Bimal Acharya

Review of syngas fermentation processes for bioethanol

Bioethanol is recognized as an important renewable and sustainable transportation fuel. Although synthesis gas (syngas: CO, H2, CO2) produced from lignocellulosic biomass (forest or agricultural biomass) is being used in the production of bioethanol by both chemical catalytic and biosynthetic processes, the latter are noted to have more advantages. In the biosynthesis process, such as the fermentation of syngas, bioethanol is produced along with acetate, butanol, butyrate, methane, peptone, and formaldehyde. Although progress has been made on research and development for the utilization of syngas on fermentation technology, the major barriers for the commercialization still include low yield, expensive biological catalyst, slow kinetics, low gas–liquid mass transfer, and challenges with catalytic separation and recycling. This paper presents a review on fermentation product impurities, microorganisms, chemical reactions, separation techniques, bioreactor types, fermentation conditions, gas–liquid mass transfer, current status of the technology and economics. It seems selection of the appropriate microorganism, nutrient medium, and appropriate hollow fiber membrane biofilm reactor might lead toward achieving an increased mass transfer efficiency for commercialization of the bioethanol.

Ethanol production by syngas fermentation in a continuous stirred tank bioreactor using Clostridium ljungdahlii

ABSTRACT Ontario biomass could be thermochemically processed by dry and wet torrefaction to produce high quality solid biofuel. These solid fuels or raw biomass could also be gasified to produce syngas. This study analyzes and demonstrates a successful and efficient way of producing bioethanol from syngas fermentation using Clostridium ljungdahlii in a laboratory scale continuous stirred tank bioreactor having an innovative gas supply and effluent extraction structures. At the beginning of the experiment, a batch process was conducted to grow microorganisms and allow the growth of the microorganisms to reach to maximum cell density in a reactor without supplying a gas. Ethanol production was observed by supplying two different gas compositions which included 100% CO and simulated syngas, mimicking the composition of syngas extracted from lignocellulosic biomass having 60% CO, 35% H2, and 5% CO2. CO and syngas were fermented with different gas flow (5–15 mL/min), effluent flow (0.25–0.75 mL/min), and media flow rates and stirrer speed (300–500 rpm) at atmospheric pressure and 37˚C. The gas flow rate, media and effluent flow rate, pH level, and stirrer speed were controlled during the fermentation process. The exhaust gas was reused for the improvement of residence time using a loop-back system for improving the gas–liquid mass transfer. Excessive foam was observed during the fermentation process which was controlled using diluted antifoam-204. Maximum cell concentration reached 2.4 g/L. The mass transfer coefficient showed better performance during syngas fermentation than CO fermentation. More bioethanol production was observed by syngas fermentation than CO fermentation. CO fermentation produced 0.17–1.33 g/L-effluent ethanol and 8.92–23.67 g/L-effluent acetic acid whereas syngas fermentation produced 0.85–3.75 g/L-effluent ethanol and 8.89–14.97 g/L-effluent acetic acid.

Torrefaction of Ontario Biomass for Energy Applications

Abstract. In order to achieve energy sustainability, processed biomass can be a primary source of fuels for electrical or thermal power plants. Torrefaction is a mild form of pyrolysis at temperature ranging from 200a´¼C to 300a´¼C in minimum oxygen environment for a reasonable residence time, which improves the fuel characteristics of biomass. These improvements include hydrophobicity, low moisture contents, grindability, more stable against chemical oxidation and microbial degradation, as well as increase carbon content and heating value. This study compares the fuel characteristics of raw and torrefied biomass feedstocks from Ontario. Torrefaction occurred in a locally designed and fabricated tubular continuous reactor with an inert environment by supplying Nitrogen gas throughout the experiment. Herbaceous/agricultural biomass undergoes a more rapid mass loss compared to woody biomass during torrefaction. Carbon contents increase from 10-20% with an increase in torrefaction temperature, whereas the oxygen decreases by 10-20% with an increase in temperature. Heating value of the torrefied biomass was 10-35% higher than the raw biomass. It was observed that all torrefied products displayed hydrophobic characteristics and remained unaffected from the biodegradation when immersed in water after torrefaction. The characteristics of torrefied biomass show great potential for the sustainable energy applications.