Release of Inorganic Constituents from Leached Biomass during Thermal Conversion

Abstract

Leaching of inorganic materials has recently been shown to substantially improve the combustion properties of biomass fuels, especially straw but including other herbaceous and woody fuels. Leaching with water removes large fractions of alkali metals (typically >80% of potassium and sodium) and chlorine (>90%). Smaller fractions of sulfur and phosphorus are also removed. Alkali metals are heavily involved in ash fouling and slagging in combustion and thermal gasification systems. Chlorine is a facilitator of alkali volatilization, and contributes to corrosion and air pollution. The presence of these elements has reduced or eliminated the use of certain biomass fuels in many combustion applications, even where such use might provide significant environmental benefits. Leaching could mitigate the undesirable effects of biomass ash in thermal systems. Reported here for the first time are comparative studies of volatile inorganic species evolving from leached and unleached biomass fuels during thermal conversion. Leached and unleached samples of rice straw, wheat straw, switchgrass, commercial power plant wood fuel, and banagrass (Pennisetum purpureum) were tested in bench-scale combustion studies using an alumina-tube flow reactor housed in a variable temperature furnace and coupled to a molecular beam mass spectrometer (MBMS) system. Sugarcane bagasse, as the leached byproduct of sugar production, was also tested. The MBMS system was used to monitor the combustion products, including inorganic vapors, directly and in real time during each batch combustion event. Total relative amounts of HCl(g), SO2(g), NaCl(g), KCl(g), and other species were compared for leached and unleached samples. The MBMS results were consistent with the levels of alkali metals and chlorine in the samples as determined from the proximate, ultimate, and ash analyses of the samples. The more alkali and chlorine in a given sample, the more gas-phase HCl, KCl, and NaCl detected with the MBMS during combustion of that particular sample. The MBMS results clearly support earlier results, which indicated that leaching biomass effectively reduces or eliminates the release of alkali metal vapors during combustion.

A Review of Thermal Co-Conversion of Coal and Biomass/Waste

“…The plant growth conditions, species, and the overall sulfur availability during growth dictates the sulfur content as well as the ratio between organic and inorganic sulfur. Unlike chlorine (Cl) and K, sulfur removal by pretreatment processes like aqueous leaching will only take out the sulfur not organically bound to the structure of the plant [228][229][230][231][232]. Dayton et al [232] observed a bimodal distribution of SO 2 release during unleached wheat straw combustion, with a peak during the devolatilization phase and a second more intense peak during the char combustion stage.…”

Switchgrass for bioethanol and other value-added applications: A review

“…Leaching of switchgrass with water was proposed by Dayton et al (1999) to remove alkali metals and chlorine prior to combustion. The study reported that leached biomass produced less alkali metal vapors and resulted in reduced char formation during combustion.…”

Characterization of char derived from various types of solid wastes from the standpoint of fuel recovery and pretreatment before landfilling

“…In thermal recycling aspect, MSWIs show significant low energy efficiency (13 to 14%) unlike thermal power plants because they have to keep lower steam temperature to prevent severe boiler corrosion (Malkow, 2004). Besides, RDF or biomass occasionally requires a special boiler system for its combustion (Dayton et al, 1999;Jensen et al, 2001). …”