Energy from biofuel

• Main Author: Khor, Adela
• Format: Dissertation
• Language: English
• Published: University of Sheffield 2006

Coal energized the 19"' century while oil worked for the 20"' century. The question that remains pending is the future energy source. With the expanding population and growing demands from industrialization of countries such as India and China, total reliance on fossil fuel is not sustainable. The urgent need for a green and sustainable fuel prompted research into energy crops as an alternative to fossil fuels. There is a need to identify a suitable biomass species, which can provide high-energy outputs in order to replace conventional fossil fuel. The type of biomass used would then depend on the energy conversion process and the form in which the energy is required. Theoretical and experimental studies are also required to investigate the relevant parameters for future process optimisation. In response to this demand, the first part of this PhD study focuses on examining the parameters affecting the behaviour of solid bed combustion in a fixed bed reactor. Extensive experimental investigation aimed at studying the behaviour of various biomass materials in fixed bed combustion was successfully completed. The parameters investigated in this experimental programme were the influence of primary air flow rates and particle sizes. Complementary conversion technology studied in this PhD is the solid bed gasification in a counter-current fixed bed reactor. The proposed gasifier is part of a two-stage integrated unit that ultimately aims to utilise biomass fuel such as wood chips and fuel pellets derived from agricultural waste. Experimental work was carried out to increase the understanding of the underlying principle of the gasification process. Parameters studied were the steam/air ratio, reactor temperature and particle size. The results obtained from the combustion tests showed that the burning rate, ignition velocity and flue gas composition were governed by the primary air flow. Single stage combustion occurred at higher air flow rates. It was also observed that the ignition front speed was inversely proportional to the bulk density. Larger particles have a lower ignition front speed and burning rate. The average burning rates of herbaceous fuels were 3-4 times slower compared to other biomass materials such as willow wood, miscanthus pellets and RDF waste. Channelling occurred around the side wall of the reactor for small miscanthus pellets and at several locations in the bed for larger particles (35mm cube). However, the observed channelling effect is severe in the test cases for uncut straw. The study of the gasification process showed that the hydrogen yield is influenced by the steam/air ratio, total flow rate of reactants, reactor temperature and particle size. The production of methane was negligible and no higher hydrocarbons were detected. A computational code known as the Fluid Dynamics of Incinerator Combustion code (FLIC) was used to interpret the experimental data. In this model, the initial solid waste undergoes step changes in the volume of its components consisting of moisture, volatile, fixed carbon, ash and internal pore space. The code was adapted to incorporate the steam-char reactions and gas phase reactions for the modelling of the gasification process. Comparison between the experimental results and model predictions showed good agreement. The combustion and gasification characteristics such as the burning rate, ignition rate, temperature profiles and gaseous emissions showed similar trends.