Over 30% and ∼2% (on a carbon basis) organic matter can be leached from mallee leaf and wood by water, respectively, producing acidic leachates containing organic acids. As a result, there are significant differences in the leaching characteristics of both organic and inorganic species in biomass between batch and semi-continuous leaching operations. Under conventional batch leaching, the acidic leachate continuously contacts the biomass for a prolonged period, resulting in the leaching of at least some water-insoluble inorganic species (e.g., organically bound) from biomass. Therefore, the batch leaching method clearly overestimates the amount of water-soluble inorganic species in biomass and exhibits two-step leaching kinetics, i.e. a rapid leaching step for an initial short period followed by a slow leaching step for a relatively long period. This study further develops a semi-continuous leaching method to address this issue via minimizing the contact between the leachate and the biomass sample. The semi-continuous leaching quantifies the true water-soluble inorganic species in biomass. Its leaching kinetics include only the first rapid leaching step, with the disappearance of the second slow-leaching step due to the absence of the interaction between acidic leachate and biomass. The results suggest that in the sequential extraction scheme used in chemical fractionation, semi-continuous (instead of batch) water leaching method should be used for quantifying water-soluble inorganic species in biomass. Attention should also be paid to the potentially substantial loss of fuel materials when utilizing water leaching as a pretreatment method to remove inherent inorganic species in biomass for fuel quality improvement. As result of overestimating water-soluble inorganic species and loss of organic matter, care must be taken when using water batch washing as a method for studying the effect of the inherent water-soluble inorganic species on thermochemical reactions of biomass.

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This study investigates the evolution of fuel properties and aging of a series of bioslurry fuels prepared from fast pyrolysis bio-oil and biochar at different biochar loading levels (up to 20 wt %) for a storage period of 29 days. The results demonstrate that, at room temperature, the storage of bioslurry results in a reduction in the acidity [total acid number (TAN)], a reduction in the viscosity, and an increase in the water content of the bio-oil phase. In comparison to the blank bio-oil samples, the presence of biochar leads to more severe changes in the fuel properties of bioslurry. After 29 days of storage, the bioslurry fuels are still acidic. An increase in the biochar loading level further decreases the TAN and viscosity of bio-oil phases and increases the water content of bio-oil phases. The storage of bioslurry also results in undesired redistribution of alkali and alkaline earth metallic species between the biochar and bio-oil phase in bioslurry, via the leaching of these inorganic species from the biochar into the acidic bio-oil by two-step kinetics.

A drop-tube furnace with a novel double-tube configuration was successfully developed to directly determine char yields during the pyrolysis of a wide range of solid fuels (mallee wood; mallee leaf; rice husk; biosolid; and subbituminous, bituminous, and anthracite coal) at a gas temperature of 1573 K. The char yield from pyrolysis of mallee wood and mallee leaf is <5%, ∼13% for rice husk, ∼16% for biosolid, ∼45% for subbituminous and bituminous coal, and ∼75% for anthracite coal. The retentions of Na, K, Mg, and Ca in biomass chars are <50%. About 35% of Na and K and ∼66–85% of P and refractory species in biosolid are retained in the char. In contrast, the retentions of major inorganic species in coal chars are >85%. This study shows using total ash as ash tracer results in 45–220% overestimation of char yields for biomass fuels and 13–27% for coals due to partial evaporation of ash. Similarly, selecting Na and K results in overestimation of biomass char yields by at least 2.5 times and selecting P leads to overestimation of biomass char yields by at least 80% because of substantial release of these species during pyrolysis. Similarly, selecting Mg, Ca, Al, Fe, Ti, or Si as tracer also results in inaccurate estimation of char yields due to partial release of these elements during pyrolysis. It is noted that for Si, which is often used as a tracer, the overestimation of char yields is 9–16% for coals but can be substantial (17–50%) for the case of biomass samples because of the substantial Si release during the pyrolysis of biomass (especially mallee wood with ∼32% of Si released). Clearly, for the solid fuels studied, no single element can be reliably used as tracer for calculating char yield during pyrolysis at high temperature. The new experimental method developed in this study fills this critical gap and enables direct determination of char yield during solid fuel pyrolysis in drop-tube furnace at high temperature.

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