Forest residues to energy : local air quality, health risks and greenhouse gas emissions

Local impact assessment of biomass-based district energy systems (DES) is still in its infancy. There has been a lack of appropriate assessment methods for parameters with broad variability on local scale, and lack of DES impact assessments. This study investigates how would: 1) the inclusion of site-specific terrain, land use and microclimatic characteristics, variable population density and breathing rates affect accuracy of assessments on local air quality and health; 2) an incremental increase of PM₂.₅, NOx and CO concentrations from DES contribute to ambient air quality and population exposure, 3) life-cycle GHG emissions from DES contribute to global warming, and 4) the introduction of biomass affect economics of DES compared to the fossil fuel-based DES. Utilizing dispersion modeling the study established an assessment approach which confirmed the need for inclusion of population dynamics, site-specific microclimatic characteristics, and diurnal circulation patterns. Otherwise, health risks could potentially be underestimated by more than 20%. Applying this approach on a small-scale biomass gasification plant (BRDF), the study concluded that the health impact was the highest for NO₂ (677 DALY) when all energy was produced by biomass, and for PM₂.₅ (64 DALY) if all energy was produced by natural gas. Complete replacement of Power House (PH) by one biomass plant can result in almost 28% higher impact compared to 513 DALY when both BRDF and PH are operational. NO₂ emissions from the BRDF exceeded the air quality objectives (BCAQO) in all seasons except during summer. Although overall incremental contribution of PM₂.₅ is at least one order of magnitude lower than BCAQO, the maximum PM₂.₅ emissions from the PH could adversely add to the already high background concentrations. Meeting energy demand solely by an expanded full-scale BRDF from locally supplied biomass reduces GHG annually to 3.81E+06 kg CO₂eq from 7.08E+07 kg CO₂eq when energy was produced solely by the current PH. An introduction of biomass increased total costs by $19 M compared to existing PH, but saved $8.4 M in carbon tax over plants’ lifetime. $3.3 M of societal damages could be avoided over plants’ lifetime in case of combined use of natural gas and biomass.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat