The increased demand for energy from biomass enforces the utilization of new biomass fuels (e.g., energy crops, short-rotation coppices, as well as wastes and residues from agriculture and the food industry). Compared to conventional wood fuels, these new biomass fuels usually show considerably higher ash contents and lower ash sintering temperatures, which leads to increased problems concerning slagging, ash deposit formation, and particulate matter emissions. One possibility to combat these problematic behaviors is the application of fuel additives such as kaolin. In contrast to the usual approach for the application of additives based on an experimental determination of an appropriate additive ratio, this study applies novel and advanced fuel characterization tools for the characterization of biomass/kaolin mixtures. In the first step the pure biomass fuels (softwood from spruce and straw) and the additive were chemically analyzed. On the basis of the analysis theoretical mixing calculations of promising kaolin ratios were conducted. These theoretical mixtures were evaluated with fuel indexes and thermodynamic equilibrium calculations (TEC). Fuel indexes provide the first information regarding high temperature corrosion (2S/Cl) and ash melting tendency (Si + P + K)/(Ca + Mg + Al). TEC can be used for a qualitative prediction of the release of volatile and semivolatile elements (K, Na, S, Cl, Zn, Pb) and the ash melting behavior. Moreover, selected mixtures of spruce and straw with kaolin were prepared for an evaluation and validation of the release behavior of volatile and semivolatile ash forming elements with lab-scale reactor experiments. The validation of the ash melting behavior was conducted by applying the standard ash melting test. It could be shown that the new approach to apply novel and advanced fuel characterization tools to determine the optimum kaolin ratio for a certain biomass fuel works well and thus opens a new and targeted method for additive evaluation and application. In addition, it helps to significantly reduce time-consuming and expensive testing campaigns.

To examine relevant combustion characteristics of biomass fuels in grate combustion systems, a specially designed lab-scale reactor was developed. On the basis of tests performed with this reactor, information regarding the biomass decomposition behavior, the release of NO x precursor species, the release of ash-forming elements, and first indications concerning ash melting can be evaluated. Within the scope of several projects, the lab-scale reactor system as well as the subsequent evaluation routines have been optimized and tests with a considerable number of different biomass fuels have been performed. These tests comprised a wide variation of different fuels, including conventional wood fuels (beech, spruce, and softwood pellets), bark, wood from short rotation coppice (SRC) (poplar and willow), waste wood, torrefied softwood, agricultural biomass (straw, Miscanthus, maize cobs, and grass pellets), and peat and sewage sludge. The results from the lab-scale reactor tests show that the thermal decomposition behavior and the combustion behavior of different biomass fuels vary considerably. With regard to NO x precursors (NH 3 , HCN, NO, N 2 O, and NO 2 ), NH 3 and, for chemically untreated wood fuels, also HCN represent the dominant nitrogen species. The conversion rate from N in the fuel to N in NO x precursors varies between 20 and 95% depending upon the fuel and generally decreases with an increasing N content of the fuel. These results gained from the lab-scale reactor tests can be used to derive NO x precursor release models for subsequent computational fluid dynamics (CFD) NO x post-processing. The release of ash-forming vapors also considerably depends upon the fuel used. In general, more than 91% of Cl, more than 71% of S, 1−51% of K, and 1−50% of Na are released to the gas phase. From these data, the potential for aerosol emissions can be estimated, which varies between 18 mg/Nm 3 (softwood pellets) and 320 mg/ Nm 3 (straw) (dry flue gas at 13% O 2 ). Moreover, these results also provide first indications regarding the deposit formation risks associated with a certain biomass fuel. In addition, a good correlation between visually determined ash sintering tendencies and the sintering temperatures of the different fuels (according to ÖNORM CEN/TS 15370-1) could be observed.

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