Small scale biomass burning stoves and boilers are growing in popularity in the UK and abroad, owing in part to renewable heat incentives and policies. However, combustion in domestic scale appliances is often inefficient and uncontrolled in comparison to larger systems, leading to high emissions factors of gaseous pollutants such as CO, NOx and PAH, as well as fine particulate matter (PM). Evidence is presented from 105 source apportionment studies from 31 developed countries showing that the impact of residential solid fuel (RSF) combustion on air quality is more wide spread that previously thought. Wood burning contributes to up to 95% of wintertime ambient PM in some rural communities in New Zealand, which is used as a case study throughout this work. Modelling work has shown that emissions from heating stoves may be underestimated in global climate models (GCMs) and UK residential wood consumption is forecast to increase by a factor of 14 between 1990 and 2030. By 2030, annual emissions of black carbon (BC) from UK wood stoves and fireplaces are predicted to exceed 3000 tonnes which is higher than the traffic sector. BC is the most important component of RSF radiative forcing, accounting for over 77% total warming effect. Model inventory and literature emission factors (EFs) for over 150 pollutants have been compared and contrasted, and compiled into a new RSF emissions inventory.
Results are presented from experimental work on the emissions testing of a 6 kWth multi-fuel stove and three high quality cook stoves, burning a range of over 25 conventional and novel fuels including wood, coal, agricultural residues and torrefied wood briquettes. Despite a large resource of agricultural residues being available with lower EFs than open burning, their suitability as a residential solid fuel is uncertain. For example, straw briquettes had a measured density less than half that of wood logs and reed briquettes showed evidence of ash melting in the stove bed due to a high sodium, silica and chlorine content. It was found that PM and CO emissions were correlated to the content and composition of volatile matter within the fuel and NOx is linearly dependent on fuel nitrogen content. Mean whole cycle PM EFs ranged from 2.1 g kg-1 for wood logs to 4.2 g kg-1 for coal and 0.5 g k-1 for smokeless fuel. Torrefaction of wood has the potential to significantly reduce emissions, with PM EFs 49% lower than wood logs. However, emissions from all fuels were highly dependent on the duration of the flaming phase of combustion, during which EFs may be a factor of 5-9 higher than the smouldering phase. Heat treatment such as torrefaction removes 10-15% of volatiles, shortening the flaming phase and removing key species involved in the chemical soot formation pathways, which are discussed in detail.
The physical and optical properties of collected particulate samples collected were also examined using electron microscopy and spectroscopy, which are useful for GCMs. Flaming phase particles had a high average EC/TC ratio (>0.9), a high carbon:oxygen ratio (93:5.4) and an Ångström Absorption Exponent (AAE) near 1 (0.9-1.2). After emission, it was found that particles undergo a significant increase in branching and oxygenation (C:O 88:10). The morphology of particles was also found to change following the injection of plasma into the flue, which is evaluated as a promising retrofit abatement technology.
