• Energy Research

Coproduction of biofuels with biochar (the carbon-rich solid formed during biomass pyrolysis) can provide carbon-negative bioenergy if the biochar is sequestered in soil, where it can improve fertility and thus simultaneously address issues of food security, soil degradation, energy production, and climate change. However, increasing biochar production entails a reduction in bioenergy obtainable per unit biomass feedstock. Quantification of this trade-off for specific biochar-biofuel pathways has been hampered by lack of an accurate-yet-simple model for predicting yields, product compositions, and energy balances from biomass slow pyrolysis. An empirical model of biomass slow pyrolysis was developed and applied to several pathways for biochar coproduction with gaseous and liquid biofuels. Here, we show that biochar production reduces liquid biofuel yield by at least 21 GJ Mg(-1) C (biofuel energy sacrificed per unit mass of biochar C), with methanol synthesis giving this lowest energy penalty. For gaseous-biofuel production, the minimum energy penalty for biochar production is 33 GJ Mg(-1) C. These substitution rates correspond to a wide range of Pareto-optimal system configurations, implying considerable latitude to choose pyrolysis conditions to optimize for desired biochar properties or to modulate energy versus biochar yields in response to fluctuating price differentials for the two commodities.

Coproduction of biofuels with biochar (the carbon-rich solid formed during biomass pyrolysis) can provide carbon-negative bioenergy if the biochar is sequestered in soil, where it can improve fertility and thus simultaneously address issues of food security, soil degradation, energy production, and climate change. However, increasing biochar production entails a reduction in bioenergy obtainable per unit biomass feedstock. Quantification of this trade-off for specific biochar-biofuel pathways has been hampered by lack of an accurate-yet-simple model for predicting yields, product compositions, and energy balances from biomass slow pyrolysis. An empirical model of biomass slow pyrolysis was developed and applied to several pathways for biochar coproduction with gaseous and liquid biofuels. Here, we show that biochar production reduces liquid biofuel yield by at least 21 GJ Mg(-1) C (biofuel energy sacrificed per unit mass of biochar C), with methanol synthesis giving this lowest energy penalty. For gaseous-biofuel production, the minimum energy penalty for biochar production is 33 GJ Mg(-1) C. These substitution rates correspond to a wide range of Pareto-optimal system configurations, implying considerable latitude to choose pyrolysis conditions to optimize for desired biochar properties or to modulate energy versus biochar yields in response to fluctuating price differentials for the two commodities.

Abstract The fluid flow, heat transfer, and gas-phase chemical reactions for a natural-draft plancha-type biomass cookstove are studied at steady state with a commercial CFD code, ANSYS Fluent™. Different firepowers (in the range of real operating conditions), modeled as different flow rates of wood volatiles entering the 3D computational domain, were investigated. Firepower was found to have minimal effect on the air flow rate through the cookstove and the efficiency, but to strongly affect stove temperatures and heating rates. The main results were duplicated by a simple analytical model with one tunable parameter, and with simplified combustion, heat transfer, fluid properties, and pressure losses. The analytical model highlights the importance of the air mass flow rate through the cookstove, which is affected by design choices. The largest diferences between the CFD model and the analytical model occurred at the lower firepowers, when temperatures were so low that combustion was incomplete.

Abstract The fluid flow, heat transfer, and gas-phase chemical reactions for a natural-draft plancha-type biomass cookstove are studied at steady state with a commercial CFD code, ANSYS Fluent™. Different firepowers (in the range of real operating conditions), modeled as different flow rates of wood volatiles entering the 3D computational domain, were investigated. Firepower was found to have minimal effect on the air flow rate through the cookstove and the efficiency, but to strongly affect stove temperatures and heating rates. The main results were duplicated by a simple analytical model with one tunable parameter, and with simplified combustion, heat transfer, fluid properties, and pressure losses. The analytical model highlights the importance of the air mass flow rate through the cookstove, which is affected by design choices. The largest diferences between the CFD model and the analytical model occurred at the lower firepowers, when temperatures were so low that combustion was incomplete.

Abstract In this study, a pyrolysis biomass cookstove with separate combustion and pyrolysis chambers (two-chamber stove) is investigated and compared to a widely-used char producing cookstove design (top-lit updraft, TLUD). The influence of pyrolysis fuel type (pellets of hardwood, corn stover, or switchgrass) on CO, NO, CO2 and particulate emissions, and the time dependence of particulate size distribution are quantified. Water boiling tests are conducted in a hood with pine wood as the combustion fuel for the two-chamber stove. Thermal and modified combustion efficiencies, and char yields and elemental compositions are reported. The sensitivity to fuel choice is far lower in the two-chamber stove than in the TLUD, thus making the two-chamber stove design well suited to challenging waste biomass fuels. The NO emission factors are positively related to the nitrogen content of biomass pellets, whereas the particulate emission factor (measured only for the two-chamber stove) follows an order of hardwood

Abstract In this study, a pyrolysis biomass cookstove with separate combustion and pyrolysis chambers (two-chamber stove) is investigated and compared to a widely-used char producing cookstove design (top-lit updraft, TLUD). The influence of pyrolysis fuel type (pellets of hardwood, corn stover, or switchgrass) on CO, NO, CO2 and particulate emissions, and the time dependence of particulate size distribution are quantified. Water boiling tests are conducted in a hood with pine wood as the combustion fuel for the two-chamber stove. Thermal and modified combustion efficiencies, and char yields and elemental compositions are reported. The sensitivity to fuel choice is far lower in the two-chamber stove than in the TLUD, thus making the two-chamber stove design well suited to challenging waste biomass fuels. The NO emission factors are positively related to the nitrogen content of biomass pellets, whereas the particulate emission factor (measured only for the two-chamber stove) follows an order of hardwood

Gaseous C 2 H 5 Cl (ethyl chloride) was injected into the post-flame zone of a turbulent combustor, with equivalence ratios and residence times in the range of those encountered in hazardous waste incinerators. Temperatures below those normally associated with incineration were selected to simulate incinerator failure modes. Samples were withdrawn from the reactor and analyzed using a Fourier transform infrared spectrometer (FTIR) coupled to a longpath cell (60 cm base path length). For the highest-temperature case (T max =1225 K), destruction of the injected C 2 H 5 Cl was rapid, and the only observable product species were HCl, CO, H 2 O, and CO 2 . For cooler injection temperatures (T max =1012, 932 K), C 2 H 4 , C 2 H 2 , and C 2 H 3 Cl were observed as well, with a C 2 H 3 Cl/C 2 H 4 ratio between 0.25 and 0.5. C 2 H 5 Cl injection was simulated numerically, using the experimental temperature profiles and modeling the reactor as a plug-flow device. The reaction mechanism developed by Karra et al. was expanded to distinguish between chlorinated C 2 isomers. Rates for the modified reactions were chosen so that the combined reaction rate of the isomers was unchanged. This modification greatly improved the agreement between numerical and experimental results for C 2 H 3 Cl, as it opened new channels for its production.