The global demand for energy is rising and coal is still expected to play a strong role in the provision of baseload power in most countries for the foreseeable future. With increasing global awareness of greenhouse gas (GHG) emissions and rising concerns about the impact of rising CO2 levels in the atmosphere, there is a self-evident need for clean coal technologies that can mitigate the environmental implications of the use of coal, both above and below the surface, while making the inherent energy content of coal available to a host of new middle-class consumers.
This has been the cornerstone challenge that the team at Verso Energy in Australia has been looking to answer since 2010. The traditional model of extracting coal and burning it seems increasingly outdated and one that, while providing much-needed power, is at odds with society’s drive to reduce GHG emissions and airborne pollutants. Financing for such high-capital projects becomes increasingly complicated in markets where the degree of visibility out to a typical 20-year project life has already been dramatically reduced, particularly in western markets.
Verso Energy’s deep dive into the world of clean coal technology took it through many options, including underground coal gasification (UCG), towards a little understood cross-section of work that was being driven from the US. This body of work was centred on the relatively recent discovery that natural gas in shallow coal seams is mostly biogenic in nature: in other words, produced as a result of the interaction between anaerobic microbes and a carbon-rich source of food, in this case coal. By comparison, deeper coal seams, typically of a higher rank, generate gas through thermogenic processes. The level of biogenic gas production is dependent on the degree to which the coal is bio-available (a function of quality and porosity mainly), as well as the supply of other common nutrients within the seam, which serve to enhance the underlying microbial reactions (similar to plant growth). It was observed that the biogenic gas contents of coals with high rates of nutrient-laden groundwater recharge were often elevated, and it was postulated that the application of favourable nutrient media to the coal may be able to produce gas in real time.
Figure 1. CBM stimulation.
Taking these concepts onboard and combined with Verso Energy’s access to drilling expertise through its parent company, Mitchell Group, the team focused its attention on this proposition – one that blends biotechnology with the understanding of coal chemistry, as well as modern drilling and stimulation techniques. The company’s perspectives developed into a belief that a model using microbial augmentation could bypass the somewhat passive native microbiological systems to develop a viable commercial proposition. Further, the use of this technology – termed microbial coal conversion (MCC) – could be readily applied to coalbed methane (CBM) wells, as well as coalfields, and scaled in a systematic manner such that an affordable and clean alternative to coal mining could be developed.
A key technical challenge that Verso Energy has solved is the ability to adapt microbial consortia to different types of coal and different underground conditions. While early competitor efforts to stimulate biogenic gas production in coal seams were by the supply of nutrients alone, Verso Energy has always felt that the required levels of efficiency can only come from carefully tailoring seed consortia to each individual site. In particular, the formation of a stable biofilm on the internal surfaces of the coal is paramount. That biofilm needs to contain a range of microbe species in equilibrium, with coal degrading microbes at the coal interface and acid producing microbes and methanogens feeding on successive breakdown products. The difficulty lies in trying to achieve such a biofilm using the native microbiology due to its enormous natural diversity. Verso Energy goes through an extended laboratory adaptation using coal samples from each new site to allow a stable biofilm to be developed in simulated underground bioreactors. The resultant tailored biofilm contains all the required microbial species for conversion of the specific coal substrate in the right proportions. This seed can then be scaled up quickly, ready for field applications, and provides the right basis for establishing a productive coal seam bioreactor. This process takes the bio-technologists at Verso Energy approximately three months for new coals, but has been demonstrated through laboratory pilots to have a huge impact in terms of stable production of methane-rich biogas.
Figure 2. CBM restoration.
The bio-conversion process is identical to commonly used anaerobic digestion models, with the exception that Verso Energy’s ability to tackle specific coals via co-feeding is believed to be the first time that such a recalcitrant substrate has been successfully digested. Design options exist to complete the entire process within the coal seam for direct gas generation, or conduct the process as two discrete steps in which the primary objective is to recover high levels of short-chain fatty acids in effluent from the subsurface. The effluent can then be processed to yield biogas using existing wastewater digester technology.
Verso Energy has focused on the use of horizontal intercept wells, used widely in CBM fields, for deployment of its two-step bioconversion process. A controlled circulation process through the horizontal sections increases the degree to which microbes and nutrients contact the coal seam and allows the composition of the circulating fluids to be controlled with increased precision. The coal solubilisation step is decoupled from the gas production step, which can be conducted in a surface plant similar to a wastewater biogas plant. The result is that coal can be recovered as an inert liquid, reducing problems associated with gas seepage in shallow coal beds.
Verso Energy’s most recent success in its work on MCC has been its process development unit in its Brisbane laboratory that has been continuously running on Victorian brown coal (lignite) since January 2016. The process development unit has yielded important engineering design inputs, as well as an active adaptation of the microbial consortia, hardening them to the coal substrate. The company is now confident that, both economically and functionally, a greater degree of conversion can be achieved sub-surface with less effort. The objective is to enhance nature’s processes, but to do so efficiently and with a high degree of control. Gas production from the laboratory bio-reactor has grown through 2016 and a steady rate of gas production equivalent to 20 m3(gas)/t(coal)/year is now being sustained.
What are the potential applications?
Three different applications of MCC technology have emerged for Verso Energy. First, the bio-remediation of coal-related tailings ponds. By creating lagoon digesters, MCC can be used to both reduce the overall footprint of the ponds, as well as to generate power from the resulting biogas: low cost and low risk, but an important tool in the arena of environmental protection, especially in emerging markets, where tailings ponds are often large and heavily contaminated, as well as close to local populations.
Second, a bio-CBM enhancement model is considered in which MCC is used to stimulate CBM wells. The MCC process of injecting microbes and nutrients after the dewatering phase can be repeated several times in order to saturate the coal with biogenic methane, extending the average life of a CBM well significantly and reducing the need to develop new fields. This application is expected to have strong market pull given its low cost of application, as well as the need across many CBM basins to find ways of extending production significantly. In markets such as China, Indonesia and Australia, where ambient gas prices are consistently north of AUS$6/million Btu, low-cost CBM stimulation can support emerging and existing projects on a repeatable cycle and dramatically improve both the rates of production, as well as the underlying value of the reserves.
Bringing fresh technology to bear in Eastern Europe’s nascent CBM sector can create a foundation of commercially viable local gas production, creating sought after supply security.
Third, and the largest opportunity by far, is the in-situ application of MCC to the vast global resources of coal that lie below mining limits. Targeting appropriate seams at depths between 200 – 600 m, there are potential applications of MCC in a host of countries, such as in China, India, Indonesia, South Africa, Australia and Poland. With the UCG industry facing a number of environmental and cost challenges, MCC steps in as a viable, low-cost, benign technology that can be readily scaled in size.
With over 90% of the world’s coal resources deemed as being un-mineable, MCC can serve to move society away from a historic model of large polluting coal-fired power projects to one in which a smaller-scale, cleaner pathway from carbon source to power source takes precedence. Capital is better allocated towards a viable long-term business and emissions are negated as future coal-fired projects are avoided.
Verso Energy, in conjunction with Aum Energy, jointly seek to bring MCC to market and to make a step-change improvement in the environmental impact of the use of coal, while maintaining many of the important aspects of energy supply security and social impact.
Rohit Vedhara, Aum Energy, Singapore, and Gary Love, Verso Energy, Australia.
This article first appeared in World Coal November. To read this and much more, register to receive a copy of the issue here.
Read the article online at: https://www.worldcoal.com/special-reports/14112016/change-is-now/