Introduction
Fugitive methane is not only a greenhouse gas but also a wasted energy resource if not utilized. Methane is the second largest contributor to global warming from anthropogenic gases, after carbon dioxide. It is estimated to be 23 times more potent than CO2 in terms of trapping heat in the atmosphere over a time frame of 100 years. Annual global anthropogenic methane emissions are projected to increase to 7904 million tonnes of CO2 equivalent by 2020. Coal mine methane emission accounts for 6% of the global anthropogenic methane emissions<FootNote>
Methane to Markets. Global methane emissions and mitigation opportunities. September 2008
</FootNote>. Every year, approximately 2.8 × 1010 m3 of methane (equivalent to 460 million tonnes of CO2) is emitted to the atmosphere from coal mining activities around the world<FootNote>
Franklin P. Methane from coal mines presents opportunity to recover energy and generate revenues. Coalbed Methane Outreach Program, US Environmental Protection Agency, 30 September 2009
</FootNote>. Based on the global coal mine methane emission data<FootNote>
U.S. EPA. CMM Global Overview. Prepared by the U.S. Environmental Protection Agency Coalbed Methane Outreach Program in support of The Methane to Markets Partnership, July 2006
</FootNote>, approximately 88% of all methane emissions are found in only ten countries. China’s mine methane emission ranks number one, followed by the United States, India, Ukraine and Australia. Methane concentrations vary across mines. Methane concentrations in ventilated air range from 0.1% to 1.2%. The maximum concentration allowed in ventilation air is set by government safety regulations and is in the range of 0.75% to 1.3% in almost all major coal producing countries. In both Australia and the United States remedial action must be taken when the methane concentration in ventilation air reaches 1%, and 0.75% for China.
China is the biggest coal producer in the world with an annual coal output of 2.79 billion tonnes in 2008 [
1]. A recent survey [
2] indicated that China’s coal mines emitted up to 1.8×10
10 m
3 of methane into the atmosphere in 2007, equivalent to 260 million tonnes of CO
2-e. Ventilation air methane accounts for over 80% of the methane emitted. In 2006, the overall drainage gas volume from 286 highly gassy state-owned coal mines (out of 621 gassy coal mines) reached 2.614×10
9 m
3, and approximately 6.15×10
8 m
3 of the drainage gas was utilized, equivalent to 23.53% of the drainage gas in total. Although gas drainage efficiency in China has been increased from 15% in 1998 to 26% in 2004, much of the captured gas is poor in quality. It is estimated that over 70%-80% of the drainage gas has a methane concentration of less than 30%, which cannot be used by conventional gas utilization technologies, including gas engines and turbines [
2]. Therefore, it is important to conduct coal mine methane emission reduction from both ventilation air and emitted poor drainage gas in China to significantly reduce global mine methane emissions, and use the methane as a clean energy source. Ventilation air methane (VAM) capture, mitigation and utilization are major problems because VAM represents the largest proportion of methane emissions from coal mines and because the air volume flow rate is large and the methane resource is dilute and variable in concentration. A typical gassy mine in Australia produces ventilation air at a rate of approximately 150 to 400 m
3/s with a methane concentration of 0.1%-1%.
This paper overviews existing and developing technologies for the mitigation and utilization of low concentration coal mine methane, and presents the research progress in developing an innovative lean burn catalytic turbine technology for fugitive methane mitigation and utilization. This turbine system can be powered with about 1% methane in air. This technology has potential to capture and utilize a large part of the fugitive methane from coal mines, landfills, oil and natural gas systems, waste water, and animal waste management. In addition, it can be applied to other low heating value gases including biogas and combustibles in industrial off-gases.
Technologies for dilute methane mitigation and utilization
A range of technologies is needed for cost-effectively capturing, utilizing and mitigating diluted mine methane depending on mine site specifications. Depending on specific mine site condition, several technologies could be configured in a combination way for a VAM mitigation and utilization plant to maximize its economic performance. For example, as shown in Fig. 1, a combined plant could consist of VAM capture units, lean-burn turbine units and VAM mitigation units. The VAM capture units can process part of the ventilation flow to enrich VAM, which could be then used by the turbine units to generate power for the plant operation and pushing ventilation air (VA) through all of the units. Waste heat from the turbines could be used for desorption in the capture units and other applications.
VAM mitigation and utilization require either treatment as a dilute gas, or concentration to levels that can be used by conventional methane-fuelled engines. Effective technologies for increasing the concentration of methane are not available but are being studied. Considerable work has focused on the oxidation of methane in very low concentration processes classified as either thermal oxidation or catalytic oxidation through the following reaction,
Table 1 summarizes VAM mitigation and utilization technologies by fundamental mechanism and technical principle.
Ancillary uses
Ancillary uses of VAM generally involve substituting ventilation air for ambient air in combustion, for example, in conventional coal fired power stations, waste coal fluidized bed boilers, gas turbines and gas engines. Energy recovery is feasible for ancillary-use technologies though it does impose risks on the gas turbines and gas engines associated with inherent dust loading of ventilation air. When ventilation air replaces ambient air as combustion air in conventional pulverized coal fired power station boilers, there exist several potential operational issues including possible boiler damage. Damage results from the sudden temperature rise following an increase in CH
4 concentration resulting in slagging and fouling and the release of slag. These issues need to be addressed before full-scale implementation of ventilation air substitution is feasible. Additionally, the lack of availability of power stations convenient to mine sites limits the suitability of this technique [
3].
Waste coal/methane fluidized bed combustion has been studied on a laboratory scale, and it was reported that methane can be fully oxidized in the combustor [
4]. However, the quality of the waste coal should be a key factor in stabilizing combustion, and combustion feasibility needs to be studied at a pilot-scale to obtain necessary operational parameters and experience before its wider application.
A significant part of the air supply to conventional gas turbines is used for cooling and dilution. As a result, a significant fraction of methane will bypass the combustor if ventilation air is used, unless the turbine system has separate compressors for combustion, for cooling and dilution. In general, only small amounts of VAM can be used by conventional gas engines with a requirement for gas cleaning to remove particles. Conventional gas engines using mine ventilation air as combustion air have been demonstrated at Appin, Australia, but their use has ceased. This may be due to an economic issue related to gas cleaning, as the gas engines require strict cleaning limits.
Principal uses
Principal uses of VAM involve combustion of the methane in ventilation air as the primary fuel. Some technologies require supplementary fuel when recovering energy to generate power if the primary methane concentration is too low.
Thermal flow reverse reactors (TFRR) and catalytic flow reverse reactors (CFRR) employ flow-reversal principles to transfer combustion heat to a solid medium, and then back to incoming air to raise its temperature to the ignition temperature of methane. The two systems differ only in the use of a catalyst. Catalytic monolithic reactor (CMR) technology is a honeycomb-type monolithic reactor which is often used, and is known for its low pressure drop at high mass flows, high surface area, and high mechanical strength [
5], however it requires a recuperator.
TFRR, RTO, CFRR and CMR technologies are technically feasible for VAM mitigation when methane concentration meets the minimum operational requirement. The TFRR technology has been demonstrated in Zhenzhou Coal Mine and CONSOL Energy’s Coal Mine for the VAM mitigation [
6]. Operational problems are likely when using TFRR and CFRR to recover heat to generate electricity, because it is difficult to maintain continuous power generation with a variable methane concentration, unless the methane concentration is maintained constantly and over minimum methane concentration requirement for the power generation. Based on catalytic combustion experimental results in a CMR laboratory-scale rig, the CMR burner can be continuously operated when the methane concentration is greater than 0.3% and the air is preheated to 500°C by a recuperator using CMR flue gas. It is likely that the flow reverse reactors would require similar concentration of methane for continuous operation. The ~5.5 MW MEGTEC TFRR pilot-scale demonstration plant at West Cliff has been commissioned, but the commissioning results have not been published. Recently, Biothermica Technologies Inc. has been trying a VAMOX system for the VAM mitigation at Jim Walter Resources’ Mine No. 4 in Alabama [
7]. In fact, the VAMOX system is also a type of TFRR, but its configuration in a U shape is somewhat different from that of the MEGTEC system. Dürr System, Inc. has recently promoted an Ecopure
TM RTO system for the VAM oxidation [
8], but it seems there is no real trial on ventilation air yet.
Lean-burn gas turbines being developed across the world include EDL’s recuperative gas turbine and CSIRO’s lean-burn catalytic turbine. The Energy Development Limited (EDL) gas turbine can operate continuously when methane concentration exceeds 1.6%, and the air is preheated to 700°C before combustion. It requires the addition of substantial quantities of methane to the ventilation air to reach a sufficient concentration. Unfortunately, EDL development work on this turbine technology has ceased. Reduction of the minimum methane concentration at which a turbine system can operate has substantial advantages in reducing the reliance on supplementary fuels.
CSIRO designed a 1% methane catalytic combustion gas turbine system [
9] based on methane catalytic combustion experimental data and the design criteria for a turbine system. A 1% methane turbine can use a much greater proportion of ventilation air compared with a 1.6% methane gas turbine unit. A 25 kWe demonstration unit has been successfully commissioned at CSIRO’s QCAT laboratories, and the experimental results have demonstrated this lean burn catalytic gas turbine technology process. Thermodynamic analyses indicate lean-burn turbines can be operated at lower methane concentrations, perhaps to 0.8% [
10], but it may be difficult to generate power efficiently at this concentration depending on heat recover effectiveness.
Biofiltration has been used for landfill gas and has been studied for the biologic oxidation of VAM. Based on published data, the space velocity of the biofilter is approximately 1/
h, which requires an extremely large compost size to take the required amount of ventilation air. The cost of mitigating methane emissions using a biofilter from animal husbandry is about USD$ 26 per tonne of CO
2 equivalent reduction. Researchers will need to overcome this issue to make this technique cost-effective for VAM mitigation [
11,
12].
Concentrators have been applied in several industries to capture volatile organic compounds. A concentrator could enrich methane in mine ventilation air to levels that meet the requirements of utilization technologies, and could act as a buffer to cope with variations in methane concentration and ventilation air flow rate. Experiments conducted by Environmental C & C, Inc. (ECC) on an adsorbent in a fluidized bed concentrator were disappointing, and trials have ended. CSIRO has investigated a new concept for enriching methane in ventilation air [
13].
Shengli Engine Machinery Plant has developed and demonstrated a lean burn gas engine technology at mines in China, which takes the poor quality drainage gas of a methane concentration of higher than 7% but safety is of a concern because its pipelines operate in the explosive range of 5%-15% methane. Also, this technology cannot use poor drainage gas with a methane concentration of less than 7%.
Ventilation air methane catalytic turbine (VAMCAT)
Since the research program began in 2001, different phases of the technology innovation and development have been conducted in CSIRO: the methane catalytic combustion experimental study, process design and simulation, research and development of the 25 kW prototype VAMCAT unit, and current mine site trials of the 25 kW turbine unit. The development of this novel turbine system has required multi-disciplinary skills in the areas of thermodynamics, combustion, catalyst, turbo-machinery, aerodynamics, heat transfer, fluid dynamics, high speed reduction gearbox, high speed shaft coupling, and control and monitoring etc. Several main tasks for developing the 25 kW prototype unit were undertaken in collaboration with a number of Chinese partners including Shanghai Jiaotong University.
Design of VAMCAT system
As the methane is always contained in the mine ventilation air, there should be no by-pass air and no cooling air for the turbine. Detailed ventilation air methane stream characteristics can be found in Ref. [
14]. Then, based on an analysis of the methane catalytic combustion experimental trials of commercial catalysts [
15], a process design for the ventilation air methane catalytic combustion gas turbine (VAMCAT) has been devised by CSIRO [
9]. This turbine system can be powered with about 1% methane in air, perhaps lower to 0.8%, and it can also be run at a higher methane concentration depending on application site specifications.
The process design for the low concentration methane catalytic combustion gas turbine system is relatively simple conceptually. The dilute methane stream is compressed, and then heated in a recuperator using the turbine exhaust gas, and then enters into the catalytic reactor, and the combusted gas is then expanded in a turbine. However, there are certain constraints on the design required to utilize the very low energy density fuel. Heat recovery from the exhaust gas is needed to preheat the air up to the temperature required by the catalytic combustion process, and this is feasible even when the compressed feed is at relatively low temperatures. This fits well with a decision to restrict the feed temperature to the turbine to less than 850°C, so special materials are not required in the turbine and cooling air is not required. The system is more efficient with lower compression, and the optimal design is a balance between getting a net power output and realistic temperatures. Figure 2 illustrates the ventilation air methane catalytic combustion gas turbine process for mitigating and utilizing the ventilation air methane.
The VAMCAT system is an unique system mainly for the mitigation of fugitive methane to reduce greenhouse gas emissions; meanwhile, the heat is recovered for generating power. Hence, this turbine system differs from the conventional gas turbine cycle system in a number of aspects as compared in Table 2. Detailed thermodynamic analysis of the VAMCAT has been presented in Ref. [
16], and major thermodynamic parameters of the VAMCAT system are summarized in Table 3.
25 kW prototype VAMCAT demonstration unit
With the support of the Department of Climate Change under the Australia China Bilateral Climate Change Partnership the 25 kWe prototype VAMCAT unit has been designed, manufactured and commissioned in CSIRO Laboratories from June 2006 to September 2009. Several main components for the 25 kW prototype unit including the compressor, turbine, high speed reduction gearbox and recuperator, were designed and manufactured in collaboration with a number of Chinese partners. Figure 3 displays the 25 kW VAMCAT unit assembled at CSIRO QCAT Laboratories. An electricity sink consisting of a bank of heating elements was used to destroy the electricity generated by the unit. The commissioning results demonstrated the VAMCAT technology process, and the unit can be operated with about 1% methane in feeding air stream with a power output of approximately 25 kW.
Mine site trials and demonstration of the 25 kW prototype VAMCAT unit
With the support of the Department of Climate Change under the Australia China Bilateral Climate Change Partnership, CSIRO works with Huainan Coal Mining (Group) Co. Ltd. to conduct site trials and demonstration of the 25 kW prototype VAMCAT unit. The site trials project began in March 2010, and it is planned that the trials will be completed in December 2011. The effect of the characteristics of mine ventilation air flow on the performance parameters of the unit will be tested. Xieyi Coal Mine has been selected for the site trials, and site infrastructure construction is nearly completed. Figure 4 exhibits selected ventilation air shaft and drainage gas plant for the site trials of the 25 kW prototype VAMCAT unit at Huainan.
Conclusions
With the aim of developing more efficient, cost-effective technologies for mitigating and utilizing the fugitive coal mine methane, this paper reported major research results obtained so far at CSIRO on the novel lean burn catalytic combustion gas turbine, which can be powered with about 1% methane (volume) in air.
The 25 kWe demonstration unit has been successfully commissioned at CSIRO Laboratories, and the experimental results have demonstrated this lean burn catalytic gas turbine technology process, and the unit can be operated with about 1% methane in feeding air stream with a power output of approximately 25 kW. Mine site trials and demonstration of the 25 kW unit is underway at Huainan Coal Mining Group, and it is planned to be completed in December 2011.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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