Effect of different inorganic iron compounds on the biological methanation of CO₂ sequestered in coal seams

Highlights

• FeS₂ has the potential to increase the bio-methanation of sequestered CO₂.

• Change law of liquid phase parameters keep consistent with the biomethane generation.

• Microorganisms will react with coal and iron to promote the production of biomethane.

• Coal seams with high sulfur are suitable for the research of MECBM projects.

Abstract

To study the effect of different inorganic iron compounds on the biological methanation of CO₂ sequestered in coal seams, a biogas production experiment was carried out through use of bituminous coal D (China) and indigenous microorganisms, by adding bicarbonate and inorganic iron compounds. It is found that the addition of FeCl₂ or FeS₂ promoted methane production. FeS₂ resulted in the highest methane concentration (30.54%) and methane production (0.191 mmol/g), which increased by 30.46% and 38.11%, respectively. The pH of the experimental group was lower than that of the control group during the whole process. The chemical oxygen demand (COD) first increased and then decreased, and the COD peak value greatest increased by 3.07% after FeCl₂ was added. The ammonia nitrogen content in the liquid phase with FeCl₂ or FeS₂ was improved obviously (11.01% and 6.76%), and the microorganisms were increased accordingly. Scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) showed that the microorganisms reacted with coal and iron after addition of Fe to generate several amorphous substances on the surface of coal, and further enhanced biomethane production. This study provides a new method for the utilization of CO₂ sequestered in coal seams and the rational development of high sulfur coal.

Introduction

Coal is a heterocyclic macromolecular organic compound, which contains C, H, O and variable amounts of other elements. The hydrocarbon compounds and lignin derivatives of coal can be degraded by microorganisms by serving as carbon source for their growth and as a H₂ source for methanogenesis [1,2]. Scott put forward the concept of microbial enhanced coalbed methane (MECBM) for the first time, which mainly uses microbial action to degrade coal by injecting methanogenic archaea and nutrients into coal seams [3]. Biogenic coalbed methane (CBM) is an important component of natural gas resources worldwide. Indeed, approximately 20% of global CBM is biogenic gas. CO₂ reduced biogas is one of the main types of biogenic CBM. At present, most biogenic CBM reservoirs are produced by CO₂ reduction [[4], [5], [6]]. Moreover, methanogens have been shown to degrade coal to CH₄ under suitable environmental conditions by using exogenous CO₂, and almost all types of methanogens can produce methane by reducing CO₂ [7,8]. Since the CO₂ adsorption capacity of coal is greater than that of methane, CO₂ can be used to replace methane in the coal seam, so that methane changes from an adsorbed state to a free state and enhanced coalbed methane [9]. Therefore, by combining the microbial method and the CO₂ displacement CBM method, injecting carbon dioxide into coal seams can achieve both CO₂ storage and CBM production [10,11]. However, injecting CO₂ into coal seams has been shown to increase the concentration of water-soluble CO₂ and carbonate, as well as to affect the energy and metabolic pathways of microbial metabolism. These factors affect the occurrence and metabolic activity of microorganisms, change the structure and composition of endogenous microbial communities in coal seams, and thus affect methane production [[12], [13], [14], [15]]. In our previous work, sodium bicarbonate was used to simulate CO₂ sequestered in coal seams and conducted biogas production experiment, proved the feasibility of CO₂ bio-methanation. It was found that ethers are preferentially utilized by microorganisms, improving the hydrolytic ability and hydrogenotrophic methanogenesis. However, the methane production (<0.14 mmol/g) and the production rate were low in the process of biological methanation [16]. In addition, the acidity of coal seam increases as the CO₂ concentration increases, which greatly affect the metabolic activity of endogenous microorganisms; and these conditions are very unfavorable for the production of biomethane by CO₂ reduction.

As a necessary trace element in the biochemical reaction of microorganisms, Fe has various forms and valency, which can promote the enzyme synthesis, electron transfer and proton transfer process, and change the microbial communities’ structure. Among these forms, proper Fe⁰ can improve the activity of lactate dehydrogenase, can be used as an electron donor to produce hydrogen, and can also be used as nutrient for microbial growth [17]. However, when the Fe⁰ concentration is too high, it inhibits the microbial activity [18]. Fe²⁺ can reduce the oxidation-reduction potential (ORP) and buffer organic acids, and also promote the production of coenzyme F₄₂₀, to affect the methane production rate [19,20]. Fe²⁺ is easily oxidized to Fe³⁺ to improve the methanogens activity. However, it was also reported that Fe³⁺ induced the competition between reducing bacteria and methanogens for acetate and hydrogen consumption, and might inhibit temporarily the methanogens activity [21]. Available electron donors are necessary and critical in the two types of methane generation (CO₂ reduction and acetate fermentation) [22]. Elemental iron is considered an alternative electron donor for methanogens and can be used as an electron donor during the reduction in CO₂ to methane production, thereby improving the methanogenesis performance of various methanogens [23,24]. In recent years, scrap iron and various forms of iron compounds have been extensively studied in anaerobic digestion and remediation of environmental pollutants, and adding rusty waste iron to sludge anaerobic digestion systems has been shown to increase the diversity of bacteria, increase the abundance of iron reducing bacteria and facilitate the degradation of complex substrates [25,26]. Adding goethite and FeCl₃ to continuous stirred-tank reactors (CSTR) can promote anaerobic digestion of algae to produce methane [27]. High sulfur coal is an important coal resource in China, accounting for approximately 8% of China’s coal reserves. Currently, high sulfur coal is mainly used after desulfurization by various desulfurization technologies, but several problems still exist, such as low desulfurization efficiency, secondary pollution and low-cost performance [28]. Whether Fe added as an alternative electron donor for methanogens in anaerobic environments can stimulate the bioconversion of CO₂ to methane in coal seam environments has not been verified, and existing knowledge regarding the effects of different inorganic iron compounds on biomethanation of CO₂ is very limited. In addition, the in situ utilization of underground high sulfur coal can be realized by biological methods remains to be studied.

Using bituminous coal D from Qianqiu Coal Mine in Yima, Henan Province as the substrate in the laboratory, methanogenic archaea enriched by indigenous microorganism in mine water were used as the microorganisms’ source to simulate biogas production from coal. Bicarbonate, along with different inorganic iron compounds, were added to samples to simulate CO₂ sequestered in coal seams, after which changes in liquid-phase test indexes such as the biogas production efficiency, fermentation broth pH, COD, ammonia nitrogen and microbial content during anaerobic reaction were evaluated based on comparison with a control group without added iron compounds. The changes in the coal surface morphology and structure before and after the biological reaction were also discussed to analyze the effects of the iron compounds on the biomethanation of CO₂ in coal seams to provide a feasible method for promoting this process and the development and utilization of high sulfur coal.