Emission Of Mercury From Biomass Fuels Burning In Rural China

Because of the high toxicity, mercury could cause severe pollution and damage towater, soil and organisms when its concentration is high. Atmospheric mercuryoriginated mainly from anthropogenic sources, one of which is the burning of biomassfuel. About500million tons of biomass fuels are consumed in rural China each year,which produce6¹0tons of mercury. The impact of Hg emission from burning ofbiomass fuels on atmospheric mercury concentrations is significant. Biomass pellet isa new type of clean and efficient biomass fuel. It spreads rapidly due to its lowmoisture, convenience of usage and low pollution. Earlier studies showed thatbiomass pellet produced1/3CO₂and1/30CO compared to that of the traditionalbiomass fuels when a unit of effective heat is generated. Research on pollutionemission characteristics of biomass pellet will be helpful to the improvement andpromotion of the new fuel.Direct combustion is the most important way for biomass to release its Hg toatmosphere, and the emission factor method is currently the international mainstreammethod to estimate the atmospheric mercury emissions. However, the emission factorwhich was used in China was mainly from foreign studies or the mercury content inbiomass. In this study, a traditional brick stove and a biomass pellet stove were builtand installed in Changping County to conduct the combustion experiment. OntarioHydro method and carbon balance method were used to measure the emission factorsof total mercury and different forms of mercury (Hg²⁺, Hg⁰and Hg_p). The moisture,Modified combustion efficiency （MCE） and oxygen supply et al. which impact CO,CO₂and Hg emission were analyzed, and the mercury emissions inventory of biomassfuels combustion in rural China was created.CO₂and CO are important parameters for analyzing mercury emission. The study on CO₂and CO could help improving combustion efficiency of biomass fuels, andcould provide references for mercury emission estimation. Results showed that theEFs of CO₂and CO were1649.4g/kg and47.8g/kg for firewood, and1503.2g/kg and52.0g/kg for crop residues, respectively. For pellet biomass fuels, the EFs of CO₂andCO were1708.0g/kg and4.4g/kg for pellet pine, and1552.1g/kg and17.9g/kg forpellet corn, respectively. Regression models can be applied to predict the EFs. Theagreement between the measured and calculated EFs was evaluated using the RSD foreach regression result in these regression models. For the CO₂EFs, the RSD was0.6±0.5%for uncompressed fuels and0.04±0.02%for pellet fuels. For the CO EFs,the RSD was9.3±7.9%and3.3±2.5%for uncompressed and pellet fuels, respectively.There are mainly three kinds of Hg (Hg²⁺, Hg⁰and Hg_p) in the flue gas of thebiomass fuels combustion. Results showed that the total mercury EFs of17kinds offirewood combustion were9.27±8.41ng/g, and the proportion of Hg²⁺, Hg⁰and Hg_pwas0.05:0.83:0.12. The mercury EFs of eight kinds of crop residues combustion were6.54±3.47ng/g, and the proportion of Hg²⁺, Hg⁰and Hg_pwas0.05:0.70:0.25. Hg EFsfor firewood and crop combustion are lower than previous studies by20⁴0ng/g. Thetotal mercury EFs of two biomass pellet combustion were11.00±4.68ng/g and theproportion of Hg²⁺, Hg⁰and Hg_pwas0.09:0.90:0.01.The study compared the impacts of moisture and oxygen conditions. In the rangeof measured moisture, when the moisture increased, the total mercury EFs of biomassfuel combustion showed a significantly decreasing trend. However, proportion of Hg_pin total mercury increased as the moisture increased significantly. In certain oxygensupply range, the total mercury EFs of crop combustion showed an increasing trend asthe oxygen supply increased.Based on the mass of crop residues and firewood consumed as residential energysources in China and Hg EFs obtained from controlled tests, the inventories of Hgemissions for biomass burning in rural areas were calculated for each province ofChina. In2007, the total mercury emissions to the atmosphere of biomass fuelcombustion in rural China was5.0tons, of which firewood and crop residuescombustion accounted for2.2tons and2.8tons respectively. Overall, the total mercury emissions of biomass fuels in rural China showed a clear upward trend from2000to2004, and it began to keep stable and dropped slightly from2004to2007.The main reason is the use of alternative energy sources and the migration of ruralpopulation to urban areas.