Biomass and sewage sludge are attracting increasing interest in power plant technology as a source of carbon dioxide-neutral fuels. A new way to reduce the consumption of fossil fuels could be the co-combustion or co-gasification of coal and biomass or coal and sewage sludge. In both cases, pyrolysis is the first step in the technical process. In order to obtain detailed information about the pyrolysis of coal/biomass and coal/sewage sludge mixtures as well as unblended fuels, the ‘Institut für Verfahrenstechnik und Dampfkesselwesen (IVD)’ at the University of Stuttgart has carried out investigations using an electrically heated entrained flow reactor.
One application of substitution of fossil fuels could be the utilization of pyrolysis gas or gas generated in a gasification process as a reburn fuel in conventional boilers fired with fossil fuels. Investigation showed that generated gas from coal, biomass and sewage sludge pyrolysis and gasification have high NOx reduction efficiencies compared to methane or low calorific gases using it as a reburn fuel in coal fired boilers. In order to take advantage of this pretreatment process the release of organic as well as of mineral compounds during the pyrolysis or gasification has to be investigated. For coal pyrolysis and gasification the reactions are known since there was a lot of research all over the world. Biomass or sewage sludge have other structures compared to fossil fuels and contain alkali, chlorine and other problematic compounds, like heavy metals. The release of those elements and of the organic matter has to be investigated to characterize the gas and the residual char. The optimum process parameters regarding the composition of the generated gas and the residual char have to be found out.
The IVD has studied the co-pyrolysis of biomass and sewage sludge together with a high volatile hard coal. The main parameters to be investigated were the temperature of the pyrolysis reactor (400°C–1200°C) and the coal/biomass and coal/sewage sludge blends. Besides co-pyrolysis experiments test runs with unmixed main fuels were carried out with the hard coal, straw as biomass, and a sewage sludge. It was expected that the high reactivity of biomass and sewage sludge would have an effect on the product composition during co-pyrolysis.
The test runs provided information about fuel conversion efficiency, pyrolysis gas and tar yield, and composition of pyrolysis gas and tar. Besides gas and tar analysis investigations regarding the path of trace elements, like heavy metals, alkali, chlorine and nitrogen components, during the pyrolysis process varying different parameters have been carried out. The fuel nitrogen distribution between pyrolysis gas, tar and char has been analyzed as well as the ash composition and thus the release of mineral components during pyrolysis.
Increasing reaction temperatures result in a higher devolatilization for all fuels. Biomass shows a devolatilization of up to 80% at high temperatures. Hard coal shows a weight toss of approx. 50% at same temperatures. Sewage sludge devolatilizes also up to 50%, which is nearly a total release of organic matter, because of the high ash content of about 50% in sewage sludge.
Gaseous hydrocarbons have a production maximum at about 800°C reaction temperature for all feedstocks. Carbon monoxide and hydrogen are increasingly formed at high pyrolysis temperatures due to gasification reactions.
Mineral elements are released during straw pyrolysis, but within the hot gas filtration unit further recombination reactions and condensation of elements on panicles take place. There is no release of mineral elements during sewage sludge pyrolysis and only a slight release of heavy metals at high pyrolysis temperatures.
The effect of co-pyrolysis depends on the feedstocks used in association with the panicle size. The co-pyrolysis test runs showed that a synergetic effect exists when using sewage sludge and hard coal. There is a higher char production related to the unmixed fuels; gas and tar formation are lowered. Co-pyrolysis test runs with biomass and coal did not show this effect on the pyrolysis products. Reasons for this behaviour could be a difference in particle size and material structure which influences the devolatilization velocity of the fuels used or the relatively short residence time in the entrained flow reactor. It seems possible that coal pyrolysis is influenced by the reaction atmosphere, generated in co-pyrolysis. In the co-pyrolysis of coal and sewage sludge, the sludge degases much faster than coal because of the structure of sewage sludge and its small panicle. The coal pyrolysis taking place afterwards in the reaction tube occurs in a different atmosphere, compared to the mono-pyrolysis experiments. The devolatilization of coal in the co-pyrolysis experiments together with straw was not disturbed by the gaseous products of straw pyrolysis, because the large straw particles showed a delayed degasing compared to the coal particles.