Optimal evaluation of crop residues for gasification in Ghana using integrated multi-criterial decision making techniques

Abstract

Lack of sustainable feedstock quantity and quality has been reported to be one of the major challenges confronting operations of gasifier plants in Ghana. In this paper, TOPSIS (Technique for order of Preference by Similarity to Ideal Solution) COPRAS (Complex Proportional Assessment) and VIKOR (Multi-criteria Optimisation and Compromise Solution) were used to select optimal crop residues for sustainable gasification in Ghana among thirteen residue types. The Analytical Hierarchy Process (AHP) was used as weighting criteria for the three Multi-Criteria Decision-Making (MCDM) techniques. Thirteen criteria based on particle size, proximate, ultimate analysis, calorific values, and quantity of residues were used. Five scenarios were considered; ranking of individual feedstock, consideration of the combination of the feedstock grouped under four categories. The ultimate and proximate analysis of the various crop residues as determined were generally within recommended levels as reported in the literature. Cashew shells and cassava peels have the highest heating value and annual technical residue potential of 23.4 MJ/kg and 880,750 tonnes respectively. Ranking of the individual crop residues confirmed cassava peels as the best alternatives of all the three MCDM techniques. The average rankings of the alternatives from the three MCDM in the order of best to worst are cassava peels, maize stalk, cocoa pod husk, maize cobs, rice straw, shea nut husk, groundnut husk, Palm kernel shells, EFB, rice husk, coconut husk/shells, cashew shells, and shea nut cake. Rankings of the alternatives under the four categories considered showed that feedstock blends containing four residues or more offer better opportunities for sustainable gasification than individual feedstock. Statistical analysis shows that there is a very strong, positive correlation between VIKOR, COPRAS, and TOPSIS. The strongest correlation is between VIKOR and TOPSIS with spearman's rank correlation index of 0.98. The analysis was sensitive to the weight of the strategy of group utility, recoverability ratio, residue-to-product ratio and annual crop production figures. It is recommended that optimal gasifier design and operational conditions taken into consideration the various feedstocks and their combination as determined from this study must be studied.