• 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.

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

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.

ABSTRACTThis study investigates the differences between oxycombustion and conventional air combustion through methane flame extinction measurements and calculations. We report global extinction strain rates for methane with O2/N2, O2/CO2, and O2/CO2/H2O flames for a range of calculated stoichiometric adiabatic flame temperatures. Tests were performed in a subatmospheric opposed-jet burner, with pre-vaporized water. For a given oxidant diluent, global extinction strain rates increase strongly with increasing flame temperature and with pressure. At a given temperature, the effect of oxidant composition depends on whether dissociation is included in computing the stoichiometric adiabatic flame temperature. When dissociation is included, O2/CO2 and O2/CO2/H2O flames are roughly 30% harder to extinguish than O2/N2 flames. When dissociation is not included, the oxidant composition has a minimal effect. Calculations with the CHEMKIN suite of programs yield the same ranking of extinction strain rate for the diffe...

Pyrolysis of centimeter-scale wood particles is of practical interest and provides a sensitive test of pyrolysis models, especially their thermochemistry. In this paper we present an updated comprehensive pyrolysis model including chemical reactions and transport of heat and species, implemented independently in two different software environments. Results of the model are compared to experimental results of three independent sets of centimeter-scale experiments. Temperatures, mass losses, and rate of production of several gaseous and light tar species are included in the comparisons. Predictions and experiments agree qualitatively and in most cases have reasonable quantitative agreement. We also report comparisons of model predictions to literature data obtained in other regimes (thermogravimetric analysis and homogeneous tar cracking) in order to demonstrate that predictive capabilities of the model have not been compromised by the modifications presented here.

Abstract The levelized cost of hydrogen for municipal fuel cell buses has been determined using the DOE H2A model for steam methane reforming (SMR), molten carbonate fuel cell reforming (MCFC), and wood gasification using wastewater biogas and willow wood chips as energy feedstocks. 300 kg H2/day was chosen as the design capacity. Greenhouse gas emissions were calculated for each for the three processes and compared to diesel bus emissions in order to assess environmental impact. The levelized cost per kilogram for SMR, MCFC, and gasification is $5.12, $8.59, and $10.62, respectively. SMR provided the lowest sensitivity to feedstock price, and lowest levelized cost at various scales, with competitive cost to diesel on a cost/km basis. All three technologies provide a reduction in total greenhouse gases compared to diesel bus emissions, with MCFC providing the largest reduction. These results provide preliminary evidence that small scale distributed hydrogen production for public transportation can be relatively cost-effective and have minimal environmental impact.

Abstract Methanol is one of the fuels that are an alternative to petroleum-based liquid transport fuels. This paper assesses the feasibility of co-production of methanol and biochar from thermal treatment of pine in a two-stage process; pyrolysis or gasification to produce biochar and volatiles, and the processing of the volatiles to produce methanol using process data for large-scale conversions based on natural gas. Three concepts were studied: (i) slow pyrolysis at 300 °C; (ii) slow pyrolysis at 450 °C; and (iii) gasification at 800 °C, all of them followed by processing of the volatiles into syngas and the conversion of the syngas into methanol. Gasification was able to generate methanol at or below current (2012) prices of methanol produced from fossil fuel ($422/t) from a plant size of 100 t/h upwards. Pyrolysis is not competitive without valuing the biochar as a product. Considering both biochar and methanol as marketable products improves the viability of slow pyrolysis concepts. Their profitability is sensitive to the biochar selling price between, with a break-even at a biochar price of about $220/t for the pyrolysis at 300 °C and about $280/t for pyrolysis at 450 °C.