Combustion phenomena in biomass gasifier cookstoves
Date
2016
Authors
Tryner, Jessica, author
Marchese, Anthony J., advisor
Willson, Bryan, committee member
Yalin, Azer P., committee member
Peel, Jennifer, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Approximately 2.8 billion people (~40% of the global population) rely on solid fuels, such as wood, charcoal, agricultural residues, and coal, for cooking. Exposure to emissions resulting from incomplete combustion of solid fuels leads to many adverse health impacts. These health impacts have motivated the development of solid-fuel cookstoves that reduce user exposure to carbon monoxide (CO) and fine particulate matter (PM2.5). In recent years, rating systems and emission rate targets for solid-fuel cookstove performance have been proposed. The aspirational targets included in these systems (e.g., Tier 4 in the ISO IWA tiers) have encouraged the development of cookstoves that reduce emissions of CO and PM2.5 by more than 50% and 95%, respectively, compared to a baseline three-stone fire. In a top-lit up draft (TLUD) gasifier cookstove, solid biomass fuel is gasified and the resulting gaseous fuel is mixed with secondary air above the fuel bed to produce the flame that heats the cooking surface. Household biomass cookstoves that utilize gasifier designs have attracted interest due to their demonstrated ability to emit less CO and PM2.5 per unit of energy delivered to the cooking surface than other cookstove designs. Unfortunately, highly variable performance has also been observed among gasifier cookstoves, and some have been found to emit more CO and PM2.5 than a three-stone fire. Accordingly, three studies were conducted to: (1) identify the sources of the observed variability; (2) characterize the manner in which stove design, fuel properties, and operating mode influenced performance; (3) gain insight into how secondary air velocity affected fuel-air mixing and the flame dynamics in the secondary combustion zone; and (4) evaluate whether or not the reductions in emission rates that are sought could be achieved with the TLUD design. In the first study, five natural draft TLUD design configurations were tested with two fuels (corn cobs and Lodgepole pine pellets) to investigate the variability in performance that had been observed in previous studies. The results indicated that stove design, fuel type, and operator behavior all influenced emissions. Four of the five configurations exhibited lower emissions when fueled with Lodgepole pine pellets than when fueled with corn cobs. Furthermore, large transient increases in CO emission rates were observed when stoves were refueled during operation by adding fresh biomass on top of the hot char bed that was left behind after the previous batch of fuel had gasified. An energy balance model was also developed, using temperature data collected from thermocouples mounted on each configuration, to identify the factors that contributed the most to sub-unity efficiency. The results illustrated that up to 60% of the energy input to the stove as fuel could be left over as char at the end of the test, and whether or not the energy in this char was subtracted from the energy in the fuel consumed during the test when calculating the thermal efficiency of a given configuration had a large effect on the calculated efficiency value. The manner in which cookstove design, fuel properties, and operator behavior affected TLUD performance was investigated in more detail in a second study. Seventeen different stove geometries, 4 primary air flow rates, 4 secondary air flow rates, 5 secondary air temperatures, 4 fuel moisture contents, and 4 different sfuel types were tested in a modular test bed using a procedure specifically designed to capture the low emissions observed during normal operation and the high emissions observed during refueling and char burnout. The lowest high-power emissions measured during normal operation were 1.6 g/MJd-1 CO (90% confidence interval (CI) = 1.1-2.1) and 18 mg/MJd-1 PM2.5 (90% CI = 17-19). These values were well below the Tier 4 targets of 8 g/MJd-1 CO and 41 mg/MJd-1 PM2.5, but post-refueling emissions were always above the Tier 4 targets. Higher secondary air velocities resulted in lower emissions. Changes in fuel type influenced the composition of the producer gas entering the secondary combustion zone during normal operation and sometimes resulted in order of magnitude changes in PM2.5 emissions. Temperature measurements taken in the fuel bed indicated that the stove operated as an inverted downdraft gasifier during normal operation and as a conventional updraft gasifier after refueling. Overall, the results suggest that efforts aimed at reducing users' exposure to CO and PM2.5 emissions from solid fuel combustion need to take fuel type and operator behavior, in addition to stove design, into consideration. The third study was designed to investigate the effects of secondary air velocity on the fuel-air mixing process and flame dynamics in the secondary combustion zone by employing high-speed imaging techniques. Images of OH* chemiluminescence, acetone (which served as a fuel tracer) planar laser-induced fluorescence (PLIF), and OH PLIF were collected at multi-kHz repetition rates in a burner designed to generate a two-dimensional replica of the secondary combustion zone in a gasifier cookstove. This burner featured two opposed planar jets that formed an inverse non-premixed flame in which the air and fuel were in cross flow. Images were collected for various air and fuel velocities. Regular deflecting oscillation of the jets, which has been reported previously for isothermal, non-reacting, unconfined opposed planar jets, was observed in some cases but appeared to be suppressed by convection in the vertical direction and buoyancy effects in other cases. The acetone PLIF images revealed that a high air jet velocity resulted in more extensive mixing of the air and fuel below the height of air injection. As a result, the reaction zone was located further below the top of the burner in comparison to the low air velocity case. These results suggest that higher air jet velocities may lead to lower emissions from gasifier cookstoves as a result of better fuel-air mixing and a lower reaction front location that allows more time for CO and PM to be oxidized before reactions are quenched by the cold cooking surface; however, the literature suggests that unconfined opposed axisymmetric jets do not exhibit deflecting oscillation behavior and, as a result, there are limitations associated with the use of opposed planar jets as a model for the secondary air jets in a gasifier cookstove.