As the earth transitions to a low carbon sustainable future, the diversity of biomass fuels must be expanded to accommodate the growing energy demands. This can have advantages such as encouraging the use of waste feedstocks to capitalise on economic gains as well as disadvantages such as the associated environmental and operational problems from biomass high in inorganics and minerals. Globally the heating sector is the largest sector of the energy industry accounting for 50% of energy consumption [IRENA, 2018]. Of this 14% comes from traditional biomass used in heating stoves. This an area of increasing interest since these systems are having a detrimental effect to air quality from high emissions of NOx, SO2, unburnt hydrocarbons and particulate matter.
In this thesis the aim was to understand the relationship between the fuel properties and emissions from combustion on a domestic stove. This included analysing the impacts of pre-treatment on the performance and emissions. Traditional wood fuels, willow and spruce logs, were used as benchmarks and compared to their torrefied counterparts. The results showed that torrefied fuels increased the emissions of CO2 (spruce 65-90 and willow 67 to 78 kg GJ-1) but reduced the emissions of CH4 (spruce 0.37 to 0.18 and willow 0.18 to 0.04 kg GJ-1). Reductions in NOx from torrefied fuels was the result of a shift in N partitioning and retaining more fuel-N in the char which was released as N2 from reducing conditions during char burnout. Similarly, sulphur retention in the ash increased because of the increased Ca/S ratio of torrefied fuels, thereby reducing SO2 emissions. Particulate matter emissions were also reduced from using torrefied fuels. The torrefaction process reduced the emissions of soot forming volatiles such as eugenol and vanillin which can contribute to soot formation by both the hydrogen abstraction carbon addition and cyclopentadiene methods.
Waste spent coffee grounds, bracken and agricultural residues were analysed in comparison to wood logs and briquettes as potential novel fuels for the domestic market. The very fine particle size and high calorific value (21.1 MJ kg-1) of the spent coffee resulted in high flue gas temperatures (peak temperature 600C). However, the high fuel-N content (1.84 wt.% db) resulted in excessive emissions of NOx (190 g GJ-1) which was more analogous with coal and peat than biomass. Bracken is currently a large management problem globally as it is so dominant and fast growing, the most used management technique involves harvesting and burning it in large open fires. Commercial bracken briquettes when combusted were difficult to ignite because of the density of the briquettes (1250 kg m-3). These were broken down into four evenly sized segments and it was apparent the size, shape and density of the briquettes was preventing efficient combustion (mass conversion increased by 20%). By reducing the size of the briquettes, and increasing the surface area to volume ratio, the emissions of CO decreased from 180 to 140 g kg-1 of fuel and the total organic carbon emissions from 1.3 to 0.8 g kg-1 of fuel. The SO2 (1.7 g kg-1) and HCl (0.6 g kg-1) emissions from bracken briquettes were much higher than compared to barley straw (0.8 and 0.3 g kg-1), wheat straw (0.5 and 0.2 g kg-1), miscanthus (0.8 and 0.3 g kg-1) and wood briquettes (0.3 and 0.06 g kg-1). This was because of the high S (0.2 wt.% db) and Cl (0.14 wt.% db) contents of the fuel.
Because of the strong correlation between the fuel mineral contents and the emissions, SRC willow grown on contaminated land was pre-treated by washing to investigate the removal efficiency of problematic species and the impacts on the emissions. The ash content of the pre-treated willow reduced by 27%; this was lower than seen in previous work, however the method used was more applicable to industry methods of pre-treatment. High removal efficiencies of Pb (69%), S (55%) and Cu (47.5%) were observed and this came from a combination of solid mineral removal from soil and bark as well as some leaching. When combusted the washed willow had reduced emissions of CO (5.2 reduced to 2.45 kg GJ-1), THC (0.22 reduced to 0.125 kg GJ-1) and PM (0.12 reduced to 0.063 kg GJ-1). However even though the fuel-N content decreased, the NOx emissions were 25% higher for the washed willow, this was mostly likely due to the reduced Na content which has been shown to catalyse NOx reduction reactions during char combustion.
The overall outcome of this thesis is a stronger understanding of the chemical and physical properties of fuels that influence the emissions from combustion on stove systems. Several useful correlations have been identified between the composition analysis and the emissions data that include the majority of fuels used in this thesis. These correlations are useful in identifying the suitability of novel fuels and also in identifying the advantages of pre-treatment for industry and the potential expansion of their fuel inventories.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)