Effects of biomass-generated producer gas constituents on cell growth, product distribution and hydrogenase activity of Clostridium carboxidivorans P7T
Introduction
Due to the steadily increasing demand for ethanol, extensive research is being performed to develop an economically viable process for ethanol production. The gasification–fermentation process utilizing the fermentation of gasified lignocellulosic biomass to ethanol is being explored owing to the low cost and availability of biomass. The process involves the conversion of biomass to producer gas (a mixture of CO, CO2, H2 and N2), following which the producer gas is converted to ethanol using microbial catalysts. The term “producer gas” denoted in this work refers to biomass-generated producer gas. It has been found that anaerobic bacteria such as Clostridium ljungdahlii and C. autoethanogenum can be used to convert CO, CO2 and H2 to ethanol and acetic acid [1], [2]. The research described in this work utilized a novel Clostridium species, recently identified as C. carboxidivorans P7T [3], to convert producer gas to ethanol and acetic acid [4].
In previous studies [5], certain effects of producer gas fermentation were observed. The process involved growing cells in a batch system under continuous flow of synthetic producer gas, following which the system was changed to a continuous liquid flow in which fresh media was added and products/cells were removed with no cell recycle. The term “synthetic producer gas” refers to a mixture of purchased compressed gases with a similar CO, CO2, and H2 composition as the producer gas. After the cells reached a steady concentration, the synthetic producer gas was replaced with the producer gas that had been cleaned with two cyclones followed by two 10%-acetone scrubbers, all in series. Following the producer gas introduction, the cells stopped consuming H2 almost immediately and the cells stopped growing after a delay of approximately 1.5 days. The cessation in cell growth led to cell washout from the reactor as a result of the continuous operation. In addition, an increase in ethanol production was also observed.
Producer gas via gasification typically contains tars, ash, and certain gaseous components [6], [7]. It was hypothesized that one or more of these potential “contaminants” induced cell dormancy, stopped H2 utilization, and affected product distribution. This work assessed whether tars, ash, ethylene, ethane, acetylene, and/or nitric oxide contributed to the above conditions. In addition to the producer gas cleaning described above, the inclusion of filters was also assessed to determine if any of the conditions could be eliminated.
Section snippets
Biomass and producer gas
Producer gas was obtained by gasification of switchgrass. Switchgrass is a sustainable perennial herbaceous crop [8] which is advantageous owing to its high yields, low nutrient requirements, and geographically wide distribution [9]. The switchgrass was harvested, baled, chopped and then gasified in a fluidized-bed reactor as previously reported [5]. The exiting gas was passed through two cyclones in series to remove particulates (such as ash) and then through two scrubbers in series. Each 4-ft
Chemostat studies—cell growth
The cell concentration profile for one chemostat study is shown in Fig. 2. In the first stage, the cells grew and the cell concentration began leveling off on Day 8. Following the initiation of continuous liquid flow on Day 8 (stage 2), the cell concentration remained essentially constant. There was not much change in the cell concentration during the third stage following the introduction of producer gas. The 0.025 μm filter negated the previously observed decline in cell concentration observed
Conclusions
When fermenting biomass-generated producer gas with C. carboxidivorans P7T, results showed that tars promoted cell dormancy and a redistribution of ethanol and acetic acid production. However, cells could adapt and grow in the presence of tars following prolonged exposure. Preliminary studies showed that nitric oxide inhibited the hydrogenase enzyme. The additional cleaning of producer gas using a 0.025 μm filter prevented growth inhibition although the filter cleaning did not eliminate the
Acknowledgements
This research was supported by Aventine Renewable Energy, USDA-CSREES IFAFS Competitive Grants Program award 00-52104-9662, USDA-CSREES Special Research Grant award 01-34447-10302, and the Oklahoma Agricultural Experiment Station. Special thanks are also given to Ralph Tanner, University of Oklahoma, for isolating and providing C. carboxidivorans P7T.
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