Combustion of Switchgrass in Biomass Home Heating Systems: Emissions and Ash Behavior

Emissions and efficiency of three commercially available furnace types, horizontal feed, underfeed, and dropdown feed, were characterized using switchgrass pellets, and the results were compared to wood pellets. The efficiency of the furnaces was determined using a mass balance method. The gaseous emissions, including carbon monoxide (CO), nitrogen oxides (NO_x), and sulfur dioxide (SO₂) measurements, were conducted using the Testo 350 portable emission analyzer on all three furnaces at high- and low-load conditions. The particulate matter (PM) emissions were measured gravimetrically from filter measurements. There were no major differences in combustion efficiency noted among the three furnaces for both switchgrass and wood at high loads. PM emissions were higher for switchgrass compared to wood under both high- and low-load conditions likely as a result of the higher ash content. The CO emissions varied between furnace ranging from 21 to 53 g/kg for wood and from 10 to 28 g/kg for switchgrass under high load. Under low-load conditions, the emissions from wood were comparable to switchgrass. SO₂ emissions were similar to wood, except in furnace 2, where switchgrass exhibited much higher SO₂ concentrations. NO_x emissions were typically higher for switchgrass for all furnaces when compared to wood. PM and ash was analyzed for their chemical composition using inductively couple plasma-mass spectroscopy, atomic absorption spectroscopy, and X-ray fluorescence. It was found that most of the non-volatile elements and heavy metals remained in the bottom ash. Sulfate and potassium contributed to about 25-50% of the fine particle (PM_2.5) mass. More clinker formation was observed in the horizontal auger feed (furnace 1) as a result of alkali release and subsequent melting of potassium silicate. The furnace with the least operational problems during switchgrass combustion was further studied with a catalytic retrofit to reduce the emissions. After the retrofit, the CO emissions were reduced by about 80%. However, an increase in NO_x emissions was observed likely as a result of the formation of thermal NO_x. The catalysts also oxidized some of the soot particles (elemental carbon) and hydrocarbons (organic carbon and volatile organic compounds) formed from the incomplete combustion and slightly reduced the PM emissions.