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

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 degrees C had similar ignition temperatures to a representative bituminous coal. Hydrochars produced at 220 degrees C showed enhanced reactivity, whereas 240 degrees 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 degrees C showed the optimum performance in terms of minimizing fuel segregation and inhibition, lowering the oxidation activation energy barrier, and reducing slagging and fouling.

Automated summary

Clean pretreatments such as washing or hydrothermal carbonization can mitigate the problems of slagging, fouling, and fuel segregation in the co-combustion of wheat straw and coal. Water washing can remove most of the potassium and phosphorus, reducing the risk of slagging and fouling. Hydrothermal carbonization can increase the heating value of wheat straw and remove organically-bonded calcium and magnesium. Hydrochars have a higher potassium retention rate and specific surface area. Hydrochars produced at 200 and 220 degrees Celsius show the optimal performance.