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International Journal of Energy for a Clean Environment
SJR: 0.195 SNIP: 0.435 CiteScore™: 0.74

ISSN Imprimir: 2150-3621
ISSN En Línea: 2150-363X

International Journal of Energy for a Clean Environment

Formerly Known as Clean Air: International Journal on Energy for a Clean Environment

DOI: 10.1615/InterJEnerCleanEnv.v6.i3.60
pages 289-328

GENERATION OF A GASEOUS FUEL BY PYROLYSIS OF BIOMASS AND SEWAGE SLUDGE FOR USE AS REBURN GAS IN COAL-FIRED BOILERS

C. Storm
Babcock Borsig Service GmbH, Duisburger Str. 375 D-46049 Oberhausen, Germany
H. Spliethoff
Institute for Energy Systems, Technical University of Munich, Germany
Klaus R. G. Hein
Institute of Process Engineering and Power Plant Technology (IVD), University of Stuttgart, Pfaffenwaldring 23, D-70550, Stuttgart, Germany

SINOPSIS

Biofuels, including biomass and sewage sludge, attract increasing interest in power plant technology as a source of carbon dioxide-neutral fuel. Besides using solid pulverized biofuel as an additional fuel in coal-fired boilers, a further possibility to run a combined coal and biofuel process is to pre-pyrolyse or pre-gasify biomass or sewage sludge and to use the gas as reburn fuel in the coal-fired boiler. This process has the advantage of keeping undesired components, such as alkaline, chlorine, and heavy metal compounds, away from the coal-fired boiler. Besides the environmental aspects of air pollution, operational and economical problems, such as slagging, fouling, corrosion, and contamination of ash, become the focus of attention.
By thermal pre-treatment of biofuels using pyrolysis or gasification, the problems of a direct co-combustion process can be minimized. Within the pre-treatment process, the solid feedstock is separated into a high-energy gas and a solid residue. The gas can be used as a reburn fuel in coal-fired boilers with excellent NOx reduction properties, and the solid residue can be further processed or disposed of. The aim of the thermal pre-treatment is to bond the problematic components into the solid pyrolysis residue in order to avoid operational problems within the boiler. The separation of coal and biofuel ash in this process enables different ways to use both residuals.
At the Institute for Process Engineering and Power Plant Technology, University of Stuttgart, the pyrolysis and gasification of different biofuels has been investigated, with special emphasis on the reburn efficiency of the gas and on the release of inorganic compounds. The pyrolysis and gasification experiments have been carried out in a lab-scale entrained flow reactor (height 2.5 m, diameter 80 mm, 30 kWel, residence time 2−5 s) and in a lab-scale fluidized bed reactor (height 3 m, diameter 108 mm/135 mm, 20 kWel) in inert and oxidizing atmospheres. The gas composition, the composition of the solid residues, and the release of different components have been analyzed at reactor temperatures between 400°C and 1300°C. For reburn experiments, a lab-scale combustion reactor (pulverized fuel, height 3 m, diameter 200 mm, 30 kWel) was used.
The test runs provided information about fuel conversion efficiency, pyrolysis gas and tar yield, and composition of pyrolysis gas and tar. Studies regarding the path of inorganic elements, such as alkali, chlorine, and nitrogen components, during the pyrolysis process have been carried out. The fuel-nitrogen distribution among pyrolysis gas, tar, and char has been analyzed, as well as ash composition and the release of inorganic components during pyrolysis.
Detailed results of investigations using pyrolysis gas from coal (Greul, 1997; Greul et al., 1996, 1997), sewage sludge (Rüdiger et al., 1996a; Storm et al., 1998a) or biomass (Rüdiger et al., 1996b; Storm et al., 1999a) for NOx reduction are published elsewhere. To get information about the path of different feedstock components during pyrolysis, the quality and quantity of pyrolysis char has been analyzed and reported in (Storm et al., 1998a,b; 1999a,b).


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