Reducing Ultrafine Particle Emissions Using Air Injection in Wood-Burning Cookstoves

Abstract

In order to address the health risks and climate impacts associated with pollution from cooking on biomass fires, researchers have focused on designing new cookstoves that improve cooking performance and reduce harmful emissions, specifically particulate matter (PM). One method for improving cooking performance and reducing emissions is using air injection to increase turbulence of unburned gases in the combustion zone. Although air injection reduces total PM mass emissions, the effect on PM size-distribution and number concentration has not been thoroughly investigated. Using two new wood-burning cookstove designs from Lawrence Berkeley National Laboratory, this research explores the effect of air injection on cooking performance, PM and gaseous emissions, and PM size distribution and number concentration.Both cookstoves were created using the Berkeley-Darfur Stove as the base platform to isolate the effects of air injection. The thermal performance, gaseous emissions, PM mass emissions, and particle concentrations (ranging from 5 nm to 10 µm in diameter) of the cookstoves were measured during multiple high-power cooking tests. The results indicate that air injection improves cookstove performance and reduces total PM mass but increases total ultrafine (less than 100 nm in diameter) PM concentration over the course of high-power cooking.

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

“…However, the fraction of emitted particles smaller than 30 nm was higher for the forced-air gasifier stoves and the naturaldraft gasifier fueled with pellets compared to the other biomass stoves (Shen et al 2017). Rapp et al (2016) used a Fast Mobility Particle Sizer (FMPS) to measure the size distribution of particles emitted from a three-stone fire, an improved side-feed wood stove, and two side-feed stoves with forced-air injection that were all operating at high power to boil water. The air-injection stoves emitted 48% and 69% less PM 2.5 mass than the conventional side-feed stove (g PM 2.5 per kg fuel burned), but emitted 1.5 and 2.2 times as many 10 nm particles (Rapp et al 2016).…”

“…Exclusive reliance on mass-based targets for particulate emissions from other combustion sources, such as diesel engines, has been criticized (Kittelson 1998), in part because ultrafine particles (i.e., particles with d < 100 nm), which contribute very little mass to PM 2.5 , may be more detrimental to human health than larger particles (Oberd€ orster 2000; Health Effects Institute 2013). Similarly, some researchers have expressed concern that efforts to reduce the health risks associated with HAP may be hindered if "improved" biomass cookstoves are introduced that emit lower levels of PM mass but higher levels of ultrafine particles (Just et al 2013;Rapp et al 2016).…”

“…The number size distribution of ultrafine particles emitted from different cookstoves has been reported previously (Zhang et al 2012;Hawley and Volckens 2013;Just et al 2013;Tiwari et al 2014;Rapp et al 2016;Shen et al 2017). Hawley and Volckens (2013) and Just et al (2013) both used a Scanning Mobility Particle Sizer (SMPS) to measure the aerosol size distribution emitted from a three-stone fire, a side-feed wood cookstove, and a forced-air gasifier cookstove operating under steady state.…”

“…Rapp et al (2016) used a Fast Mobility Particle Sizer (FMPS) to measure the size distribution of particles emitted from a three-stone fire, an improved side-feed wood stove, and two side-feed stoves with forced-air injection that were all operating at high power to boil water. The air-injection stoves emitted 48% and 69% less PM 2.5 mass than the conventional side-feed stove (g PM 2.5 per kg fuel burned), but emitted 1.5 and 2.2 times as many 10 nm particles (Rapp et al 2016). These results are concerning because particles with mobility diameters between approximately 10 and 30 nm deposit in the alveolar region of the lungs with maximum efficiency (Hinds 1999).…”

“…Zhang et al (2012) measured the size distribution of particles emitted from a traditional wood-burning cookstove in the field and identified changes in the size distribution with time. The laboratory studies described previously measured the size distribution of particles emitted from stoves that were operating steadily in a single mode (Hawley and Volckens 2013;Just et al 2013) or primarily reported emissions data integrated over the course of a single cooking task (Rapp et al 2016;Shen et al 2017). This study is unique in that real-time data on the size distribution of particles emitted from a forced-air gasifier cookstove during several steady and transient operating modes is presented to illustrate: (a) how the aerosol formation, growth, and oxidation processes can vary with operating conditions; (b) the range of emissions performance that might be observed in real-world settings; and (c) the relative contributions of different operating modes to the total emissions measured during a test.…”