United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-83-006 March 1983
&EPA Project Summary
Survey and Analysis of
Current European
Technologies for
Wood Combustion
Werner Martin and Daniel R. Koenigshofer
This report summarizes current Euro-
pean technologies and air pollution
policies pertaining to wood combustion.
Twenty concepts for wood burning
equipment are described. Also included
are section diagrams, size ranges, and
comments regarding efficiency and
convenience. Many of the system types
are not currently available on the U.S.
market. A list of manufacturers and
their addresses is included. The report
also contains emission data for many of
the systems described. European emis-
sion regulations and test procedures are
presented. Finally, a brief overview of
health research related to suspected
carcinogenic materials in wood com-
bustion emissions is presented. This
information was gathered both in the
U.S. and in Europe by engineers familiar
with wood combustion on both conti-
nents. The work was funded by EPA's
Industrial Environmental Research Lab-
oratory to supplement their other ac-
tivities relating to air pollution from
wood combustion.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Unlike in the U.S., use of wood as a fuel
has not dramatically increased in recent
years in Europe. Rather, it has always
been an important fuel for certain appli-
cations. This long history combined with
the relative scarcity of wood has resulted
in a wide variety of efficient wood burning
equipment on the European market. Some
of the systems are not presently available
in the U.S.
This report examines 20 wood firing
systems. Their operation is described and
displayed graphically. Typical manufac-
turers are listed and comments are made
regarding their efficiency, ease of opera-
tion, and applications. Pollutant emission
data are presented for all system types.
Finally, specific regulations and support-
ing health research are discussed briefly.
Trends in European fuel wood use for
the most part are a continuation of
previous applications. Only in Sweden is
there strong encouragement for increased
wood combustion. This is manifest in the
recent construction of several wood-fired
district heating plants in Sweden. Else-
where wood continues to be an important
fuel in rural areas where it is often burned
with coal. Wood related industries (such
as cabinet shops, furniture, and pulp and
paper) are the prime consumers of waste
wood. They, too, often supplement the
wood with coal. Another interesting trend
is found in urban/suburban areas where
multi-fuel boilers have become popular.
The unique aspect of this is that wood is
seen as an auxiliary, emergency fuel to
back up oil or gas.
In the residential and small commercial
market, down- and cross-draft units have
displaced up-drafters. Thermal storage in
masonry or water has also become rela-
tively common. Industrial wood combus-
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tion equipment appears to be moving
toward cyclone burners and fluidized-bed
combustors. Several companies have
gasifiers in the pilot stage. The main
emphasis appears to be to use the gas to
fire internal combustion engines, often to
generate electricity.
Emission regulations tend to be simple
and not particularly stringent. Great
emphasis is placed on enforcement.
Probably the most remarkable aspect of
the pollution control strategies is the
requirement of regular inspection and
maintenance by licensed chimney sweeps
and other technicians. This policy has
resulted not only in decreased emissions,
but also in improved efficiency and safety.
Boilers must be inspected and tested
annually. Those failing must undergo the
required adjustments to be certified.
Through the inspection/maintenance
program the user gains an understanding
of the equipment and its proper operation.
This may be as valuable as the adjust-
ments.
As in the U.S., equipment manufac-
turers strenuously oppose emission stan-
dards. Their main contention is that to
meet any standard the equipment must
be designed for a very specific type of fuel
when in fact the consumer demands
versatility. In Switzerland and West
Germany, multi-fuel boilers have outsold
single-fuel gas or oil boilers for the past 5
years. This flexibility in fuel types usually
results in a decrease in overall efficiency.
Since wood is relatively scarce, Euro-
pean officials do not appear particularly
concerned about emissions from wood
combustion. With the exception of
Sweden, research on health effects of
Polycyclic Organic Matter (POM) and Poly-
cyclic Aromatic Hydrocarbons (PAH) has
been aimed at auto and industrial emis-
sions and occupational exposure rather
than at wood combustion. Sweden has
done some research specifically on PAH
emissions from wood heaters. Unfortu-
nately, the results were inconclusive.
Air Pollution Considerations
Most standards applicable to wood
combustion derive from early European
concern about particulate emissions from
coal combustion. Since many types of
coals were used, categorization was
necessary to accommodate their different
emission characteristics. Percent volatile
matter became an accepted distinguish-
ing feature. Wood then was included with
other high volatile fuel such as brown
coal briquets. Since smoke output is
generally proportional to volatile matter
content, wood and other high volatile
fuels became known as "smokey fuels."
Eventually regulations and equipment
design reflected this distinction between
"smokey" and "smokeless" fuels.
The authors reported that down-draft-
ers produced significantly less pollution
than up-drafters. This early knowledge
was the basis for German regulations for
small stoves, and resulted in the universal
application of the "slow combustion" or
Dauerbrenner stove.
Since emissions from wood combus-
tion have not been of great national
concern in any of the European countries,
regulations relating to wood are a part of
those pertaining to particulate emissions
from coal. As in the U.S., in recent years
there has been increasing concern about
emissions from residential appliances.
Visibility problems have been docu-
mented in alpine valleys which are sub-
ject to frequent inversions. To date,
however, these problems have not re-
sulted in any regulatory action. There has
also been increasing concern about the
effects of PAH emissions from wood
combustion as reflected by recent con-
ferences and health research.
In Germany combustion emissions are
regulated by the Federal Air and Noise
Protection Law of 1974. Switzerland is
still awaiting passage of its federal en-
vironmental protection law so emissions
are controlled by guidelines established
by insurance companies and professional
organizations (e.g..engineering societies).
Some local governments have passed
ordinances but enforcement is possible
only through nuisance complaints.
In Sweden only large equipment (>900
MJ/h) is regulated by the federal En-
vironmental Protection Act. The National
Environmental Protection Board can,
however, recommend standards or regu-
lations for any pollution source.
Reported Emissions
Most of the emission data found in
Europe are produced by government
agencies who are responsible for testing
and certifying equipment. Most of this is
done using VDI (Association of German
Engineers) standards. References may be
found in Appendix E of the full report.
Manufacturers are required to test their
own equipment and verify that it meets
the regulations.
Table 1 summarizes available emission
data.
Residential and
Commercial
Two fireplaces were tested by Rudling
and Ahling (1). Braskamin I was a fire-
place without a warm air heat exchanger,
but with a glass door and outdoor com-
bustion air. Braskamin II had a built-in
warm air heat exchanger. Pollutant con-
centrations in the stack gases of the two
fireplaces were very similar. The higher
thermal efficiency of the Braskamin II,
however, resulted in loweremission rates
per unit of heat output (mg/MJ out).
Emission rates from Braskamin II were
about half those from I. Note that in both
cases the reported efficiency was much
higher than commonly reported in the
U.S. literature.
All of the residential boilers described
in the full report had to meet the West
German TSP standard. Capacities and
efficiencies were based on information
from the manufacturers; therefore, no
specific data was available. All that is
known is that the standard was met.
A detailed emission analysis of two
types of residential boilers was also
performed by Rudling. Emissions from a
prechamber boiler were compared with
those from a standard-grate-fired boiler.
Both of them were fired with wood chips,
controlling for firing rate, moisture con-
tent, and excess air. Tables 2 and 3
contain the significant data from that
study.
In the standard-grate residential boiler
the only factor influencing TSP and POM
was the excess air level. The highest CO
value was accompanied by extremely
high PAH values. The moisture content of
the wood and the firing rate appeared to
have little influence on the pollutant
concentrations.
The prechamber system was composed
of the standard-grate-fired boiler used in
the first system, but with a gravity fed
prechamber. It was difficult to draw
conclusions from the data presented in
Table 2, since too many variables changed
from one test to the next. Again, the
excess air level, as indicated by the COz
concentration, seemed to be the most
important variable associated with the
changes in TSP and PAH concentrations.
In Test 4 the excess air seemed to be too
high to maintain adequate combustion
temperatures at the given firing rate.
Rudling concluded that increased firing
rates would produce higher TSP and POM
emissions and that higher moisture con-
tent would result in higher PAH emis-
sions.
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Table 1. Summary of Emissi
Equipment Type
Residential
Fireplaces
Braskamin 1
Braskamin II
Tile Stoves
Boilers
Multifuel (1 Firebox)
Multifuel (2 Fireboxes)
Solid Fuel
Solid Fuel w/DHW tank
Pre-Chamber (for Logs)
Pre-Chamber (for Chips)
Commercial
Boilers
Handfired/Susp. Comb.
Pre-Chamber, Handfired
Moving Grate (Chips)
Industrial
Boilers
Retort, Stoker
Cyclone Burner
FluidizedBed
Inclined Moving Grate
Handfired/Susp. Comb.
Dutch Oven Type
Gasifiers
ion Data of Wood-Fi
Output
Capacity
MJ/h
21.5
43.0
2.6-6.3
61-600
61-600
60-1420
60-146
125-2000
125
277-2-103
250-3-1 03
1.4-6-103
3-5-103
1-50-103
1-50-103
16-103
2.6-16.2-103
3-1 03
ired Equipmt
Efficiency
%
40-45
70-80
75-85
65-85
70-85
70-85
50-85
70-85
80-85
70-80
75-85
75-85
75-84
84-88
>nt in Europe (fc
Manufac-
turers
1.2
1.2
1.2
1
3
14
4.5.6
15
6
7.8
9.10
16
6.11
6
6
11.12.13
>r manufacturers, see Appendix D of full report)
Average Emissions
(ug/J)
TSP
5.7-170
16-78
<79*
<79*
<79*
<79*
<79*
45-7600
<156*
< 79*
<756*
794
26-77
2.0
28
77-155
103-252
POMs Others Remarks
W/J
0.3-17 CO:0.6-7.3 (5)
0.3-7 CO:1. 1-0.8 (5)
Capacity in
MJ/m2h{6)
lower efficiency
for oil
lower efficiency
summer
0.004-8.3 CO:0.06-5 (5)
w/ multi-cyclones (4)
0.001 w/elec. precip. (7)
w/baghouse (10)
fired w/wet bark (8)
(4)
(4)
CO/7. 26 pollutants measured
Ald.:0. 76 in exhaust gas of/C-
engine (9)
*Standard of West Germany.
It was difficult to compare emissions
from the two systems since moisture
content and firing rates were not well
controlled. At design capacity (7 kg/h)
TSP concentrations in the prechamber
system were reported to be lower by a
factor of 1.5 to 5. PAH concentrations
were also reported to be much lower in
the prechamber configuration.
Industrial
There is a wide variety of industrial
combustion equipment. Retort under-fed
stokers are very common in the lower
capacity ranges. They reach efficiencies
up to 85% and can handle dry and green
chips. TSP emission is not excellent
because the forced combustion air carries
all the ash through the boiler. Manufac-
turers claim, however, that the particulate
matter passing through the multi-cyclone
is exclusively light fly ash.
Tests of a Swedish cyclone burner
revealed very low TSP and POM emis-
sions (4). The burner is part of a district
heating system in southern Sweden
(Vaxjo) and burns a mixture of wet bark
and green chips. The cyclone burner
exhausts to a normal oil-fired boiler. The
system is equipped with an electrostatic
precipitator.
Afluidized-bed plant has been tested in
Eksjo, Sweden (5). The plant is equipped
with a baghouse and fires wet bark and
green chips and, for a few hours every
day, municipal solid waste. This multi-
fuel approach with fluidized-bed combus-
tors has created great interest in Sweden.
The Swedish National Board of Environ-
mental Protection conducted an emission
test on that particular unit and found
average TSP concentrations as low as 5
mg/m3, or approximately 2 mg/MJ.
Inclined moving grate boilers are often
found in pulp and paper mills where wet
bark and even waste water treatment
plant sludge is burned. One particular
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Table 2. Emissions from a Standard-Grate-Fired Wood Chip Boiler
Parameter
Units
Test 1
Test 2
Test 3
Firing Rate
Moisture Content of Wood
Stack Gas Temperature
CO2
CO
H,Cy
TSP
PAH
kg/h
%
°C
%
ppm
ppm
mg/m3 (at 1O%COd
tig/ m3 (at 10% COd
7.0
14
200
8.0
8,000
NA
360
16.000
7.0
14
200
5.7
6,000
NA
120
25,000
13.0
36
190
5.0
6,500
NA
260
2,800
Table 3. Emissions from a Prechamber Wood Chip Boiler
Parameter
Units
Test 1 Test 2 Test 3 Test 4
Firing Rate
Moisture Content of Wood
Stack Gas Temperature
COa
CO
r/xCy
TSP
PAH
kg/h
%
°C
%
ppm
ppm
mg/m3(at 1O%COd
Iig/m3(at 10% CO,)
4.2
21
150
6.5
60
30
72
<6.8
7.0
48
190
6.7
700
50
72
42
7.0
48
190
7.0
2.100
110
80
220
7.0
48
180
4.0
2,500
400
2,300
2,300
boiler of this type has been tested for
particulate matter (6). Average TSP emis-
sion was found to be only 28 mg/MJ
using simple mechanical filters.
The Swiss Federal Materials Testing
and Research Institute (3) has done an
emission field survey among typical wood
fired industrial boilers in Switzerland.
Dutch-oven, suspension burner, and stok-
er retort boilers were the most prevalent.
The investigators suggested that a na-
tionwide TSP emission standard of about
85 mg/MJ could be justified since more
than half of the tested installations met
such emission levels. In some cases 85
mg/MJ can be reached using only a
settling chamber.
Combustion Technologies
The combustion equipment presented
here is divided into three categories:
Residential Space Heating, Commercial
Applications, and Industrial Applications.
The equipment described represents a
good cross-section of the technologies
available on the European market. For
each piece of equipment, a brief explana-
tion of its specific use and a description of
the equipment is followed by a list of its
special features and their influence on
efficiency, emissions, and convenience.
Residential Equipment
Unlike in the U.S., free-standing metal
wood stoves are not extremely popular.
Instead, wood is usually burned in one of
four types of appliances: a masonry/tile
stove, a combination cookstove and boil-
er, a multi-fuel boiler, and, in some cases,
fairly efficient fireplaces.
Since most of the forests are owned by
local government, wood is normally gath-
ered and prepared by the user rather than
by commercial wood dealers. Towns issue
permits to cut trees which have been
marked by the town forester. This results
in professionally managed forests which
produce far greater yields than most
privately owned forests in the U.S. In
Switzerland, a law requires planting new
trees on a one-for-one basis even on
private land. A permit is required to clear
cut anywhere. This concern for preserva-
tion is also manifest in the wood burning
equipment commonly found in Europe.
Commercial Equipment
Wood working shops have created a
demand for wood residue burners. These
are used to burn sanderdust, sawdust,
shavings, and all sizes of scrap wood.
Space heating, wood dryers, and hot
water needs are the most common end
uses. The great variation of fuel size
created in these shops has resulted in
rather complex combustion equipment
design. Shavings, sawdust, and sander-
dust are burned in suspension, whereas
larger pieces of wood are fed manually
and burned on a grate.
Industrial Equipment
The larger combustion installations are
located either in industrial or utility
plants, including district heating plants.
Wood fired boilers are most often found in
the pulp and paper and in the wood
working industry. A more recent applica-
tion of wood fueled boilers is in district
heating or public cogeneration plants,
where wood waste is available.
There are only a few major manufactur-
ers of industrial-size wood-fired boilers in
Germany, Switzerland, and Scandinavia.
There, specialization in wood and/or
other solid fuel fired equipment has
brought them international recognition.
References
1. Rudling, L. and B. Ahling. Chemical
and Biological Characterization of
Emissions from Combustion of Wood
and Wood Chips in Small Furnaces
and Stoves; for the National Swedish
Environmental Protection Board by
University of Stockholm, September
1980.
2. Swiss Firewood Commission. "Tile
Stove Central Heating Systems."
Advisory Office for Wood Combus-
tion, SVW, Solothurn, Switzerland.
3. Swiss Federal Office for Environ-
mental Protection. "Suspended Par-
ticulate Matter Emissions from Auto-
matically Fed Installations Burning
Wood Waste." Measurements done
by Federal Material Testing and
Research Laboratory (EMPA),
Dubendorf, Switzerland, November
1975.
4. National Swedish Environmental Pro-
tection Board. Measurement Proto-
col of Emission Test at Vaexjoe
District Heating Plant, unpublished,
1980.
5. National Swedish Environmental Pro-
tection Board. Measutement Proto-
col of Emission Test at Eksjoe Flui-
dized Bed Plant, unpublished, 1980.
6. Papermill Utzensdorf. Measurement
Protocol for Lambion Bark-Fired Boil-
er. Association for Enhancement of
Water and Air Pollution Control,
Zurich, Switzerland, May 1979.
-------
7. TUV Baveria. "Emission Measure-
ments on a Gasifier Using Wood
Waste and Fueling a Gas Engine."
DI-WUL 30-stra-ru 116 4317, Tech-
nischer Uberwachnugsverein Bayern
e.V., 8000 Munchen 40, West
Germany.
Werner Martin and Daniel R. Koenigshofer are with Integrated Energy Systems,
Inc., Chapel Hill, NC27514.
Michael Osborne is the EPA Project Officer (see below).
The complete report, entitled "Survey and Analysis of Current European
Technologies for Wood Combustion," (Order No. PB 83-156 729; Cost:
$11.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•fr U. S. GOVERNMENT PRINTING OFFICE: 1983/659 -095/1904
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