STUDY OF BIOGASIFICATION OF COAL TO METHANE FOR EX SITU AND IN SITU APPLICATIONS

Author

Ji Zhang, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2017

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

Liang, Yanna

Abstract

Burning coal to generating electricity is certainly not environmentally friendly and sustainable. But considering its abundant and inexpensive nature, coal is not going to be completely replaced by other forms of fuels or disappear from the energy market in a short while. Thus, better and clean technologies, rather than combustion, need to be developed to utilize coal for generating valuables for the society. Specific for this study, coal bioconversion was investigated for the purpose of producing methane from coal. The first part of the dissertation characterized microbial communities dedicated for conversion of coal to methane in situ and ex situ. To enhance methane production in situ in bituminous coal seams, distribution of microorganisms in the formation water collected from a coalbed methane well (CBM) was investigated. Based on next generation 16S rDNA sequencing, both Bacteria (231species) and Archaea (33 species) were identified. Among the bacterial domain, polymer-degrading, benzoate, fatty acid and sugar utilizing bacteria were dominant. Among the archaea domain, the major methanogens (89.8%) belonged to the order of Methanobacteriales that are hydrogenotrophic. To develop a microbial consortium for ex situ coal bioconversion, the original microbial community was adapted to ground coals for five months in a laboratory environment. DNA sequencing revealed the presence of 185 bacteria species and nine archaea species that were dramatically different from those in the original formation water. In particular, the majority (90.4%) of methanogens were within the order of Methanomicrobiales. To increase methane production, two nutrient solutions were tested. Solution #2 which targeted methanogens provided a methane yield of 111 ft3/ton in 20 days, which translated to a 5.6 ft3/ton-day. In addition, the adapted consortium was found to be aerotolerant. The second part of the dissertation presented a suitable recipe for biogasifying coal collected from the Illinois basin. This recipe was developed based on formation water from local CBM well. To develop this recipe, the formation water’s chemical composition was analyzed first. This analysis revealed that the formation water had a distinct geochemical signature including a low concentration of sulfate, nitrate, calcium, and magnesium, but a high concentration of sodium, potassium, and chloride. To maximize methane yield from coal, a Box-Behnken design necessitating 29 reactors was adopted to find the optimal nutrient conditions. The optimal condition provided by the Design of Expert (DOE) software was: Fe powder at 74 mM, methanol at 97.9 mM, ethanol at 100 mM, and trace mineral supplement solution at 100%. Under these conditions, the predicated methane yield and content was 1,417.35 ft3/ton and 80.7%, respectively. To confirm these results, a verification experiment was conducted, where a methane yield of 1,390.22 ft3/ton with a methane content of 84.6% were observed, which were fairly close to the predicted values. In the last part of the dissertation, a 3-liter fermentor was established to evaluate Illinois coal biogasification in a larger reactor over a longer duration compared to those reported in the literature. During the one-year study, the developed recipe was added three times to sustain methane release from coal powders. After the fermentation was terminated, the residual coal and fermentation broth were characterized in detail. The cumulated methane production was 5,171 ft3/ton with a methane content of 75.36%. Compared to the untreated coal, the coal residue appeared to be finer and highly degraded with less carbon but more sulfur and ash. Based on mass balance, volatile and fixed carbon decreased 15.9% and 29.6%, respectively, using the untreated coal as the baseline. According to GC/MS analysis, the fermentation broth contained mainly three groups of compounds: fatty acids, aromatics, and hydrocarbons. In addition, the fermentation broth was found to have effects on flocculation and contain compounds that possess surface-active properties. Further investigations are needed to identify these chemicals responsible for these activities and develop ways to enhance coal biogasification based upon results obtained then.