Improvement of fuel qualities of solid fuel biochars by washing treatment

Abstract

Raw biomass washing prior to pyrolysis treatment has been employed to mitigate the ash-related problems encountered during resultant biochar combustion. However, wide-spread application of this approach is limited by high energy consumption and low washing efficiency. In the present study, the biochar washing instead of raw biomass washing was attempted to overcome aforementioned problems, and fuel qualities of washed biochars were investigated for the first time. The results showed that major ash forming metals can be efficiently removed by all tested washing agents (de-ionized water (DW), acetic acid (AA) and citric acid (CA)), especially for acid washing agents AA and CA. As a consequence, the ash content of the biochars decreased, and the slagging and fouling issues were dramatically mitigated. Due to metal removal, the washed biochars exhibited improved combustion properties, especially for the biochars derived from agricultural biomass. In addition, noticeable decreases in nitrogen and sulfur contents were observed following washing treatment, suggesting the additional benefit of reducing emissions of nitrogen and sulfur pollutants during washed biochar combustion.Compared to raw biomass washing prior to pyrolysis, the significantly increased washing efficiency and fuel quality and decreased hydrophilicity of the biochars indicate that pyrolysis combined with subsequent biochar washing is more suitable to produce solid fuel biochars with high fuel quality from different source of waste biomass.