HUNGARIAN JOURNAL 
OF INDUSTRIAL CHEMISTRY 

VESZPREM 
Vol. 30. pp. 119- 126 (2002) 

PRESENT STATUS AND PERSPECTIVES OF CO-COMBUSTION IN GERMAN 
POWER PLANTS 

W. SCHEURER*, U. RICHERsl**, H. SEIFERT1** andK. R. G. HElN* 

(Universitat Stuttgart, IVD, Pfaffenwaldring 23, 70569 Stuttgart, GERMANY 
1Forschungszentrum Karlsruhe GmbH, Institut fiir Technische Chemic, D-76021 Karlsruhe, GERMANY 

Received: December 03, 2001 

Various approaches to the future waste management policy in Germany are currently under discussion. One problem 
arising in this connection, this is the suitability of existing furnaces for the co-combustion of waste. The use of sewage 
treatment sludge in power plants is already being practiced on a technical scale. Co-combustion in power plants is of 
interest also because of the C02 problem, as renewable resources can also be used for this purpose. -
This ruiicle documents the technical status of co-combustion in Germany and the available quantities of selected 
supplementary fuels. Moreover, experience accumulated in German coal fired power plants in using supplementary fuels 
is compiled. Future possibilities are assessed. 

Introduciion 

As a consequence of political criteria, waste 
management in Germany will be characterised in the 
future by a high degree of separation of waste streams 
for which optimum ways of utilisation will be sought 
after. Where recycling makes no sense, thermal 
treatment of waste is a possibility. 

Until the late eighties, emissions constituted the 
main point in debates about thermal waste treatment. 
Nowadays, it is the costs of waste management, which 
are becoming more and more important. Moreover, 
greater attention is being devoted to the efficiency of 
waste management plants against the background of the 
C02 problem and the greenhouse effect. 

One possibility of thermal waste treatment or 
regenerative fuel combustion with high efficiency and at 
relatively low cost is co-combustion in existing power 
plants. In Germany, various waste materials are used in 
coal frred power plants among other facilities. 

In this study, the current state of knowledge about 
co-combustion in power plants is described. Further 
potentials of co-combustion are presented on the basis 
of existing power plants. 

Electricity Generation 

The studies were performed on the basis of an analysis 
of the German electricity market. In 1998, 982 German 
power plants generated a total of 520,000 million kWh 
of electricity for use in industry and private households. 
A 27% share was generated by the combustion of hard 
coal, a 25% share by the combustion of brown coal [1]. 
These quantities did not change greatly throughout the 
nineties. 

Analysis of the installed electric capacity of German 
power plants shows by far the largest fraction (89.5%) 
to be owned by public electricity utilities. Table I shows 
more detailed information about electricity generation. 

In considering the possibilities of more extensive co­
combustion of waste materials, the furnace technology 
and plant sizes must be taken into account. According to 
[2], public utilities in Germany generate 99% of their 
power output on sites with more than 50 MW electric 
power. The furnace technologies used by public utilities 
are listed in Table 2. 

According to Table 2, the dominating firing systems 
used are pulverised-fuel furnaces, for which two modes 
of operation are possible. Wet-bottom furnaces. in 
which the ash is removed as molten stag and dry-bottom 
firings in which the ash is present as solid particles. ln 

Contact information:"' Phone: ++49 711685 3762, Fax: ++49 711 685 3491 
** Phone: ++49 7247 82 4656, Fax: ++49 7247 82 6715 



120 

Table 1 Installed Electrical Capacity and Power Production in German Coal-fired Power Plants 

industry German railways total public power utilities 
installed capacity 1998 

brown coal 
17861 MW 780MW 110MW 18751 MW 

(36.2 %) (1.6 %) (0.2 %) (38.0 %) 

hard coal 
26248MW 3700MW 595MW 30543MW 

(53.2 %) (7.5 %) (1.2 %) (62.0 %) 

Total 
44109MW 4480MW 705MW 49294MW 

(89.5 %) (9.1 %) (1.4%) (100 %) 
power production 1998 

brown coal 
122450 million kWh 4790 million kWh 

(45.7 %) (1.8 %) 

hard coal 
122000 million kWh 15530 million kWh 

(45.6 %) (5.8 %) 

total 
244450 million kWh 20320 million kWh 

(91.3 %) (7.6 %) 

Table 2 Firing Systems in Germany 

firing system 
stoker furnace 

fluidised-bed furnace 
multi-fuel furnace 

pulverised-coal furnace 

share 
2,2% 
2,3% 
6,9% 
89,1% 

Germany, dry-bottom furnaces dominate, holding a 
share of 59.2%. Wet-bottom furnaces are installed on 
40.8% of the sites. Because of this dominating role, this 
study will concentrate on pulverised-coal furnaces. 

A look at the geographic distribution of power plants 
over the territory of Germany shows the expected 
relationship with the population density. Highly 
populated areas mainly have hard-coal-fired power 
plants, while brown-coal-fired power plants are to be 
found in brown-coal mining areas so as to avoid long 
transport routes. New coal-fired plants are being built 
mainly to replace old brown coal fired units [1] [3]. 

The liberalisation has caused an upheaval in the 
electricity market in Germany. It is safe to assume that 
existing overcapacities will result in power plants being 
shut down. Particular some power plants installed as 
standby facilities to guarantee the continuity of 
electrical power supply are no longer required as a 
consequence of mergers of large electricity companies. 
However~ no reliable data are available for this ongoing 
process. 

Legal Framework 

In Germany~ power plants and waste incmeration plants 
must meet different emission limits. Emissions from 
power plants with thermal powers in excess of 50 M\vth 
are regulated by the Ordinance on Large Power Plants 
[4]. while waste incineration plants must meet the lower 
limits of the Ordinance on Incinerators for Waste and 
Similar Combustible Materials (5}. 

Where waste materials are used as fue1s in power 
plants. the emission limits must be calculated on the 
basis of the emission limits and criteria contained in the 
two ordinances. This calculation. the so called mixing 
rule~ takes into account the calorific values. specific flue 
gas volumes+ and other data. One problem arising in this 

550 million kWh 127790 million kWh 
(0.2 %) (47.7 %) 

2470 million kWh 140000 million kWh 
(0.9 %) (52.3 %) 

3020 million kWh 267790 million kWh 
(1.1 %) (100 %) 

connection is that the Ordinance on Incinerators for 
Waste and Similar Combustible Materials contains 
requirements on pollutants not listed in the Ordinance 
on Large Power Plants. For this reason, extensive 
measurements of emissions of a power plant may be 
necessary to obtain a permit for co-combustion of 
waste. A summary of chosen limits is included in 
Table 3. 

The EU has decided a new directive on waste 
incineration with criteria about the co-combustion of 
waste in power plants. As far as waste incineration is 
concerned, the limits contained in the EU [6] directive 
are based on the requirements of Ordinance on 
Incinerators for Waste and Similar Combustible 
Materials. In co-combustion the emission limits for heat 
shares of recovered fuels up to 40% are calculated by a 
mixing rule always starting at a minimum share of 
10 %. The EU directive states emission limit values for 
the power plants Cprocess to be used in the mixing rule 
together with the emission limit values for waste 
incineration Cwaste to calculate the allowed emission 
limit value. For the pollutants not listed in the 
Ordinance on Large Power Plants, the directive contains 
total emission limit values. 

Supplementary Fuels and their Properties 

In Germany, various management pathways exist for 
sludge arising in the treatment of municipal and 
industrial sewage. In Germany, sewage sludge is 
covered by waste management legislation. Next to 
agricultural uses, composting, and landfilling, thermal 
treatment has become an established disposal pathway. 
Especially municipal sewage treatment sludge is 
important when it comes to co-combustion in power 
plants. because most of the residues from industrial 
sewage treatment plants are being incinerated already. 

Relatively exact data exist about the treatment of 
municipal sewage in Germany. Accordingly, the 10 522 
municipal sewage treatment plants in operation [7] 
annually produce 2 642 200 t of dry sludge. Treatment 
pta~t sizes and sewage sludge arising yield/output differ 
regtonally and; as expected, depend on the population 
density. Taking into account the sludge fraction from 
municipal sewage treatment plants already incinerated, 



121 

Table 3 Emission limits'for waste incineration plants and power plants 

13th BimScbV 17th BimSchV EU-Directive 

Co-Combustion 
Power Plants Waste Incineration Wa~te Incineration (C waste) Power Plants 

>300MWth 
Cement Kilns 

Daily Average Daily Average Half Hour 
DaiJy Average Half Hour 

Daily Average Daily Average 
(6% 0 2) (11 % Oz) Average 

(11% Oz) Average 
(6% 02) (10% Oz) Com_Eound Unit (11% Oz) (11 % Oz) 

co mglm3<t 250 50 100 
50 x<t3 x(9 

150 (:! 100 

org. compounds as mglm3<t 10 20 10 x<J3 w<n total Carbon 20 

Particulate Matter mg/m3 <1 50 10 30 10 30 30(1:! 30(11 

S02/ S03 as SOz mg/m30 400 50 200 50 200 200(12 5o<u 
Sulfur emission 

% 15 
factor 

5(12 

NO u. NOz as NOx mgfm3<I soo<3 200 400 200 400 200 (300°0) (lZ soo<ll 
HCl mg/m30 100 10 60 10 60 x<J3 10(11 

HF mgfm3<t 15 1 4 1 4 x<t3 1 (11 

ITlandCd mglm3<t o.o5<4 o.os<6 0.05(6,ll 0.05(6,!1 

Hg flg/m30 0.03 0.05 0.05 (6 0.05(6,ll 0.05(6' 11 

L Sb, As, Pb, Cr, 
Co, Cu, Mn, Ni, V, mg!m3<t 0.5 (4 u.s (6 0.5 (S,ll 0.5 {8,11 

Sn 
PCDD/PCDF nglm3<t 0.1 (5 0.1 (7,{S 0.1 (7,11 0.1 (7,ll 

(1 reference Standard Cubic Meter, dry (7 
(2 concentration based on the mass which can't be violated by more than (S 

10 % of all measurements within a time period of 24 hours (9 
0 

average over sampling time (at least 6 h; maximum 8 h) 
withoutSn 
to be set by the competent authority 
biomass 

(3 for the emission control the state of the art should be applied g 
1 

(4 average over sampling time (at least 0.5 h; maximum 2 h) total emission limit value for this process (C emission) 
emission limit value to be used for mixing rule (C process) 
mixing rule with the limits as stipulated and C waste 

(5 average over sampling time (at least 6 h; maximum 16 h) (12 

(6 average over sampling time (at least 0.5 h; maximum 8 h) (l3 

a maximum of 2.3 million t of sewage treatment sludge 
(dry) can be available for co-combustion in power 
plants. 

The consideration of wood for co-combustion in 
coal-fired power plants demands a distinction among 
very different wood fractions. Forest wood residues, 
arising in the tending of woods normally contain no 
pollutants. The quantities available are influenced by 
forest areas in a region, weather conditions, such as 
heavy storms, and by other factors. 

The quantities of forest wood residues available in 
Germany are based on statistical calculations in which, 
among other factors, the forest areas of the regions 
considered, the tree species, and ecological aspects are 
taken into account. It may be assumed that 
approximately 7.5 million t of raw wood can be used in 
power plants in Germany. [8] (9] 

Moreover, the optimum solution to the disposal of 
waste wood is being discussed in Germany. Waste 
wood arises in a variety of industrial sectors or private 
households and may carry a variety of pollutants, such 
as wood preservatives or paints containing heavy 
metals. 

Determining the quantities of waste wood in 
Germany is difficult for a variety of reasons. Different 
interpretations of the term "waste wood" are used and 
sawdust and similar materials from the wood industry 
are not always taken into account. Additional new legal 
regulations, in particular on regional level have to be 
taken into account. Moreover, the amount of waste 
Wood is influenced by local waste management The 
quantities of waste wood which can be used for co-

combustion, consequently, can be determined only with 
major uncertainties. An estimate conducted on this basis 
results in a total quantity of 4.1 to 11.7 mil1ion t per 
annum. A breakdown into the different fractions of 
waste wood is shown in Fig.]. 
Straw is another supplementary fuel of interest arising 
as a by-product in the cultivation of wheat, barley etc .. 
Straw so far has mostly been left on the fields or used in 
stock farming. In the industrialised countries, straw has 
never been used for energy production, but that use is 
already being pushed in Denmark by law. 

The quantities of straw that could be used for co­
combustion are determined by the present agricultural 
uses of existing areas, by weather conditions, and by 
soil properties. The quantities of straw are determined in 
a similar way as the amounts of forest wood residues. In 
Germany, a total of approx. 41.4 million t of straw arise 
annually [8] which, however. cannot be fully used for 
co-combustion. If the quantities required in stock 
farming and other areas are taken into account, then, 
according to [8], only 15% of the total amount, namely 
5.9 million t of straw per annum, would be available for 
thermal utilization. Other authors assume that between 
10.1 and 17.2 million t of straw can be used annually 
[9]. 

Estimates of the potential of recovered fuels from 
post user waste materials in Germany are shown in 
Table 4. The potential is quoted with 5.2- R.4 million 
tons/a. In this table production residues (pre~user 
wastes) are not quoted due to the limited data available. 
Estimates received from waste management companies 



122 

packaging (max) 

packaging {min) 

furniture, etc .. (min) 

construction and demolition (max) ~~~~-~~~~·~~~~~~·-

0 2 3 4 5 6 7 
amount [million metric tons/yr.] 

Fig.l Range of waste wood arising from packing, used 
furniture and the construction sector 

Table 4 Potentials for recovered fuels from high calorific 
fractions of selected waste materials [10] 

Total 
High calorific potential for 

fraction recovered fuels 
[million t/a] 

[%] [million t/a]* 
household waste 11.1 40-50 2.7-4.4 
industrial waste 

similar to 4.8 50-60 1.4-1.2 
household waste 

bulky waste 1.8 75-85 0.8- 1.2 
sorting residues 0.8 65-75 0.3-0.5 

(DSD) 
total 18.5 5.2-8.4 

* calculated from 60% of the lower and 80 % of the higher estimate 
of the hiah calorific fraction, without considering the possible 
additionat' streams that might go into Municipal Solid Waste 
(MSW) incinerators 

Table 5 Fuel analysis of coals and selected supplementary fuels 

hard coal brown coal 
LHV* (raw) [MJ/kg] 28 9 
moisture (raw) [%] 5.1 50.4 
volatile matter (dry) [%] 34.7 52.11 
ash (dry) [%] 8.25 5.1 
FixedC (dry) [%] 57.1 42.83 
c (dry) [%] 72.48 65.9 
H (dry) [%] 5.64 4.9 
N (dry) [%] 1.28 0.69 
s (dry) [%] 0.94 0.39 
Cl (dry) [%] 0.128 <0.1 
0 (dry) [%1 11.1 23 
ash fusion temperature [oCJ 1250 1050 
*Lower Heating Value **Refuse Derived Fuel 

average at 2 million tons total, which would yield about 
I miliion tons/a recovered fuel. 

The co-combustion of sewage sludge, wood and 
straw in a power plant effects the power plant operation. 
These influences are determined by the fuel 
characteristics and the chemical compositions. Table 5 
lists the most important fuel data for a number of 
selected examples. 

Most of the supplementary fuels considered mostly 
have lower calorific values than hard coal. As a rule~ 
waste wood has a humidity of approx. 10% and a 
calorific value on the order of 17 MJ/k.g. In the case of 
sewage treatment sludge. the calorific value is between 
2 and 12 MJ/k.g. depending on the water content. The 
water content and the ash fraction have a major 
influence on the calorific value of bio fuels, for which 
IS to 18 MJ/kg is to be expected. 
The large ash share in sewage sludge increases the 
amounts of fly ash by co-combustion. Although the a h 
content of straw can be compared to that of regular 
fuels. the low melting points may cause problems in 
operation as a result of caking. Moreover the chlorine 
content must be considered which has a major impact 
on corrosive processes. Especially straw normally has a 
relatively high chlorine content. Waste wood may 
contain up to 3'R- nitrogen and heavy metals. 

wood Straw RDF* dried sewage sludge 
12.4 15 23.5 10.58 
33 10.6 4.1 3 

83.2 74.4 82.6 49.52 
0.34 6.1 12.2 45.1 
16.5 19.9 5.2 2.39 
48.7 47.4 56.8 25.01 
5.7 4.5 7.9 4.88 

0.13 0.4-0.78 0.74 3.2 
0.05 0.05-0.11 0.25 1.1 
<0.1 0.4-0.73 0.82 <0.1 

45 40.4 21.3 17.69 
1200 850 ll20 1200 

Fig.2 Schematic diagram of a typical pulverized-coal-fired 
power plant with flue gas treatment system 

Basic Principles of Co-combustion in Power Plants 

The co-combustion of supplementary fuels in power 
plants is based on the principle of using existing plants 
without major modifications. Possible ways of feeding 
supplementary fuels are shown in Fig.2. 

The coal crushed in coal mills is fed into the 
combustion chamber by air as transport medium. 
Thermal energy of the hot flue gases is transferred in the 
boiler to the water·steam system for electricity 
generation or other uses. Tht=" pOllutants are separated in 
the flue gas treatment system, which is mostly made up 
of a Selective Catalytic Reactor (SCR) de NOx reactor, 



an electrostatic precipitator and a flue gas 
desulphurisation plant. 

Supplementary fuels can be fed in two very different 
ways. One possibility, which will be the main focus in 
this study, is direct feeding to the combustion chamber 
together with the standard fuel. In this case, there may 
be need for an additional preparation step, i.e., drying or 
crushing. Another possibility is thermal pre-treatment 
by pyrolysis or gasification in a separate reactor. The 
products of this pre-treatment are then fed to the 
combustion chamber after passing varying steps of 
cleaning or heat recovery. 

Existing power plants were designed for a specific 
fuel throughput. The influence of co-combustion on the 
different plant components may be very different. One 
example to be mentioned is the fuel volume flow 
increase resulting from the co-combustion of biomass. If 
approximately 10 % of the thermal output of the furnace 
system ise supplied as biomass, the fuel volume flow 
almost doubles. Common grinding and feeding of 
biomass and standard fuel may then result in problems. 

The melting point of ashes produced in the 
combustion chamber can be reduced as a result of co­
combustion. Compared to operation with the standard 
fuel, this may cause increased slagging and fouling in 
the combustion chamber and the heat exchangers. 
Problems of this kind may arise in particular when bio 
fuels are used whose ash melting point is very low. In 
power plants which, due to their mode of operation, 
generate molten slag, low melting temperatures of the 
ashes are desirable. 

In the region of convective heating surfaces, there 
may be increased contamination or erosion when 
secondary fuels with high ash share are used. Especially 
for sewage sludge, this is an aspect of importance. Bio 
fuels, because of their lower ash fractions, are not 
problematic in this respect. 

It should also be examined whether co-combustion 
of supplementary fuels results in changes in the flue gas 
volume and the flow rate, respectively. Any change in 
the flow rate influences the heat transfer in the boiler 
and also has an impact on the flue gas purification 
system. Higher flow rates may increase erosion. 
Another critical point is the high chlorine content of 
straw or other supplementary fuels, which may give rise 
to high-temperature corrosion and wastage of the 
heating surfaces. 

The possible effects of co-combustion on the flue 
gas purification plant are determined by the placement 
of the DeNOx-System. In the case of the high-dust 
placement shown in Fig.2, deactivation of the DeNOx 
catalyst by components of the fly ash, such as alkali 
metals, arsenic, phosphorus, or fluorine, is quite 
possible. 

For co-combustion of waste materials, a low-dust 
arrangement of the DeNOx-System downstream of the 
flue gas desulphurization plant is advantageous with a 
view to potential poisoning of the DeNOx catalyst. The 
price to be paid is reheating of the flue gases to the 
operating temperature of the catalyst. 

Dust removal may cause problems especially when 
sewage sludge with its high ash content is used. In 

123 

addition to the increased amount of ash, the removal 
behaviour of mixed ashes can be different from that of 
the ashes produced by the use of standard fuel. 

The effects of co-combustion on the flue gas 
desulphurisation plant are determined by the chemical 
composition of the supplementary fueL As a 
consequence of sulphur concentrations, bio fuels reduce 
the sulphur input, while sewage sludge tends to increase 
it. 

In addition to sulphur, arsenic, lead, mercury, and 
other heavy metals can be removed in the flue gas 
desulphurisation plant. As a consequence, the utilisation 
of gypsum produced in flue gas desulphurisation can be 
influenced. In the case of chlorine containing 
supplementary fuels it must be taken into account that 
chlorine increases the solubility of gypsum. 

Status of Co-combustion 

In the course of this study, those power plant locations 
in Germany were listed which have experience in the 
co-combustion of supplementary fuels. The list is based 
on literature and other sources. Because of the large 
number of sources, no references will be indicated. 
Table 6, Table 7 and Table 8 show the locations plus 
information about the respective power plants. 

At the present time, experience in the co-combustion 
of sewage sludge has been accumulated at 17 locations. 
The operating permit for co-combustion of sewage 
sludge was granted to twelve plants, but is being used 
by only nine plants. This results in an capacity for co­
combustion of sewage sludge of approx. 350,000 t of 
sewage treatment sludge per annum. 

According to Table 7, biomass or waste wood are 
being used as supplementary fuels at two locations. 
Experiments about co-combustion have been 
documented for six other sites, and conceptual design 
studies exist for two locations. As far as the other waste 
materials are concerned, these are specific solutions for 
industrial locations where specific wastes arise in large 
quantities. There have been tests in several additional 
power plants, of which results have not been published. 
After the occurrence of BSE cases on the European 
mainland, interest shifted and co-firing of meat and 
bone meal has been established in several German 
power plants. There is not sufficient data published to 
draw an accurate picture about range of co-firing and 
the effects of co-firing on the power plants. Depending 
on the legal and political developments co-ftring of 
meat and bone meal might be a temporary solution. 

Potentials 

The use of supplementary fuels in electricity generation 
can be estimated on the basis of the primary energy of 
coal used, which amounted to 2708 PJ in 1998. Under 
the assumption that 10% of the energy could be 
substituted by a supplementary fuel with a calorific 
value of 10 MJ/kg, German power plants could use an 
annual 27 million t of supplementary fuels. Due to 



124 

Table 6 German power plants experienced with co-combustion of sewage sludge 

Sewage Sludge 

Proportion 

Site I Company Firing System Boiler Size Supl. Fuel Status Capacity 

% by wt % -th. 

Berrenrath brown coal 
235 MW., dewatered 

since 1995 
4 3.5 

max. 

Rhein braun fluidized bed cont. operation 65 000 t TS/yr. * 

Box berg 111 VEAG 
brown coal dry-

2x500MWo~ dewatered 
since 1999 3-5 approx. 1 approx. 

bottom cont. operation (6% *) 30 000 t TS/yr. 

Braunsbedra EWAG brown coal stoker 4x7 MW., 
dewatered 1996 tests up to 50 approx. 25 

100% wood performed 

Buschhaus brown coal dry-
930MW., 

dried since 1998 5 * 
<5 

approx. 

BKB bottom dewatered cont. operation 80 000 t TS/yr. 

Duisburg H. hard coal 200MWih 
dewatered cont. operation 9 <OS 

approx. 

Stadtwerke wet-bottom 365 MW., 16 000 t TS/yr. 

Farge Bremen hard coal 
356MWc~ dewatered 

permission but 
9 <05 

approx. 
Preussco Elektra !lry-bottom noo ration 10 000 t TS/yr. 

Franken II hard coal 1047 MW., 
dewatered since 1998 cont. 

5 % TS* < 1.5 
approx. 

Bayernwerke dried operation 35 000 t TS/yr. 

l:leilbronn 1933 MW., 
dewatered since 1998 4 < 1.1 approx. 

EnBW dried cont. operation (4 %*) 40 000 t TS/yr. 

Lausward 170 dried 
permission but 21 II 1994-1996 

Stadlw. Dilsseld. no !:l~Jeration (15%* 20 000 I TS/yr. 

Liinen lnnovatherm 
hard coal fluidized 

9MWc~ de watered 
since 1997 cont. coal addition until approx. 

bed operation LHV>4200 MJ/kg 100 000 t/yr. 

Mumsdorf Mibrag 
brown coal 

N.A. 
dewatered 

tests performed N.A. N.A. 
dry-bottom dried 

KarlsruheRDK hard coal 
1280 MW., dewatered 

1998 tests 
1.25 0 .07 

EnBW dry-bottom performed 

Voerde hard coal 
350 MW .. dried 

1989 tests approx. 20 
STEAG wet-bottom performed 

Wahlheim hard coal 
382 MWih dried 

1996 tests 12.5 5 
eckarwerke wet-bottom performed 

Weiher U SaarEnergie 
bard coal 

195MWih 
dewatered since 1996 cont. 5 

(15%*) 
approx. 

dried operation 10 000 t TS/yr. 

Weisweiler 
913 MWih de watered 

cont. operation 
7.5 max. 140 000 t /yr. • 

RWE starting 5/2000 

Zolling Bayemwerke 1080MW., de watered 
since 1999 9 .6 2.76 approx. 

cont. operation 5 %TS* 10 000 t TS/yr. 

tests cont. operation .A.: Not Available 

Table 7 German power plants with co-combustion of biomass and waste wood 

Biomass & Waste Wood 

proportion 
Site and Company Firing System Boiler Size Supl. Fuel Status Capacity 

%-wt %-tb. 

Afferd El.Werke hard coal fluidized 
124MW., waste wood 

since 1997 up to up to 
Wesertal bed cont. operation 100% 100% 

N.A. 

Berrenrath Rheinbraun 
brown coal 

235 MW., waste wood 
1999 tests test with 

flu.idi zed bed cont. operation approx. 8 t/h 
.A. 

Heilbronn bard coal 
1933MW111 

wood 
feasibility study 1996 20 tlh 

EnBW dry-bottom straw 8 
80000 t/yr. 

Janschwalde EAG 
brown coal .A. waste wood tests running possible: 
dry-bottom .A. .A. 

I 00 000 t/yr. 

LUbbenau brown coal 
100MWd wood chip 1993 tests 

VEAG dry-bottom 7 .A. 

Moabit bard /brown c. 240MW., wood 1995 tests performed 
BEWAG fluidized bed .A. I to 13 

Pformeim Stadtwerke 
hard coal wood, waste wood, ~ . . . 

flujdized bed bay eas•b•lity study 1997 substitution of about 20 MW111 possible 

Scbwandorf browncoa1 
straw 1996 tests up to 20 

% 
Bayem~erke dry-bottom ince 6/1999 cont. 

8 12 operation 100 000 t/a 

tests • value stipulated in the license issued 



125 

Table 8 German power plants experienced with co-combustion of waste materials 

Waste Materials 

Bremen Entsorg.betrieb several wastes 
conceptional study 

materials 

Janschwalde VEAG 
brown coal 

N.A. RDF tests running N.A. N.A. 
possible 

dry-bottom 100 000 t/yr. 

Leverkusen HKW Bayer hard coal fluidized 
105MWih polystyrene resin concepts 0.375 0.5 330 t/a AG bed 

Schwarze Pumpe VEAG 
brown coal 

N.A. tar residues 
12194-6/98 cont. 

<I 
until6/98 

dry-bottom operation 250 000 t 

Wolfs burg hard coal 
N.A. 

lacquer sludge 
cont. operation up to 

vw wet-bottom used oil 30000 t/yr. 

Gasification I Pyrolysis 

Firing System Boiler Size Supl. Fuel Status 
Technique & 

Capacity 
Proportion 

Site and Company 

Berrenrath 
Rhein braun 

brown coal 
plastics, RDF, 

tests since ' 86 
fluidized bed 

up to 30 t!h 
sewage sludge gasification 

Hamrn 
VEWEnergie 

hard coal 
770MWih 

plastics, car shred, 
Start up phase 

rotary kiln pyrolysis 
2x50 000 t/yr. 

wet-bottom industrial waste 60MW(JO %) 

tests cont. operation * value stipulated in the license issued 

Table 9 Fuel and energy potentials of the chosen supplementary fuels 

heating value 
share on primary energy consumption '> 

fuel potential energy content 
hard coal brown coal total 

[million t/a] [MJ/kg] [PJ] [%] [%] [%] 

sewage sludge 2.3 (TS) 12 28 2.0 2.1 1.0 

waste wood 8.5 I) 16.5 140 10.2 10.4 5.2 

Straw 5.9 14.5 86 6.3 6.4 3.2 

forest residue - 7.5 10 2) 75 5.5 5.6 28 

RDF-fuets 4,5 n•> 76 5.6 5.7 2 

Total 404 29.6 30.2 15.0 

I) average of 4.1 - 12.7 Mio. t/a 
2) depending on storage time 1 0 - 60 % moisture, or 6-15 MJ/kg; here: 40 % moisture or I 0 MJ/kg 
3) primary energy consumption 1998: hard coal= 1363 PJ; brown coal= 1345 PJ; total= 2708 PJ 
4) assumed average heating value 

logistical and operational limitations these heat shares 
might not be achieved in operation. 

The results of the calculation of energy potentials on 
the basis of the calorific values of the supplementary 
fuel quantities listed in this work are shown in Table 9. 

The supplementary fuel potential incorporates an 
energy volume of approx. 404 PJ, which corresponds to 
approx. 15 %of the energy input into German coal-fired 
power plants. If this supplementary fuel were used 
exclusively in lignite or hard-coal fired power plants, a 
nearly 30 % contribution would be possible. Of all the 
fuels considered, waste wood constitutes the largest 
single item. The potential C02 reduction as a percentage 
of the total releases by coal-fired power plants is 
between 12.2% for the substitution of hard coal only, 
and 17.7% for brown coal. The reason is the higher 
specific C02 emission of brown coal. 

Comparison of the capacities of co-combustion in 
German power plants on the basis of Table 6, Table 7 
and Table 8 with the fuel potential in Table 9 indicates a 
major discrepancy. Co-combustion capacities exist for 
approx. 15 % of the arising sewage sludge. In the case 
of waste wood, a co-combustion capacity of approx. 

200,000 t per annum must be compared with an annual 
amount of approx. 8 million t per annum. 

Obstacles 

Co-combustion of waste in power plants in Germany on 
a larger scale is hampered by a variety of obstacles. 

First and foremost, the fuel supply of power plants 
must be guaranteed to maintain continuous operation. In 
the case of coal and other standard fuels, supply on 
international markets is easy. In some regions of 
Germany, the supply of sewage sludge is organised by 
industrial associations or municipal authorities. On the 
other hand, however, there are no institutions in 
Germany which could reliably supply power plants with 
biological fuels, such as forest wood waste or straw. 

Availability is influenced also by the fuels 
themselves. While sewage sludge arises continuously 
the year over as a residue from sewage treatment, 
seasonal differences and risks re ulting from climatic 
influences affect the availability of traw and other bio 
fuel s. 



126 

In addition to these problems, also the requirements 
to be met in recycling combustion residues must be 
borne in mind, because the use of these materials in the 
cement and building industries must meet specific 
criteria. The high ash content of sewage sludge affects 
the properties of fly ash even at low contributions to the 
thermal power plant capacity. 

Moreover, the co-combustion of sewage sludge, 
waste wood or other supplementary fuels requires 
amendments to the operating permits which may entail 
considerable expenses. In this connection, the lack of a 
precise definition of the terms "utilization" and 
"disposal" in German legal requirements represents an 
additional problem, with ''disposal" implying 
considerably more stringent regulations. In Germany, 
the co-combustion of sewage sludge is considered both, 
utilization and disposal, depending on the permit 
stipulated in the specific case. 

Where waste materials are used, the legal 
requirements from waste incineration must additionally 
be taken into account in Germany. Applications for 
permits require measurement programs, which may be 
quite extensive, on the basis of which emission limits 
are determined by the mixing rule. 

Another problem is the capital costs. In addition to 
the costs for the storage and transport of supplementary 
fuels, the use of waste materials gives rise to costs also 
for modifying emission measurement systems and 
instrumentation and control systems. As a consequence 
of deregulation in the electricity market, capital 
investments are made only with great reluctance at the 
present time. Because of the mergers of power 
companies, which give rise to increasingly tougher 
competition, long-term planning is not possible at the 
present time. 

Another important item is fuel costs. While a cost 
benefit is likely to arise from the combustion of waste 
material, biomasses, such as forest wood residues or 
::.traw, give rise to costs between DM 75 and 225/t (dry) 

' [!OJ. 

Summary 

At first sight, the work performed confirmed that the co­
combustion of sewage sludge is a technology 
established on an industrial scale. Although there are 
precise regulations about permits for co-combustion, the 
use of sewage sludge should be examined critically with 
respect to highly volatile pollutant residues, such as 
mercury and mercury compounds. 

In addition, the existing power plant spectrum in 
Germany offers possibilities for the co-combustion of 
forest wood residues, straw, recovered fuels or waste 
wood. The co-combustion of these materials is 
technically feasible even though altered compositions of 
residues and other details have not yet been clarified. 

In addition to the technical aspects, activities 
seeking to further co-combustion are influenced by 

economic concerns against the background of the 
deregulation of the electricity market in Germany. The 
expected financial benefits make co-combustion 
attractive for the use not only of sewage sludge, but also 
of other waste materials. 

There are two main reasons why co-combustion of 
biomasses has not yet been carried out on a technical 
scale in Germany. For economic reasons, co­
combustion is possible only with financial support by 
the government. Moreover, there are no fuel markets to 
supply power plants with these fuels. 

Acknowledgement 

The authors are grateful to the German Federal Ministry 
for Education, Science, Research, and Technology for 
the financial support made available under grant No. 
0326864 and to the European Community for the 
funding under the contract number ERK5-CT -1999-
00021. 

LITERATURE 

1. Vereinigung Deutscher Elektrizitatswerke 
(VDEW e.V.): Strommarkt Deutschland 1998; 
Frankfurt am Main; 1999 

2. Vereinigung Deutscher Elektrizitatswerke 
(VDEW e.V.): VDEW-Statistik 1996 
Betriebsmittel, Frankfurt am Main; 1997 

3. Technische Vereinigung der 
Grosskraftwerksbetreiber E.V. (VGB): 
Tatigkeitsbericht 1996/1997, VGB-Verlag, Essen 

4. Dreizehnte Verordnung zur Durchfiihrung des 
Bundes-Immissionsschutzgesetzes (Ordinance on 
Large Power Plants), Juni 1983 

5. Siebzehente Verordnung zur Durchfiibnmg des 
Bundes-Immissionsschutzgesetzes (Ordinance on 
Incinerators for Waste and Similar Combustible 
Materials), Februar 1999 

6. Richtlinie 2000176/EG des Europaischen 
Parlaments und des Rates vom 4. Dezember 2000 
tiber die Verbrennung von Abfallen 

7. EsCH B. and KROGER G.: Korrespondenz 
Abwasser,l999, 46(6), 943-952 

8. KALTSCHMITf M. and WIESE A.: Erneuerbare 
Energietrager in Deutschland. Potentiale und 
Kosten, Springer-Verlag, Berlin, 1993 

9. LEmLE L., RoSCH C. and WINTZER D.: Nachrichten 
- Forschungszentrum Karlsruhe, 1997, 29(2), 113-
122 

10. BRAUNGART M., KETELHUT R. and 
GALLENKEMPER B.: Entsorgungspraxis 2000, 18(2), 
31-33 

11. SiEGLE V.: Biogene Brennstoffe in Aufbereitung 
und Verbrennung; Dissertation Universitlit 
Stuttgart, 1999 


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