Application of Novel and Advanced Fuel Characterization Tools for the Combustion Related Characterization of Different Wood/Kaolin and Straw/Kaolin Mixtures

Abstract

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.

Application of a Modeling Tool to Describe Fly Ash Generation, Composition, and Melting Behavior in a Wheat Straw Fired Commercial Power Plant

“…However, the smelting phases detected in the present study by means of DTA analysis, together with the SO 2 release detected by TGA-MS evidence the presence of K 2 SO 4 (l) in the samples. In a subsequent study [52], the SALT_F solution was eliminated from the thermodynamic equilibrium calculations in cases where the K/(Si+P) molar ratio in the raw biomass was higher than 1. In that way, it was intended to avoid an overestimation in the precipitation of K salts (carbonates and sulfates), which in turn would yield an unrealistic decay in the AMCs.…”

Effect of Additives and Fuel Blending on Emissions and Ash-Related Problems from Small-Scale Combustion of Reed Canary Grass

“…Sewage sludge (SS), limestone (LM) and lignosulfonate (LG) were added to reed canary grass (R) in a percentage of 3 wt% (blends R-SS, R-LM and R-LG, respectively). A review of the literature [10,11,20,22,[24][25][26][27]29,30] showed that additives are generally blended with biomass in proportions up to 10 wt%. However, satisfactory results were especially obtained by adding 1-5 wt% of additives.…”

“…Additionally, fuel indexes on a molar basis, based on works by Sommersacher et al [27,41] and describing the effect of given elements on alkali release, corrosion risk and ash sintering temperature, were calculated. They correspond respectively to Si/(K + Na), 2S/Cl and (Si + P + K + Na)/(Ca + Mg + Al).…”

“…Aluminum (Al) silicates based additives, such as kaolin, have been exhaustively studied and have shown an ability to abate particle emissions [11,17,[20][21][22][23][24] and ash sintering [25][26][27] during combustion of agricultural crops and residues. Kaolin mainly acts by binding alkali compounds in ash and by forming K-or Na-Al silicates that have a higher melting temperature than pure K or Na silicates [17][18][19].…”

“…Additives from waste stream resources such as sewage sludge are of particular interest since they are financially attractive [30]. Calcium based additives, such as lime and limestone, are used for reactions with HCl and SO2 and have been recognized as well as effective in reducing the slagging tendency in combustion systems by formation of high melting silicates formed of Ca, magnesium (Mg), and alkali [18][19][20]23,26,27]. Co-firing biomass with calcium based additives actually creates a diluting effect on biomass ash, which restrains physical contact and thus sintering of ash particles [18,19].…”