Article pubs.acs.org/est
Enrichment of Ventilation Air Methane (VAM) with Carbon Fiber Composites Jun-Seok Bae, Shi Su,* and Xin Xiang Yu CSIRO Energy Flagship, 1 Technology, Pullenvale, Queensland 4069, Australia S Supporting Information *
ABSTRACT: Treatment of ventilation air methane (VAM) with cost-effective technologies has been an ongoing challenge due to its high volumetric flow rate with low and variable methane concentrations. In this work, honeycomb monolithic carbon fiber composites were developed and employed to capture VAM with a large-scale test unit at various conditions such as VAM concentration, ventilation air (VA) flow rate, temperature, and purging fluids. Regardless of inlet VAM concentrations, methane was captured at almost 100%. To regenerate the composites, the initial vacuum swing followed by combined temperature and vacuum swing adsorption (TVSA) was applied. It was found that initial vacuum swing is a control step for the final methane concentration having 5 or 11 times the VAM enrichment by one-step adsorption, which is, to our knowledge, the best performance achieved in VAM enrichment technologies worldwide. Five-time enriched VAM can be utilized as a principle fuel for lean burn turbine. Also, it can be further enriched by second step adsorption to more than 25% which then can be used for commercially available gas engines. In this way, the final product can be out of the methane explosive range (5−15%).
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INTRODUCTION Methane emissions from coal mining activities worldwide were estimated at 588.6 MtCO2-e for 2010 and are expected to be 629.7 MtCO2-e for 2015.1 About 60−70% of total methane emissions from coal mining activities come from ventilation air methane (VAM), which is a primary methane emitting source.2 To reduce the fugitive methane emissions from underground coal mines, it is important for stakeholders to consider the emitted methane as an energy source for practical and profitable use, rather than a safety hazard.3 The capture, mitigation, and utilization of VAM have been an ongoing challenging issue because of the huge air volume flow rate (approximately 120−600 m3/s in a typical gassy mine in Australia) and the low and variable methane concentration (typically 0.2−1 vol %). VAM can be used as a supplemental fuel for various combustion systems (i.e., ventilation air is used as combustion air) at mine sites, and also be used as a principal fuel for flow reversal reactors and catalytic lean burn gas turbines which may require additional supplementary fuel for stable operation depending on the VAM concentration.4 Alternatively, VAM can be enriched to a level where advanced technologies such as gas engines and lean burn turbines can utilize the captured methane to produce power. Usually, commercially available gas engines require a minimum of 25% methane. Apart from conventional separation methods, other technologies such as hydration separation5 and ionic liquids (ILs)6,7 have been studied to enrich methane concentration or to separate methane from its mixtures. As described in the © 2014 American Chemical Society
Supporting Information (SI), they are not suitable for VAM enrichment. However, adsorption technology with solid adsorbents is a viable option for VAM enrichment, as most methane in ventilation air was initially in the adsorbed phase in coal seams, and subsequently desorbed from coal matrix in the course of mining activities. In our previous small-scale study with simulated VA,8 we have achieved 98% CH4 removal efficiency with our honeycomb monolithic carbon fiber composites, whose CH4 adsorption capacity was almost two times greater than that of commercial activated carbons. The efficiency of VAM capture with adsorption technology and the captured methane quality are mainly dependent on the adsorbent capacity and the capture processes involved. It is beneficial for adsorbents to possess hierarchical pore structure including micropores and macropores for capture capacity and mass transport, respectively. Such pore structure can be found in carbon fiber composites (mixture of carbon fiber and phenolic resin)9,10 and carbon/silica composites,11 which can eliminate diffusion resistance to micropores. Also, any solid adsorbents need to be in a certain form to be used for fixed bed applications. Monolithic carbons have an advantage over conventional granular carbon beds because the former has much higher thermal conductivity and better resistances to various environmental conditions than the latter, especially for Received: Revised: Accepted: Published: 6043
January 13, 2014 March 31, 2014 April 30, 2014 April 30, 2014 dx.doi.org/10.1021/es500025c | Environ. Sci. Technol. 2014, 48, 6043−6049
Environmental Science & Technology
Article
thermal regeneration.12,13 For a process level, one also needs to consider a flow resistance (i.e., pressure drop) through a fixed bed. Thus, the void volume and the adsorbent packing density in the bed can be limited to a certain level to minimize the pressure drop particularly in dealing with high volumetric flow applications such as VA. In this regard, honeycomb monolithic (HM) structures, which possess continuously open multichannels along the adsorbent length, have been adopted to have a high void fraction, a large geometric surface, a low pressure drop, a high dust tolerance and a large contact area.8,14−16 Adsorption processes with solid adsorbents have been extensively applied for gas separation and purification, including vacuum swing (VS), pressure swing (PS), temperature swing (TS), and vacuum pressure swing (VPS).17−20 For the case of VAM enrichment, pressurizing the huge volumetric flow may not be a practical and economic option. Thus, the operating pressure should be close to atmospheric pressure. Attempts have been made to enrich VAM with the VPSA process at pressures between 250 and 25 kPa (absolute), resulting in the final product with less than 1% CH4.21,22 However, apart from the pressurization step, this level of enrichment is insufficient to be utilized as a principal fuel for gas engines and even for lean burn gas turbines to have stable operation. In this work, HM carbon fiber composites were fabricated and used to capture CH4 from simulated VA at various conditions with a prototype VAM capture unit. A new vacuum and temperature swing process is employed to desorb CH4 from the composites, involving initial VS and then a combination of TS and VS (TVS). The VAM capture performance with the HM carbon fiber composites is presented in this work.
Figure 1. Large scale HM carbon fiber composite adsorbent in a rectangular shape.
methane breakpoint (0.01 vol %) occurred. When saturated with VAM, the composites were regenerated for the following capture process. The adsorbed methane can be desorbed by various ways, such as pressure reduction (or vacuum) and thermal heat. In this work, VS was first applied and then followed by TVS. The absolute power requirement and the associated energy penalty for vacuum swing are known to be both substantially lower than those of PS.24 After the initial vacuum swing at room temperature, the composites were indirectly heated up to 393 K by passing hot air (433 K). Once the composite temperature reached 393 K, the column was vacuumed to take the desorbed methane out of the column. When there was no noticeable gas stream coming out of the column, while cooled down by cooling water, the composites were purged with fluids, such as helium, nitrogen, and air. Once the composite temperature reached room temperature, the test unit was ready for the next capture process.
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EXPERIMENTAL SECTION Large Size (30 × 100 × 200 mm3) Carbon Fiber Composites. A petroleum pitch based carbon fiber, which is commercially available (Sinocarb Co., Ltd.) and known to be a better template among other carbon fibers tested previously for methane capture,8 was mixed with phenolic novolac resin (Durez 7716, Durez Corporation, U.S.A.) to make a large size honeycomb carbon fiber composites. Once the mixtures were molded, vacuum-dried, and cured at 408 K, they underwent fabrication processes such as carbonization at 923 K under an inert atmosphere and physical CO2 activation at 1223 K, which were optimum conditions obtained from our previous study.8 Although it is known that chemical activation with ZnCl2 or KOH yields higher surface areas than physical activation,23 the latter was used in this study to avoid chemical washing and treatment steps, which add to the cost of composites fabrication. The duration of CO2 activation was varied from 0.5 to 4 h to maximize the methane capture capacity of the composites. Detailed fabrication procedures can be found from our previous study.8 The HM composite adsorbents were fabricated in a rectangular shape as shown in Figure 1 and characterized as described in the SI. Methane Capture Process. With a large-scale test unit which is described in the SI, the columns filled with carbon composites were degassed at elevated temperature (393 K) to remove any preadsorbed components in the composites before the capture process started. For the methane adsorption process, once an inlet methane concentration in the simulated VA reached a target, the simulated VA was fed into the column until the methane breakthrough curve was obtained or the
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RESULTS AND DISCUSSION Fabrication of Carbon Fiber Composites. As the size and shape of composites were changed from our previous small-scale cylindrical carbon fiber composites (30 mm in diameter),8 the duration for CO2 activation was varied from 0.5 to 4 h to find an appropriate condition for the rectangular composites. The methane adsorption isotherms at 298 K are shown in Figure 2. The methane adsorption capacities at 1 atm were found to increase up to 3 h of activation and then to decrease with further activation. As VAM concentrations are very low (
