Commodity Fuels from Biomass through Pretreatment and Torrefaction: Effects of Mineral Content on Torrefied Fuel Characteristics and Quality

Abstract

Torrefaction of biomass is rapidly gaining popularity as a viable pretreatment for use with co-combustion with coal or with other thermochemical conversion processes. This work explores the effects of combining pretreatment washing techniques using water, ammonium acetate, and hydrochloric acid. Four biomasses were studied, short-rotation coppiced willow, eucalyptus, Miscanthus, and wheat straw, all in chipped or chopped form. The resultant fuels, after the pretreatments, were characterized for ultimate analysis, proximate analysis, heating value, and pyrolysis behavior (via thermogravimetric analysis), and mass and energy yields in a fixed-bed torrefier were measured. The ease of removal of certain metals, Na, K, Mg, and Ca, as well as PO43–, SO42–, and Cl– was assessed by ion chromatography on the leachates from the water washing, and influences on fouling behavior were predicted. Fuel ashes (both prior to and after torrefaction) were also assessed in the ash fusion test, a probe for slagging behavior. Water washing resulted in a high removal of alkali metal ions and chloride, particularly for the herbaceous biomass, where up to 92% of sodium and 62% of potassium were removed, together with up to 100% of the chloride. There was a general trend of higher concentrations of water-soluble species for the herbaceous biomass compared to the woody biomass, although there were a few exceptions, such as phosphate. As a consequence of water washing, the alkali indices (an index for fouling) decreased markedly. Because the ash composition changes as a result of the different washing procedures, the ash melting behavior also changes, and hemisphere temperatures (oxidizing conditions) were seen to increase substantially, by approximately 400 °C for wheat straw to 1500 °C and 290 °C for Miscanthus to 1490 °C. Different pretreatment methods also influenced the progress of torrefaction. After all washings, the fuels became less reactive to thermal degradation, and therefore, mass (and energy yields) increased during a fixed torrefaction operation. This could be explained through measurement of the pyrolysis kinetics; removal of key catalytic metal species (such as Na and K, in particular) by washing results in slower reaction rates. Water washing was seen as the most beneficial pretreatment, because it produced the most marked improvement in the torrefied fuel in terms of its ash fusion test behavior.