Effects of Air Flow Modifications on Biomass Cookstoves

Worldwide, nearly three billion people cook and heat their homes with solid fuels such as biomass. Many of these biomass stoves are inefficient and heavily polluting. The emissions produced from biomass stoves are estimated to be the largest environmental threat to human health in the world, causing approximately four million premature deaths per year. Beyond the massive impact on human health, the burning of residential biomass fuels also significantly contributes to air pollution and climate change. Due to the undesirable side effects caused by such cooking practices, there is considerable interest in improving the efficiency and reducing the emissions from biomass cookstoves with researchers around the world exploring new designs and improvements, such as air flow modifications. The availability and location of air relative to the fuel in a cookstove is a crucial parameter in combustion efficiency and the resulting pollutant emissions. By controlling and modifying the air flow within the combustion chamber, cookstove designers and researchers hope to improve efficiency and reduce harmful emissions from biomass cooking. However, while the negative effects of biomass burning have spurred much research into designing less polluting cookstoves, there is a continued need for information about the actual impacts of air flow modifications in cookstoves./nThis dissertation explores several applications of controlling or modifying the air flow in biomass cookstoves in order to develop a better scientific understanding of the impacts of the modifications on stove performance and emissions. The first application investigates passive modifications in charcoal cookstoves intended for dissemination in Haiti. There is an acute need for fuel-efficient Haitian cookstoves; charcoal is a predominant cooking fuel in Haiti but its use creates significant burdens on both the environment and the Haitian people. The improved charcoal cookstoves intended for distribution in Haiti that were chosen for investigation were designed to reduce convective heat losses to the environment and to direct air toward the cooking pot in the attempt to reduce fuel consumption as compared to the traditional Haitian stove. It was found that all improved stoves used less fuel on average than the traditional Haitian stove, with the majority also reducing the total undesirable emissions. However, the traditional stove had the fastest time to boil, an important consideration for end users. Therefore, the improved stoves may face adoption issues in the field due to slow cooking speeds and incompatibilities with the cultural cooking style of Haiti even though they reduce fuel consumption as desired./nThe second application investigates a passive ignition aid for charcoal stoves, known as a lighting cone, which is intended to decrease the time needed to ignite a charcoal bed by increasing draft through the stove. Traditional charcoal-burning stoves often have shallow and exposed charcoal beds, which ignite slowly and inefficiently due to interference from the wind and a lack of draft through the stove and the fuel bed. A lighting cone is a simple, metal cone designed to increase the exposure of the charcoal bed surface to fresh air and protect it from heat losses due to wind during ignition. A lighting cone is an accessory intended for user convenience that is already utilized by some local populations around the world; however, no scientific results on lighting cone performance exist in the literature. Therefore, the goal of this work was to determine the effectiveness of a lighting cone in decreasing the ignition time of a traditional Haitian charcoal cookstove and evaluate its impact on stove emissions and fuel consumption during the typically inefficient ignition phase. The results of this work found that the lighting cone operated as desired - ignition time was reduced by over 50%. Due to this more efficient, shorter ignition stage, charcoal consumption during ignition was reduced by over 40% and carbon monoxide emissions were reduced by over 50%. Additionally, the number of ultrafine particles emitted, an important metric for human health concerns, was greatly reduced using the lighting cone as compared to the traditional ignition method. This suggests that a lighting cone is a viable and beneficial accessory for aiding ignition in shallow-bed charcoal stoves./nThe third application investigates the use of air injection strategies to reduce emissions from biomass cookstoves. Previous studies have found that air injection, which increases turbulent mixing, is a promising technique for reducing soot or black carbon emissions from cookstoves. However, some air injection strategies have produced undesirable emissions and performance results. Therefore, further insight into the application of air injection strategies is required to ensure the modifications produce desired and positive benefits. This dissertation explores three different mixing strategies (referred to as halos in this work), which were chosen to highlight the impact of the air injection angle on the emissions and performance results. The first strategy injected air straight down toward the fuel bed, the second strategy injected air at a downward angle toward the center of the fuel bed, and the third strategy similarly injected air at a downward and inward angle, but added a tangential component to promote swirling flow. Along with the ex-situ measurements commonly utilized in cookstove research, in-situ measurements were recorded using laser diagnostic techniques to gain a deeper understanding of how the three mixing strategies were affecting the flame zone. Comparing the ex-situ and in-situ soot measurements, the results found that all of the halos appeared to operate as desired; soot was produced in the flame, increasing radiative heat transfer to the pot, but was oxidized prior to leaving the combustion zone. However, the ex-situ results also showed that the stoves with air injection had comparable or worse performance results (e.g. thermal efficiency and fuel consumption) than the same, unmodified improved stove. Therefore, deeper investigation was pursued to identify adverse effects of the air flow modifications, such as flame quenching. Injecting air straight down toward the fuel bed at the flow rates examined was found to create large reductions in soot emissions but also appeared to quench the flame and promote incomplete combustion, leading to poor performance results. The swirling flow halo reduced soot emissions without appearing to quench the flames; however, it still suffered from poor performance results, potentially due to thermal losses. Similar to the downward air injection, injecting air at a downward and inward angle reduced the soot emissions and appeared to locally extinguish the flames to some extent; however, this mixing strategy produced better performance results than the other mixing strategies. In addition to investigating the effects of air injection on stove performance, this research explored the technical feasibility of applying laser diagnostic techniques to high sooting, temporally- and spatially-variable flames such as those in biomass combustion systems. While many challenges and uncertainties arose when applying the laser-based techniques, useful insights into the mixing strategies were deduced from the in-situ measurements and further explorations of applying laser diagnostic techniques to biomass cookstoves are recommended.

Advisor: Gadgil, Ashok
Medwell, Paul