Thermochemical characterization of husk biomass resources with relevance to energy use

Biomass combustion or co-firing in modern coal power plants for steam/power generation is a carbon–neutral process, but it could pose acidic air pollutants and ash-related challenges like slagging and fouling. This paper aimed to evaluate the energetic potential of six special crop husk residues (i.e., chestnut husk, coffee bean husk, cocoa pod husk, coconut shell, peanut husk, and water caltrop husk) through their thermochemical characterization and thermogravimetric analysis (TGA) study. The thermochemical characterization included the proximate analysis, ultimate (elemental) analysis, calorific value (higher heating value, HHV), and elemental compositions of ash. The results showed that these residues have the potential for the biomass feedstocks in the boilers due to the large carbon contents (43.81–49.27 mass%), low ash contents (e.g., 0.27 mass% for coffee bean husk), and high HHVs (18.65–21.26 MJ kg−1). The HHVs were higher for the biomass husk residues with a low O/C ratio and low ash content. The chlorine, nitrogen, and sulfur contents, responsible for acidic air pollutants, were less than 1% except for chlorine in caltrop husk (1.59%). However, based on the relevant slagging/fouling indices using the data on the ash compounds and chlorine content, it was revealed that the ashes of chestnut husk, cocoa pod husk, and water caltrop husk had a high slagging tendency mainly due to the high contents of potassium and chlorine. In contrast, co-firing of biomass husks—coffee bean husk, peanut husk, and coconut shell, with coal could result in lower emissions of particulates, oxides of sulfur- and chlorine-bearing pollutants, and reduced slagging and fouling tendencies. In this regard, coconut shell is a very clean biomass fuel due to its excellent properties and large amounts.