Hydrothermal carbonization: Fate of inorganics

Hydrothermal carbonization (HTC) is a pretreatment process for making a homogenized, carbon rich, and energy-dense solid fuel, called biochar, from lignocellulosic biomass. Corn stover, miscanthus, switch grass, and rice hulls were treated with hot compressed water at 200, 230, and 260 °C for 5 min. Mass yield is as low as 41% of the raw biomass, and decreases with increasing HTC temperature. Higher heating values (HHV) increase up to 55% with HTC pretreatment temperature. Up to 90% of calcium, magnesium, sulfur, phosphorus, and potassium were removed with HTC treatment possibly due to hemicellulose removal. At a HTC temperature of 260 °C, some structural Si was removed. All heavy metals were reduced by HTC treatment. The slagging and fouling indices are reduced with HTC treatment relative to that of untreated biomass. Chlorine content, a concern only for raw and HTC 200 switch grass, was reduced to a low slagging range at 230 °C, and 260 °C. Alkali index was medium for raw biomass but decreased by HTC.