Analysis of the Utilisation of Herbaceous Biomass Streams for Small-Scale Combined Heat and Power Systems: A Comparative Study of UK and Pakistan

Abstract

The project scope combined the author`s interest, those of her industrial research partner Biogen Systems Ltd., and those of Professor John Brammer of the University of Chester, UK. The project`s main aim was to find a cost-effective, yet sustainable strategy to upgrade the quality of low-quality herbaceous biomass streams to feed Biogen`s BCHP units without any ash-related problems such as ash melting and clinker formation. This involved pursuing a theoretical understanding and lab-scale combustion experimental investigation of the ash melting characteristics of herbaceous biomass streams of significant interest (grass cuttings, miscanthus, corn cobs, and mango stones) with different additive streams in two country locations which reflect the affiliations of the author, and which are quite different: namely Pakistan and the UK. SEM-EDS and XRD techniques were used with ternary-phase diagrams for determination of elemental and crystalline phase composition analysis of combusted ash residues. Investigations revealed that the formation of low-melting eutectic mixtures of phosphates and silicates is the primary underlying reason for ash melting of non-woody biomass streams which can be avoided by the addition of anti-sintering additives. Furthermore, eggshells can successfully replace Ca-based commercially available additives when in raw and calcined form. Another main objective of this work was to develop an Excel-base financial business model to present a framework that would improve the profitability of Biogen`s BCHP units’ deployment in the UK and Pakistan. For this, different scenarios were studied, and the effects of main logistic variables were quantified on the profitability of Biogen`s units. Results of the financial model indicate that for the UK-based operation of Biogen`s BCHP units, wood chips would serve as the most economically beneficial feedstock, followed by miscanthus pellets, while for Pakistan, waste grass pellets would be the most financially suitable feedstock, followed by corn stover pellets. Food processing companies in Pakistan can generate electricity that is cheaper than the grid-sourced power by Biogen`s E3 BCHP units by utilising on-site available free-cost fruit waste (such as corn cobs and mango waste). Accordingly, the potential contribution of this research is the elimination of the technoeconomic barriers faced by the mass-scale deployment of high-temperature biomass-fuelled thermochemical systems by identification of eggshells as a potential anti-sintering additive and investigation of the effect of cost estimates for different feedstocks for E3 operation. There is a strong need for further research to be done focusing on the gaseous emissions encompassing the utilisation of grass and its mixtures with additives and other biofuels at high-temperature thermochemical systems.