Article pubs.acs.org/EF
Emission of Speciated Mercury from Residential Biomass Fuel Combustion in China Wei Zhang,† Wen Wei,‡ Dan Hu,‡ Yan Zhu,† and Xuejun Wang‡,* †
School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China Ministry of Education Laboratory of Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
‡
S Supporting Information *
ABSTRACT: Among various sources, mercury emissions from biomass fuel combustion have received growing attention. Mercury emission from biomass fuels can be estimated on the basis of the combustion amount and the emission factors (EFs). Although mercury emissions from biomass fuel combustion occur mostly in developing countries, most EFs have been measured in developed countries, leading to bias in mercury emission inventories. In this study, mercury EFs for 25 species of fuelwood, eight species of crop residues, and two types of biomass pellets were determined according to the real-life practice of residential burning. Results showed that the EFs ranges were 0.65−28.44 ng g−1 for fuelwood, 3.02−12.05 ng g−1 for crop residues, and 5.22−8.10 ng g−1 for biomass pellets. Hg0 is the dominant form of mercury emitted from biomass fuel combustion. The proportion of Hg0, Hg2+, and Hgp was 76 ± 17, 6 ± 5, and 18 ± 14% for fuelwood; 73 ± 11, 4 ± 5, and 23 ± 13% for crop residues; and 97 ± 1, 1 ± 0.2, and 2 ± 0.7% for biomass pellets, respectively. Biomass pellets can reduce mercury emissions compared with the uncompressed raw materials. On the basis of the measured EFs, inventories of mercury emission from biomass fuel combustion in rural China from 2000−2007 were estimated. The annual mercury emission ranged from 1.94 to 5.07 Mg, of which crop residues and fuelwood accounted for 62 and 38%, respectively.
1. INTRODUCTION Mercury is a hazardous environmental pollutant because of its potentially significant effects on human health. For example, the health effects for children with mercury exposure include renal toxicity, skin rashes, hypertension, and pulmonary toxicity. The U.S. ATSDR Minimal Risk Level (MRL) for chronic inhalation exposure is 0.0002 mg/kg/day, based on neurological effects. Mercury emissions originate from both anthropogenic and natural sources. Among various sources, biomass combustion has received growing attention.1,2 Studies have shown that biomass combustion involving crop residues and fuelwood is a significant anthropogenic source, especially in winter due to combustion for heat.3,4 Recently, Friedli et al. estimated that the global mercury emission from biomass combustion was averaged to be 675 ± 240 Mg per year (for the period of 1997− 2006), and the highest contributing regions were equatorial Asia (28%) and boreal Asia (15%).5 Biomass fuels, including crop residues and fuelwood, are expected to be an important energy resource as a sustainable alternative to fossil fuels.6−8 The International Energy Agency (IEA) reported that 39% of the global population relies on biomass fuels for cooking, and large amounts of biomass fuels are consumed in developing countries.9 China is a large agricultural country and has rich biomass resources. On average, the annual production of crop residues was approximately 700 million Mg in China.10 Crop residues and fuelwood are widely used for cooking and house heating, especially in rural areas, resulting in emissions of many air pollutants and severe indoor air pollution in households. Demirbas reported that biomass fuels account for 23.5% of China’s final energy consumption.11 Wang and Feng pointed © 2013 American Chemical Society
out that biomass fuels contribute 32.7% of the total energy consumption in rural areas in China.12 Biomass pellets, as a new type of compressed, homogenized, and dried biomass fuel with several advantages including handling, storage, and combustion efficiency, are emerging as a cleaner alternative to traditional biomass fuels.13 The production and use of biomass pellets have been extensive during the past decade. For example, the consumption of biomass pellets in Sweden increased from approximately 0.5 million Mg yr−1 in 1997 to 1.9 million Mg yr−1 in 2009.14 In China, the consumption of biomass pellets has rapidly increased from 0.2 million Mg in 2008 to 2 million Mg in 2010 and is projected to reach 20 million Mg in 2015.10 Mercury emissions from biomass fuels can be estimated based on the amount of biomass fuels burnt and the emission factors (EFs). Although mercury emissions from biomass fuels occur mostly in developing countries, most EFs have been measured in developed countries. When the emission inventories for developing countries are calculated using the EFs measured in developed countries, bias is inevitable due to differences in fuel properties and combustion conditions. In previous studies, great efforts have been made to estimate mercury emissions from biomass fuel combustion in China.1,15 However, because the locally measured EFs were not available, researchers usually adopted EFs measured in Europe or North America to estimate the mercury emission from biomass fuel combustion in China,15 or mercury concentrations in biomass were used as a surrogate for EFs.1 The lack of EFs for biomass Received: August 6, 2013 Revised: September 28, 2013 Published: September 30, 2013 6792
dx.doi.org/10.1021/ef401564r | Energy Fuels 2013, 27, 6792−6800
Energy & Fuels
Article
the total burning cycle lasted 47−54 min, and the average burning rate was 22 g min−1. For cornstalk pellets, the total burning cycle lasted 31−37 min with an average burning rate of 14 g min−1. The combustion experiments were repeated three times for each type of fuel. 2.2. Sampling and Lab Analysis. Sampling work was conducted in the mixing chamber. The sampling period covered the whole burning cycle, including the flaming (obvious fire) and smoldering phases (without obvious fire). The sampling started after the initial ignition and stopped when the online measured CO and CO2 concentrations dropped to the background levels. Particulate-bound mercury (Hgp) was collected on a 47-mm-diameter glass fiber filter (as total suspended particles) in the front of the sampling train, and gaseous mercury (Hg2+, Hg0) was collected in seven impingers that connected in a series at a rate of 1.5 L min−1. Hg2+ was collected in the first three impingers containing KCl solution, and Hg0 was collected in subsequent impingers (one impinger containing HNO3−H2O2 solution and three impingers containing KMnO4−H2SO4 solution). The final impinger was filled with silica gel to remove moisture from the gas. The sampling method has been widely applied in previous studies.23,24 The equipment was connected by polyethylene pipes, and the interface was sealed with Teflon tape. The particle-bound mercury and total mercury in fuel and ash samples were measured using an RA-915 mercury analyzer coupled with a PYRO-915 attachment (Lumex Ltd., St. Petersburg, Russia). All filters were baked at 450 °C for 6 h and equilibrated in a desiccator for 24 h prior to use. The samples were thermally decomposed in an atomizer chamber at 800 °C, transforming all forms of Hg into Hg0. Hg0 was then detected by the RA−915 analyzer. The RA−915 employs Zeeman atomic absorption spectroscopy using the high frequency modulation of light polarization (ZAAS-HFM). The combined use of the RA-915 with the PYRO-915 accessory for the measurement of mercury concentrations in solid samples has been well-documented.25−27 Instrument calibrations and measurements were conducted in accordance with EPA Method 7473. The detection limit was 0.45 ng g−1 for a 0.1000 g sample. Concentrations of gaseous mercury (trapped by the absorption solutions) were determined using a Lumex RA-915 coupled with an RP-91 attachment. Analysis was based on the cold vapor atomic absorption spectrometry (CVAAS) method and the absorption of 253.7 nm resonance radiation by mercury atoms. Researchers have successfully used the RA-915 analyzer with the RP-91 attachment to measure mercury concentrations in liquid samples.28,29 The detection limit was 0.11 ng L−1 for a 5 mL sample. Carbon dioxide and carbon monoxide were measured every 2 s over the whole combustion period using an online detector equipped with a nondispersive infrared sensor (GXH-3051, Junfang Technical Institute, China). The equipment was calibrated using a span gas (CO, 1.00%; CO2, 5.00%) before each experiment. The detection limit was 0.1 ppm, and the repeatability was
