Modelling higher heating value of different separated fractions from municipal and construction and demolition wastes

Abstract

Higher heating value (HHV) is an important property of biomass and wastes used to evaluate their potential conversion to useful thermal or electric energy. Because the measurement of this property requires expensive resources and is somewhat time-consuming, many works focused their attention on the determination of mathematic models relating the HHV with the composition of lignocellulosic biomass or other fuel materials, such as their ultimate and proximate analysis. These models can supply appropriate estimates of HHV but only for analogous materials, so they should not be used to compare samples with marked differences in composition or physical and chemical properties. In this work, 9 different separated fractions of municipal and construction and demolition wastes (wood, paper/card, plastics, sewage sludge and mixtures among them) were used to deduce a mathematical expression relating HHV with their contents of carbon, hydrogen, oxygen, nitrogen, sulphur and ash. For this purpose, HHV's, proximate and ultimate analysis were experimentally obtained and the results used to create three different expressions applying linear regression methods. The best expression was selected and validated by comparing deviations among the calculated results and those retrieved from the literature and from experimental measurements regarding different wastes. It was concluded that the best expression was HHV (MJ/kg db) = 0.3845×C+0.8831×H- 29.1217×S-0.0630×O-1.0063×N+0.3888×ASH-0.2546 (with C, H, S, O, N and ASH in wt% db, considering atomic ratios O/C and H/C within 0.0O/C1.2 and 0.1H/C0.2), giving an average absolute error of 8.5 % and an average bias error of -1.6 %. However, appreciable deviations may be found when estimating the HHV of polyurethane, paper/card, mixtures of paper/plastic and sewage sludge and thus the application of the expression for these materials is questionable.