Effects of operational mode on particle size and number emissions from a biomass gasifier cookstove

Interest in the size distribution of particles emitted from biomass cookstoves stems from the hypothesis that exposure to ultrafine particles is more detrimental to human health than exposure to accumulation mode or other size regimes. Previous studies have reported that gasifier cookstoves emit smaller particles than other cookstove designs under steady operating conditions. In the present study, the number size distribution of particles emitted from a forced-air gasifier cookstove was measured at 1 Hz as the stove transitioned between several steady and transient operating modes. During normal operation, when the stove functioned as a top-lit updraft gasifier, the distribution was bimodal, with peaks at 10 nm and 40 nm, when a pot of water was on the stove. The distribution became unimodal with a peak at 10 nm when the pot was removed. Once the fuel bed had completely gasified and the secondary flame extinguished, the concentration of particles increased and the peak in number concentration shifted to approximately 80 nm. After refueling, when the stove operated as a conventional updraft gasifier, the peak in number concentration decreased to 10 nm. When the secondary flame extinguished a second time, the peak in number concentration increased to approximately 100 nm before decreasing to 20 nm during the char burn-out phase. These results demonstrate that changes in operational mode influence the combustion process and produce distinct changes in the size distribution and rate of particle emissions. Copyright © 2018 American Association for Aerosol Research

生物质炊具排放颗粒物的粒径分布之所以受到关注，源于这样一项假说：相较于积聚模态（accumulation mode）或其他粒径区间的颗粒物暴露，超细颗粒物（ultrafine particles）暴露对人体健康的危害更为显著。既往研究表明，在稳定运行工况下，气化型炊具相较于其他结构的炊具，排放的颗粒物粒径更小。本研究中，我们以1赫兹的采样频率，对一台强制送风式气化炊具（forced-air gasifier cookstove）在多种稳态与瞬态运行模式切换过程中排放颗粒物的数量粒径分布进行了测量。在正常运行工况下，当该炊具采用顶燃上行气化模式（top-lit updraft gasifier）且锅具盛水置于炉上时，粒径分布呈双峰型，峰值分别位于10纳米与40纳米处。当移除锅具后，粒径分布变为单峰型，峰值位于10纳米处。当燃料床完全气化且二次火焰熄灭后，颗粒物浓度升高，数量浓度峰值向约80纳米处偏移。重新加料后，当炊具以传统上行气化模式（conventional updraft gasifier）运行时，数量浓度峰值回落至10纳米处。当二次火焰第二次熄灭后，数量浓度峰值升至约100纳米，随后在焦炭燃尽阶段（char burn-out phase）降至20纳米处。上述结果表明，运行模式的变化会影响燃烧过程，并使颗粒物排放的粒径分布与排放速率出现显著差异。© 2018 美国气溶胶研究协会（American Association for Aerosol Research）