is to
examine the potential for control within
the boundaries of the gasification process
to reduce or eliminate NOX and SOX
precursors (H2S, NH3, HCN, etc.) that are
formed in the gasifier. The target of such
control is the elimination or reduction in
scale of downstream cleanup devices
and/or combustion stack gas cleanup
controls.
Procedure
The project consists of two tasks: (1)
Theoretical Evaluation — a literature
search and theoretical evaluation of in-
process control; and (2) Applicability —
assessment of the effects of in-process
control on gasification product gas
streams and the associated impact on
the downstream processes.
The objective of Task 1 was to evaluate
the information available concerning
chemical/physical conditions and flow
characteristics of atmospheric and pres-
surized, high temperature, slagging gasifi-
cation as related to in-process control of
NOX/SO, precursors.
The results are based on an intensive
literature review and close examination
of the most applicable work that has
been done in the areas of concern. In-
formation on entrained-bed slagging
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..processes that are under development or
commercialized was collected to define
the chemical, physical, and flow condi-
tions. Five processes (KBW, Saarberg-
Otto, Shell, Texaco, and TRW) were
selected to provide an approximate range
of base case characteristics within which
gasif ier operation can be varied to achieve
effective in-process control.
The objective of Task 2 was to examine
the effects on utility and industrial users
of providing a cleaner fuel gas by means
of in-process gasifier controls. In particu-
lar, the effects on new combined cycle
plants and on retrofit utility and industrial
boilers were investigated.
Results and Discussion
Gasifier Operating Boundaries
Emission limitations for NOX/SOX define
precursor levels which must be attained
to eliminate downstream controls; particle
loading of the gasifier exit gas must be
controlled to protect the gas turbine.
Operating conditions will play a dominant
role in optimizing in-process control. The
prominent control variables in the gasifier
and the operating conditions which in-
fluence these variables are:
• Oxidant staging: depends on burner
design.
• Temperature: depends on oxygen/
coal ratio, steam/coal ratio, oxidant,
feed stream temperatures, coal
composition, and heat losses.
• Residence time in the gasifier/slag-
ging zone: depends on reactor
volume, gasification temperature,
coal particle size, and flow rates for
coal, steam, and oxidant.
• Temperature zones (gasifier and
cooling): depend on reactor design
(volume reactor geometry, wet or
dry cooling, slag removal).
These gasifier operation and design
considerations were examined as they
relate to possible in-process controls. The
tentative conclusions regarding the flex-
ibility of gasification processes for the
application of in-process controls are:
• Compatibility of oxidant feed for in-
process controls with base case
oxidant feed methods will need to be
determined. At least one process
employs staged oxidant feed for NOX
control.
• There is some latitude in manipula-
tion of O2/coal and steam/coal ratios
if needed for in-process control.
• There is considerable latitude in how
much the CO/H2 ratios can be varied
and still produce an acceptable pro-
duct gas.
• Compatibility of residence times and
temperatures required for optimum
in-process controls with base case
residence times will need to be
determined.
• Two temperature zones within the
gasifier provide an opportunity for
staged injection of capture materials
that will avoid or minimize sintering.
Due to a paucity of reported data, a
base case untreated gas composition was
pieced together from several design and
actual operation sources; this composition
is believed to represent a base case rea-
sonably well.
Control Theory and Experience
Of the several types of reactions that
take place in an entrained bed gasifier,
only coal devolatilization, combustion of
volatiles, and combustion and gasification
of resulting char play a significant role in
the generation and fate of nitrogen and
sulfur species. Studies at gasifier condi-
tions are usually carried out for either
nitrogen or sulfur species, but not both.
Therefore as an initial step it was neces-
sary to examine the reactions separately.
The coal devolatilizes rapidly at gasifier
temperature to form char and a gas frac-
tion consisting primarily of CO, H2, C02,
and CH4. Significant quantities of HCN
and NH3 are also often reported. The
quantity of volatiles produced has been
shown to be a function of particle size,
heating rate, pressure, and coal type.
Most workers in the field generally
believe that HCN is the primary nitro-
geneous intermediate formed during
pyrolysis of fuel nitrogen compounds.
Subsequently, HCN reacts with other
gaseous species to form NH3, NO, and N2.
Since both HCN and NH3 in gasifier exit
gas will be converted to NOX by sub-
sequent combustion of that gas, the ob-
jective of in-process control in the gasifier
will be to convert a maximum fraction of
fuel nitrogen to N2. Several studies have
been conducted involving fuel-rich com-
bustion of coal in which these same
phenomena have been examined.
For the sulfur species case, as with
nitrogen coal devolatilization, combustion
of volatiles and combustion and gasifica-
tion of the resulting char also play a
dominant role in identifying the fate of
coal sulfur. Sulfur is present in coal in
three forms: organic, inorganic, and ele-
mental. Review of work on the kinetic
and thermodynamic relationships affect-
ing sulfur specie reactions at high tem-
peratures reveals the following major
expected effects:
• Pyrolysis of coal in the fuel-rich |
reducing atmosphere leads to *
devolatilization of sulfur compounds
which readily are reduced to produce
H2S through the hydrodesulfurization
mechanism. The reaction rate is very
fast.
• COS and CS2 are formed in the
gasifier due to the reactions of de-
volatilized sulfur in the gas phase
with carbon and the reaction of H2S
with CO2. COS is detected in gasifier
effluents in small amounts, but CS2
is usually not reported.
• Some of the H2S is expected to react
with the available basic oxides of
the ash and thus be retained in the
slag.
The use of metal oxides other than
calcium as desulfurization media has
been examined by the Morgantown
Energy Research Center (MERC) for pos-
sible application as a hot gas contaminant
control in the gasifier product stream. On
the basis of thermodynamic studies, there
are indications that the oxides of barium
and strontium (and perhaps manganese)
have potential as in-process sorbents.
However, thermodynamic analyses do not
address the physical phenomena, such
as desurfacing, which may occur to solid
sorbents at high temperature, and little
information was found on physical effects
work for metal oxides other than CaO.
Combined Effort for Control of
NO, and SO, Precursors
The literature appears to address the
issue of either SOX control or NOX control.
Very few attempts to consider the effect
of alkali oxides on gasifier intermediates
such as HCN have been noted.
If CaO is added to the gasifier, com-
peting reactions between CaO and HCN
would significantly affect the concentra-
tion of HCN. Acidic HCN and basic CaO
are expected to undergo a neutralization
reaction that results in calcium cyanamide
(CaNCN) as the reaction product. One
would expect that the rate of absorption
of HCN with CaO would parallel the rate
of absorption of H2S by CaO since both
HCN and H2S are acidic gases. During
the absorption of H2S by lime, the rate of
reaction is found to depend on the re-
action temperature and the desurfacing
of CaO. Surface activity depends on
temperature and plays an important role
in the absorption of the acid gas. Probably
such a phenomenon is important in the
CaO + HCN reaction also. Absorption of
HCN by CaO will proportionately reduce
the amount of NH3, NO, and N2 in the
gasifier exit gas.
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It is expected that the sulfur and
nitrogen compounds captured by the
added CaO will be removed from the
reactor with the slag. For H2S absorption,
the rate limiting step appears to be the
internal particle diffusion process. Al-
though diffusion itself is not strongly
temperature dependent, desurfacing will
have a dramatic effect on the diffusion
coefficient. Experimental observations
suggest that CaO should be added to the
gasifier at lower temperatures. The coal
conversion requirements for an efficient
gasifier demand that it be operated at
higher temperatures. A possible compro-
mise for maximum absorption activity
and minimum desurfacing activity for lime
may be to add lime in the regions of
lower temperature away from the flame.
Probably a staged addition of lime will
have to be adopted to facilitate removal
of NOX and SOX precursors. Part of the
CaO may be added with the coal slurry to
capture H2S/HCN released in the initial
devolatilization, whereas the major por-
tion of the lime may be added in the
temperature regions of 800-1000°C.
These temperatures correspond to the
cooling zone of the gasifier.
Model Concept Development
The preceding discussion indicates
potential means of reducing NOX/SOX
precursor concentrations through in-pro-
cess control. Without an available theo-
retical model that has been validated
with actual results, the specific effects on
product gas quality and precursor con-
centrations cannot be projected. The
literature surveyed revealed that several
models exist for entrained-bed slagging
gasifiers. However, none of these models
apply to control of nitrogen and sulfur
species in the gasifier.
The feasibility of adopting an existing
model for the prediction of nitrogen and
sulfur species control effects was in-
vestigated. A model that was verified
from Texaco pilot plant data was chosen.
Applicability of In-Process
Controls
The applicability of in-process controls
was examined in relation to the limita-
tions imposed by the design specifications
of the gas turbine of a new combined
cycle plant and imposed by the emission
limits of the gas turbine. Emission limita-
tions were considered for the retrofit
applications of utility and industrial
boilers.
It appears that a modest reduction in
gasifier NH3 and HCN emissions by in-
process controls would result in compli-
ance with NSPS for NOX emissions from
low-Btu gas-fed turbines. It would per-
haps also eliminate the need for the
downstream ammonia scrubber. How-
ever, medium-Btu gas would require in
excess of 50 percent reduction of nitrogen
species from the gasifier because thermal
NOX is a significant contributor to total
NOX emissions when medium-Btu gas is
the fuel. Therefore, it is likely that NH3
scrubbing or hot gas cleanup would still
be required with in-process controls for
gas turbine applications.
Assuming current and proposed SO2
emission limitations for boilers that are
based on heat input to the coal gasifier,
gasifiers without in-process controls re-
quire substantial control device capacity
downstream of either the gasifier or the
boiler. Removal efficiency will vary de-
pending on industrial- or utility-boiler
applications, on air-blown (producing
low-Btu gas) or oxygen-blown (producing
medium-Btu gas) operation, or on coal
sulfur content. In-process controls would
limit control device capacity requirements.
For the gas turbine application, sulfur
species concentrations are restricted by
NSPS limits for sulfur in the fuel gas, SO2
emissions in the gas turbine exhaust,
and turbine design specifications to limit
turbine blade corrosion. It appears un-
likely that in-process controls for sulfur
species can achieve the level of sulfur
control required. Therefore, as with boiler
applications, the primary benefit would
be a reduction of the burden on down-
stream control devices.
Particulate matter (PM) control of
gasifier product gas for gas turbine and
boiler applications is substantially in-
fluenced by design specifications to avoid
turbine blade erosion and protect gas
burners from erosion and plugging. These
design restrictions mandate that PM con-
trols precede combustion. The use of dry
sorbent injection for in-process sulfur
control will increase the PM loading of
the gasifier product, putting further
demands on PM control devices.
Conclusions and
Recommendations
On the basis of theoretical evaluations
to understand nitrogen and sulfur species
reaction mechanisms in entrained-bed
gasifiers, some tentative conclusions have
been formulated.
For the nitrogen case, the following
process parameter changes are suggested
as a way to increase the conversion of
fuel nitrogen to N2 in an entrained-bed
gasifier:
• Operate at higher 02/coal ratios and
hence higher temperatures.
• Decrease the particle size of feed
coal.
• Inject a small amount of oxygen 50-
100 msec downstream of the gasifier
flame zone.
Under normal entrained-bed gasifier
conditions, 85 to 90 percent of the fuel
nitrogen which enters the gasifier is con-
verted to N2; the remainder exits as NH3
and HCN. The objective of the process
changes described above would be to
raise this N2 conversion to 95 percent or
greater in order to combust the fuel gas in
a new gas turbine and meet NSPS limita-
tions without additional NOX control. It is
not possible, at this time, to project the N2
conversion that would result from the
recommended changes due to the com-
plex interactions of process parameters
and their effect on gas composition.
For the sulfur species case, some of
the H2S formed during gasification is
expected to react with the available basic
oxides of the ash and be removed with
the slag. Further H2S would have to be
removed by reactions of metal oxides
with H2S. Experimental studies have
shown that CaO will absorb H2S, but
significant surface area is lost above
1000°C as a result of sintering. Con-
sideration of other materials (e.g., alkaline
cement kiln dust, alkaline fly ash, and
metal oxides being investigated in hot
gas cleanup studies) is recommended.
It was concluded that the model that
was verified from Texaco pilot plant data
could be adopted as needed. Added as-
sumptions would need to be made about
the reaction surface area of CaO. The
voidage fraction of CaO particles would
need to be reduced from any measured
values at ambient temperature to account
for sintering at temperatures to be found
in the entrained-bed gasifier.
If feasible, in-process controls are
applicable for reducing the amount of
NOX and SOX precursors to combined
cycle gas turbines and to boiler retrofits
so that the capacities of downstream
control devices can be reduced (with
subsequent cost savings), while still
meeting emissions requirements and
design specifications.
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R. C. Adams, E. F. Aul, S. Kulkarni, R. A. McAllister, and S. Margerum are
with Radian Corporation, Research Triangle Park, NC 27709.
Chester A. Vogel is the EPA Project Officer (see below).
The complete report, entitled "In-Process Control of Nitrogen and Sulfur in
Entrained-Bed Gasifiers." (Order No. PB 87-141 O32/AS; Cost: $18.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
/r
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Penalty for Private Use $300
EPA/600/S7-86/051
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