Combustion characteristics of biomass briquettes

Nearly half the worlds population is dependent on wood as their primary energy source. Therefore with deforestation becoming increasingly prevalent in many regions of the developing world, there is an urgent need to improve combustion efficiency of stoves or to find alternative fuels. Densification of loose biomass residues into briquettes is a means of upgrading the feedstock material. Briquettes are easier to store, more convenient to use and burn at a more steady and controlled rate than loose biomass. This investigation focuses on understanding some of their combustion characteristics and the relationship to the briquette design and manufacture process. The social context of the work is presented through an informal case study considering the potential of briquetting in Ghana. The initial experimental work focuses on developing a process to manufacture newspaper briquettes of consistent quality at low-pressures using a wet technique, and a method to burn them in a controlled way is suggested. These techniques were used to carry out a study on rectangular slab-shaped briquettes, looking at the effect of process variables (density, moisture content and size) on briquette burn-rate. An analytical expression for the normalised burn-rate (NBR) of a briquette in free-air, in terms of these of these variables, was found by numerical fitting. The effect of shape on combustion was also experimentally investigated using cylindrical briquettes with a central hole (holey briquettes) burning in free-air and an analytical expression was derived for their burn rate. The NBR behaviour of sawdust briquettes, rapeseed oil residue briquettes as well as slabs of pine wood was then studied. Rapeseed oil residue has a very different calorific value from that of newspaper, and a method was suggested for predicting the difference in rate as a function of difference in calorific value. In the second phase of the work, a numerical model of pyrolysis was developed. The model assumed that heat transfer through the fuel limited the rate of pyrolysis. The thermal parameters (thermal conductivity, heat capacity and thermal diffusivity) were estimated using a heat probe method, and the kinetic parameters found by numerical optimization. The model was shown to predict, for newspaper briquettes, the experimentally observed size dependent behaviour of the normalised burn rate, and the trend observed for the effect of changes in density. The model was applied to predict the effect of changes in a briquettes thermal parameters on burn rate, and an analytical expression found by numerical fitting. This provides a means of estimating relative changes in burn rate due to changes in fuel properties, and forms the basis for a pyrolysis sub-model for use in stove optimisation. Such a model has the advantage of being able to vary key, easily quantifiable and easily controlled solid biomass briquette properties relative to the behaviour and properties of a well understood fuel such as wood. In the final part of the study, some of the limitations of the numerical model are explored by completing a sensitivity study investigating the relative effect of some of the key assumptions made.