Enhancing cleaner biomass-coal co-combustion by pretreatment of wheat straw via washing versus hydrothermal carbonization

Abstract

Slagging, fouling and fuel segregation issues with co-combustion of wheat straw and coal could be mitigated by clean pretreatments on wheat straw such as washing or hydrothermal carbonization (HTC). Water washing (WW) could remove upwards of 90% of potassium and phosphorus, thus reducing the risk of slagging and fouling. The dehydration and demethylation reactions during HTC increased the heating value of wheat straw to 5.4%–35.7%, simultaneously more organically-bonded calcium and magnesium were removed. Conversely, the potassium retention of hydrochars was improved owing to the relatively higher specific surface area (3.153–4.693 m2g-1) than WS (1.181 m2g-1) and WW (1.375 m2g-1). Moreover, the retention of alkali metals and porous structure showed a dual effect on the co-oxidation property, varying with HTC temperatures. Hydrochars produced at 200 °C had similar ignition temperatures to a representative bituminous coal. Hydrochars produced at 220 °C showed enhanced reactivity, whereas 240 °C hydrochars had decreased reactivity and potentially problematic ash content. The activation energy of the HTC200 blends (40.71 kJ/mol for 20% ratio and 39.24 kJ/mol for 40% ratio) are lower than the WW blends. Overall, blends of 20 wt% hydrochars produced at 200 and 220 °C showed the optimum performance in terms of minimizing fuel segregation and inhibition, lowering the oxidation activation energy barrier, and reducing slagging and fouling.