Effect of additives on thermochemical conversion of solid biofuel blends from wheat straw, corn stover, and corn cob

Abstract

To investigate the effect of fuel blending and additives on ash melting behavior and the formation behavior of particulate matter (PM) emissions from combustion of crop residues, corn stover, corn cobs, and wheat straw as well as selected blends without and with 2 wt% additive have been examined by determining ash melting behavior in laboratory muffle furnace, ash elemental composition using ion chromatography (IC) and atomic absorption spectrometer (AAS), thermogravimetric properties of ashes using thermogravimetric analysis (TGA), and crystalline phases using powder X-ray diffraction (XRD). The results show that wheat straw starts sintering above 800 °C, corn cobs at 900 °C, whereas corn stover above 1000 °C. Fuel blending can influence the ash characteristics, but the influence is not sufficient to prevent ash sintering during typical combustion temperatures. All three additives (kaolinite (Al2Si2O5(OH)4), magnesium oxide (MgO), and calcite (CaCO3)) are successful in preventing ash sintering up to 1100 °C. At 1000 °C, K, Ca, Mg, and SO42− remain in decreased concentrations only partly in the ashes (i.e., a certain share of these components is transferred into the gas phase forming particulate matter emissions). However, Cl− is completely released into the gas phase. After heating 550 °C ashes to 1000 °C using TGA, mass losses of ~15 wt% were observed in most fuels and fuel blends with and without additives. An exception in the TGA was the blends with CaCO3; the samples show a mass loss higher than 25 wt%, which at the same time leads to an increased release of components into gas phase. Kaolinite and MgO are good K sorbents, forming new silicates in the ash such as K-Al silicates, K-Mg silicates, Ca-Mg silicates, and K-Al silicates, whereas CaCO3 facilitated K release and formation of Ca silicates, Ca-Na silicates, and Ca-Mg-Al silicates. Furthermore, MgO and CaCO3 can bind SO42− in the ashes.