(323f) Pyrolysis Investigations and Ash Melting Behaviour of Agricultural Residues Used as a Fuel in Small Furnace Installations

Agricultural residues involve various problems when used as fuel in small furnace installations. The most important reasons are high ash contents on the one hand side and the low melting points of ash on the other, which are mainly caused by a high rate of alkali metals. Therefore ash particles may stick together, tend to sintering and even melt at higher temperatures. Thus a layer, which cannot be discharged, may be built, which as a consequence interferes the combustion process. For providing the agglomeration of ash it is necessary to provide ideal combustion conditions and to avoid temperature peaks. Therefore it is essential to get more detailed information on the devolatilisation behaviour of the various fuel particles. Devolatilisation / pyrolysis of solid fuel is an important sub-step during thermal conversion. Especially in case of biomass it is of special interest, since more than 85 % of the solid may be volatiles, which will be converted during pyrolysis. In addition to furnace design and operation conditions it is possible to manipulate the fuel properties by additives in order to reduce ash melting and agglomeration. Concerning the selection of additives it was paid attention to the high content of alkaline earth metals, because these elements increase the ash melting point. One option is lime flour, which includes mainly calcium carbonate with a small share of magnesium carbonate. However, it is important to add only a smidgen of an inert substance, because the ash volume and the abrasion resistance get worse with it. For analysing the fuel properties of agricultural residues wheat straw pellets are used. Primarily the fusibility of fuel ash and the effect of additives to it were investigated with a hot stage microscope. Additionally the pyrolysis behaviour of the mixtures with highest melting points and the lowest content of additive was determined as well as the one of pure wheat straw pellets. So it was possible to compare them and interpret the changes of the devolatilisation behaviour. The analyses of pyrolysis are operated with a plant, which includes a thermo gravimetric scale (TGS) in combination with a differential scanning calorimeter (DSC). A flow sheet of this test facility is given in Figure 1. The fuel is pyrolysed in a muffle oven on a scale basket under nitrogen atmosphere and afterwards the produced pyrolysis gas is mixed with air. In the heat exchanger the pyrolysis gas and the combustion air are equalised and after a mixing comb burned on the surface of a platinum comb catalyst. After the catalyst a sampling unit is implemented, which leads the flue gas to the analysers. For synthesis analyses O2, CO2 and CO analysers as well as a flame ionisation detector are installed. In addition, the calorific value of the synthesis gas was also determined, simultaneously measuring the mass loss of the biomass. For determining the oxygen demand of the volatiles dependent on time, an oxygen mass balance of the DSC was performed. Thereby the oxygen entry is known because of the defined combustion air and the oxygen in the flue gas is measured. The oxygen demand of all volatiles was considered as the difference between these two values. Additionally the calorific value of the fuel was investigated with a bomb calorimeter in order to close the energy balance of the test evaluation and to get a verification of the established results compared with the theoretical ones. With the knowledge of the exact curve progression of calorific value and oxygen demand it is possible to improve the yield of useable heat and minimize emissions and consequently achieve low environment pollution.