Small-Scale Batch-Fed Biomass Gasification and Combustion

Abstract

Small-scale solid biomass-fuelled cookstoves are highly important to satisfy the energy demand of nearly half the world’s population. With this as a main application, biomass provides the majority of renewable energy worldwide. However, traditional cookstoves, which emit vast amounts of emissions from incomplete combustion, are still the most widely used stoves. Furthermore, recently developed types of cookstoves may not be more efficient than traditional types of stoves, highlighting the necessity for more in-depth research into such combustion systems. Emissions from incomplete biomass combustion have been linked to a number of human health and environmental problems. Particulate matter (PM) emissions contribute substantially to respiratory diseases and black carbon, the second most influential anthropogenic climate forcing emission. These problems can be addressed through widespread use of more efficient and less polluting cookstoves. One type of cookstove, called a gasifier stove, has been identified as providing great potential to achieve the required drastic reduction in emissions from incomplete combustion. Gasifier type cookstoves utilise a staged combustion process. This allows the thermochemical conversion of the solid fuel to form gases, liquids (tars) and solid char, to be separated in time and location from the combustion of the released gases and tars. A much deeper understanding of the ongoing fundamental processes in gasifier stoves is required to enable emissions reductions in future designs, which is the aim of this research. In gasifier stoves, the separation of the thermochemical conversion process of the fuel and the combustion of the released products is achieved through a staged process. Air staging may be performed by providing a limited amount of primary air from below and secondary air downstream of the solid fuel bed. When lit from the top of the fuel bed this is called a reverse downdraft process. The air supply has a major influence on both the conversion and combustion processes, but the necessary amounts of, and relationships between, the air supply stages are not fully understood. At a low primary air supply, as is mostly the case in reverse downdraft gasifier cookstoves, the fuel consumption in the thermochemical conversion process scales linearly with the air supply. Increasing air flows lead to higher temperatures in the fuel bed, which in turn alter the product distribution across gases, tars and char. Combustible gases are the preferred product over tars, which may be toxic if released, and soot precursors if combusted. Thus, an intrinsic decomposition of tars may be beneficial. The product distribution can be influenced by specific mechanisms; for example tars can decompose to form gases at high temperatures and this conversion may be supported by the presence of char. In reverse downdraft gasifier stoves, the released tars and gases, together called producer gas, propagate through a layer of hot char where the tar decomposition may be enhanced, but this influence has not been established for small-scale systems. Furthermore, the product composition is dependent on the biomass feedstock and, in application, a wide variety of fuels are being used. The ash content in fuels has been found to be detrimental in many combustion applications and, although the ash and moisture contents can vary substantially between biomass fuels, when comparing the major elements in the composition (i.e. C, H and O) on a dry-ash-free basis, most biomass fuels are similar. The influence of using different fuels with varying ash contents on the conversion and combustion processes in gasifier stoves is largely unknown. In the present thesis, four studies form the main body and focus on various aspects of the combustion processes in reverse downdraft gasifier stoves. The first study addresses the influence of the draft type (natural or forced) and the relationship between the air supply stages on the overall combustion efficiency. The second and third studies focus on the batch-fed thermochemical biomass conversion process within the solid fuel bed. Specifically, the focus is the influence of an increasing fuel bed depth, thus an increasing char layer thickness to facilitate tar conversion, and the utilisation of various feedstocks, with varying ash contents. The fourth study investigates the secondary combustion process with a particular emphasis on the influence of changing conditions in thermochemical conversion, as provided by the previous two studies. Furthermore, two additional research articles, which were published prior to the official commencement of the degree, on the influence of the air supply, as well as two conference articles, are included in the Appendices. When investigating the draft type, it was confirmed that utilising forced air achieves a much higher combustion efficiency throughout the different combustion phases in gasifier cookstoves. A relationship of 1:4, of primary to secondary air, provided the best combustion conditions for the utilised wood chips. While focussing on the thermochemical conversion process of wood pellets, a fivefold increase of the primary air led to a rise from approximately 800°C to 1100°C, of the peak temperature, and from 33% to 73%, of the cold gas efficiency. A fourfold extension of the fuel bed depth, at one specific air supply rate, led to an increased production of permanent gases and a rise of the cold gas efficiency by approximately 10%. Similarly, when utilising higher ash content fuels, wheat straw, sheep manure and cow manure, a higher primary air flow was associated with greater process temperatures and a rise in the cold gas efficiency. Conversely, a threefold increase of the air supply led to double the PM2.5 combustion emissions from wood pellets and a more than tenfold increase from cow manure. Lower primary air supply rates were associated with lower emissions from incomplete combustion, especially when utilising high ash-content fuels. Foremost, the combined production of producer gas and char at low air supply rates, when utilising forced air with sufficient secondary air, has been identified to substantially reduce the emissions of incomplete combustion. The conversion of tars and the retention of ash constituents in the produced char contribute to pollutant emissions reduction. By utilising these mechanisms low value agricultural residues and even manures may be burned almost as cleanly as high value wood pellets. Careful reactor design based on the presented results and conclusions may facilitate the development of more efficient and more fuel flexible reverse downdraft gasifier stoves.