Harnessing of Coal energy through Underground Coal Gasification (UCG) in India - Opportunities and Challenges in Processing and Utilization of Syngas

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1 Paper ID : Harnessing of Coal energy through Underground Coal Gasification (UCG) in India - Opportunities and Challenges in Processing and Utilization of Syngas Jain, P.K., Pal, A.K., Kumar Aditya and Sharma, R.K., Institute of Reservoir Studies, ONGC, Ahmedabad , pkongc@gmail.com Abstract India is blessed with large reserves of deep seated coal/lignite which can not be exploited economically with the available technology at present. The gap in demand and supply of energy is on rise for the nation. This led ONGC and Gujarat Industries Power Company Ltd (GIPCL) to join hands to exploit energy through unconventional source. Underground coal gasification (UCG) is a way of accessing the un-mineable coal for its energy contents and/or as a chemical feed stock. The coal is converted into syn-gas insitu and the gas is produced on the surface. ONGC expects to commission its first UCG pilot module by 4th Qtr of at Vastan coal mines in Gujarat in association with GIPCL. The technology for UCG is being acquired from a renowned mining institute of Russia... The site is selected through a process of detailed characterization of the lignite seams. Detailed engineering design for the surface layout has already been accomplished. The requisite approvals are being obtained. Like any other gasification, the syngas in a UCG trial also needs to be cleaned up to make it usable. The extent of cleaning required depends on the end use of the gas. The prevailing emissions and waste disposal regulations at the location concerned also determine the equipment / process to be used for cleaning.the design of surface facilities for the Vastan pilot is aimed at maintaining the optimal parameters essential for stabilized gas composition and calorific value (CV). The ultimate goal is development of the process guidelines and inputs for designing an environment friendly commercial plant. Major challenges in gas processing for the successful trial include identification of end usage of low CV syngas, separation of sticky condensate from gas, handling of carbon di-oxide and ultimately turning it into an economical proposition. Opportunities during pilot run are securing technical and economic parameters of syngas processing and determination of its economic utilization and most vital being the indigenization of the technology. The paper deals at length with likely difficulties to be handled and benefits to be accrued in the syngas processing in the trial. It covers pilot specific UCG process and facilities required at surface to refine the syngas. Introduction Underground coal gasification (UCG) is a gasification process applied to un-mineable coal seams using injection and production wells drilled form the surface, which enables the coal to be converted in-situ into combustible gas. ONGC expects to commission its first UCG pilot module by 4th Qtr of at Vastan coal mines in Gujarat in association with Gujarat Industries Power Company Ltd (GIPCL). The technology for UCG is being acquired from a renowned mining institute of Russia... The plant will be using lignite seams for gasification. The site is selected through a process of detailed characterization of the seams. Detailed engineering design for the surface layout has already been accomplished. Ministry of Environment and Forest (MoEF) has awarded environment clearance for the pilot. Other requisites for trial such as mining lease and land acquisition are under progress. The syn gas is likely to consist of carbon mono-oxide, carbon di-oxide, hydrogen, methane, N 2, H 2 S and moisture if gasified with air. The calorific value (CV) ranges from Kcal/ m 3. The CV gets diluted due to presence of N 2. 1

2 As syngas consists of number of impurities and depending upon its specific end usage, the gas needs to be cleaned. Broadly, processing of syn gas involves separation of dust particles, cooling for removal of condensate products, separation of acid gases to the level below those typically required by environment regulating agencies, followed by drying of gas before any end usage. Further steps in the process are dependent on end usage such as power generation, chemical synthesis, fertilizers etc. Processing of UCG syn gas The product syngas from Vastan pilot is expected to contain many undesirable components including acid gases, particulate matters, condensates such as tars, phenols, N 2 compounds and halides. The required level of treatment and optimal technologies will vary according to the ultimate disposition of the syngas. Purification of the syngas during pilot (Fig.-1) The hot syngas at temperature C exits gasifier and enters surface heat exchanger. The gas passes through its tubes and water is pumped through its shell. The heat exchanger acts as cooler cum separator. The condensate gets separated from gas and collected in the storage tanks by condensate pumping station. There is provision of heating the condensate when it is shipped to some other place by condensate supply pumping station. The hot water at 70 0 C temperature from the heat exchanger goes to cooling tower through its residual pressure where it is cooled to 45 0 C temperature. The cooling tower is 2 section fan driven type. Make up water is required to be added in the cooling tower to compensate for evaporation loss, water carry-over and blow down. Air Steam Compressor Gasifier Syngas Heat Exchanger Syngas Air Enclosed Flare stack Condensates Water (45 o C) Decontamination Unit Water (70 0 C) Storage Tank Cooling Tower Fig.-1: Process Flow Diagram at Vastan UCG Pilot The syngas from the surface heat exchanger enters the ground flare stack for flaring. In the course of operation of circulating water supply system under the indicated temperature mode additional treatment of water of the circulating system is made in order to prevent corrosion and generation of insoluble residue on the heat exchanger surfaces. Treatment with scale inhibitor & corrosion inhibitor is envisaged to prevent scale formation and corrosion of the circulating water supply system. 2

3 Challenges and opportunities in the processing and utilization of UCG syngas (1) Identification of end-usage The syngas produced through lignite based air fired UCG trial has been observed to be a low calorific value gas. The syngas from Vastan trial is also expected to be low CV gas. The objective of the pilot is to verify the gas composition, its CV and variation in these characteristics. It is proposed to flare the syngas during pilot run of 7-13 months as it is difficult to utilize the product gas for a short span of time. However, minimum purification of the syngas is required to avoid any adverse impact on the environment during pilot also as the prevailing emissions and waste disposal regulations at the location namely Surat also determine the equipment / process to be used for cleaning. From the experience of UCG pilots worldwide, it has been observed that air fired UCG systems have restricted end use options. Presence of higher concentration of N 2 makes the syngas unsuitable for liquid fuels The options such as Fischer-Tropsch (FT) liquids or synthetic natural gas (SNG) require low N 2 concentrations. The high N 2 concentration in the air fired UCG gas make CO 2 removal prohibitively expensive... In such system, end use is restricted to power generation. Based on the economics and complexity carried out internationally, an air fired UCG plant with power generation is the simplest UCG/end use option for a new UCG venture like Vastan. Of the UCG configurations evaluated (air-fired, oxygen-fired, and steam/oxygen-fired), the air fired system has been observed to be the cheapest in terms of cost per unit of energy. Capital costs are lower than other configurations because of huge capital costs associated with the air separation and steam plants associated with the other UCG configurations. There are basically two options in this regard-direct power generation and IGCC. Both end use systems generate electric power. IGCC possesses greater efficiency; however, the process is more complex and the capital costs are greater. CO 2 separation adds cost to the total budget. At the time of commercialization of the technology, end usage of syngas is likely to be determined on the basis of economics of the process involved of that specific end usage. (2) UCG syngas clean up The clean-up has not yet been attempted at commercial level anywhere in the world. In the existing commercial plant or pilots run so far worldwide, UCG has mostly been attempted with air combustion resulting in low CV gas. The syngas thus produced is used either in conventional power plants directly without any significant clean-up for steam generation or syngas with high N 2 for liquid fuels production as is the case in Chinchilla. Extensive and relatively expensive gas clean up has been in practice for surface gasification where gas contains heavy tars and much particulate matter. Gas analysis of UCG syngas, however, shows that the gas is much cleaner. It contains less problematic condensed liquid and also particulate matter. Sulfur is produced in the form of H2S which is easier to scrub. Product gas temperatures usually range between K. Thus high temperature clean up is not required. It is widely accepted that the existing technology itself is adequate in clean up of UCG syngas. This may be considered only a development problem. As far as UCG pilot erection in the country is concerned, clean up technology in the local chemical industry is unknown. However, deliberations with the expert resulted in the conclusion that the equipment associated with the clean up can easily be made available within the country, at-least for the pilot. No licensed technology is required as the syngas is targeted to be flared after cooling and removing the condensates. Major surface equipment for gas clean up during pilot at Vastan are: Surface heat exchanger for cooling syngas and separation of condensates Flare stack for ground flaring of syngas Cooling tower for cooling hot water from surface heat exchanger Storage tanks for condensates as condensates are not proposed for any processing during the pilot run 3

4 Decontamination of process water Compressors for air injection and other associated processes Pumps and dosing tanks Chemistry lab and auto control room Petrotech-2010 The equipment are commonly used in the domestic chemical industry and can be made readily available in the market. The application of domestic equipment in the pilot and subsequent commercial trial is expected to result in the indigenization of the clean up process of the UCG syngas. 2.1 Clean-up for IGCC during power generation The extent of syngas clean up for gas turbine generator (GTG) use will be largely determined by the emission regulations of the regulatory body holding jurisdiction over the facility site and the fuel specifications of the gas turbine supplier. These requirements typically address permissible levels of H 2 S of N 2, and particulates and present no special difficulties for UCG syngas. Contaminant Particulates/ char Tars HCl S (H 2 S, COS) Na K Other trace metals Typical allowable Conc. 5 mg/ m3 <0.5 ppm 1 ppm Table-1 Gas turbine fuel gas contaminant specifications Raw syngas is corrosive and in order to reduce the need for expensive corrosion resistant alloys will generally require some level of treatment including means to avoid condensation before it can be economically transported via pipeline. 2.2 Clean-up during gas to liquids process The feed gas specifications for liquid hydrocarbon synthesis via the Fischer Tropsch (FT) pathway are extremely stringent due to the sensitivity of the reaction catalyst. In particular, sulphur compounds and metals should be removed to very low levels. In practice, requirements vary according to process, catalyst, and licensor, and are subject to economic trade offs and so it is not possible to provide a single and universal specification. However, a typical FT feed gas specification is given below: Contaminant Typical allowable Conc S volume N 2 compounds volume Halogen compounds < 10 ppb volume Alkaline metals < 10 ppb volume Solids (including soot, ash and Total removal dust) Organic compounds ( tars Removed below dew including BTX* ) point** Class 2 organic compounds*** volume *BTX (Benzene, Toluene, Xylene) **Removal to a level at which no condensation will occur during FT compression ***Class 2 tars comprise of phenol, pyridine, thiophene 4

5 Table-2: Typical FT feed gas specifications In addition; High concentration of inert components will result in lower yields due to purging requirements. The H 2 to CO ratio requirement of the syngas varies from about 1.3 to 3 depending on the GTL technology employed. (3) Removal of particulates and tars UCG tends to produce fewer particulates in the syngas. The technology for removal of particulates and tars is well established. It involves the use of cyclones, bag house filters and electrostatic precipitators. These equipment, however, are not proposed for the Vastan pilot. The surface heat exchanger acts as cooler and condensates separator. There is likelihood of HX tubes getting clogged by condensates. However, provision of heat exchanger bypass for the syngas is available to tackle the problem. (4) Low CV syngas The product UCG gas is a mixture of H 2, CO, CH 4, CO 2 and higher hydrocarbons. The CV is in the range Kcal/ m3, for gasification with air injection (compare it with natural gas CV of 9000 Kcal/ m3). Using enriched air and controlled water/ steam injection using pressure swing technology, the CV of the syngas can be increased to about 3000 Kcal/ m3. As the quality of syngas decreases with the age of the cavity, in the commercial projects, many pairs of injection and production wells operate simultaneously, with new pairs coming on line as the coal in the old cavities gets burnt, to ensure a reasonable consistency in the quality of the gas. Sometimes, if available, the syngas is spiked with controlled quantity of natural gas to enhance the CV. (5) Variability in Gas quality and Gas production rates The UCG process is an unsteady state process. A wide variability in gas quality and production rates has been observed on an hourly or daily basis in many field experiments. The need of constant flow rate has been tackled by installing flow control valve in the production line and by injecting constant flow of air. At few pilots, 5 to 10% variation in the CV of syngas has been observed on daily basis. This falls within the acceptable limits for firing of the syngas in the large boilers. (6) Removal of sulphur compounds In the Vastan pilot, the whole of sulphur is expected to separate out during cooling of syngas in the surface HX and will remain with the condensates. However, it will be essential to remove the sulphur during commercial trial. Technology for the removal of sulphur compounds is well established. Collectively known as AGR (acid gas removal) technologies, these include absorption of the slphur containing compounds (mainly H2S and COS) by solvents such as methyldiethanolamine (MDEA process), dimethylethers of polyethylene glycol (Selexol process) and methanol (Rectisol process). In addition, catalyst-based technologies are also available for the removal of sulphur compounds using zinc oxide catalysts. (7) Safety concerns UCG is a promising technology for utilization of coal/ lignite lying idle underground. The process is a combination of mining, exploitation, gasification and syngas clean up. Some of the benefits include increased worker safety, no surface disposal of ash and coal tailings, low dust and noise pollution, low water consumption, larger coal resource exploitation and low methane emission to atmosphere.ucg involves some environmental impacts such as land subsidence and ground water reserve pollution, which serve as disadvantages. UCG syngas contains hazardous gases like CO and H2S. The clean up process and emergency response should address all the safety related issues. Means for detection of leakage of hazardous gases are installed at strategic locations in the plant and appropriate operating procedures are recommended in the event of leakage of gases. UCG could be covered under the Mines and the Oilfields Act (Regulation and development) Act, 1948 and Petroleum and Natural Gas Rules As UCG is still in its formative stage in India, there are 5

6 no UCG specific regulations and all regulations applicable to Oil and gas and mining operations are to be followed while carrying out UCG activities. Hence the project take utmost care in complying with the standards prescribed in the applicable Acts, Rules and Regulations and their amendments. Conclusions: With six operational pilot projects across the globe and three more under construction, the underground coal gasification (UCG) industry is pressing toward full-scale commercial development. Today, the technology is potentially at the transition point to main stream commercial implementation. The advantages of UCG are readily apparent: Savings from not having to mine and transport coal, capital costs lower than conventional plants, no ash disposal, and ready CO 2 storage. Moreover, there is an abundance of coal reserves not economical for mining that might be gasified below ground... SYNGAS is a versatile product that can be utilized in efficient power generation, transformed into synthetic natural gas or liquid fuels, or used as chemical feed stock. Although there are some difficulties associated with UCG implementation in India, particularly in processing of syngas, these are manageable. The technology could potentially provide all of the benefits of IGCC power generation, but with no coal mining and no surface gasification plant required. Also, UCG potentially has a built in capacity for CCS. Acknowledgements The authors wish to express their gratitude to Dr. R. V. Marathe, Executive Director-HOI, IRS for allowing us to present the paper and providing valuable suggestions during the course of the study. Sincere gratitude is due to the Petrotech Society for accepting the abstract. Authors express their sincere thanks to all the members of the UCG Group, IRS for their critical review and fruitful discussion during the course of this work. The authors also thank GIPCL officials for providing valuable inputs during various interactions held. References 1. Viability of Underground Coal Gasification in the Deep Coals of the Powder River Basin, Wyoming, June 2007, GasTech, Inc, Casper Wyoming 2. Underground Coal Gasification Independent Technical Report for International Resource Holdings Ltd, Uhde Shedden Project No Document No: RPT PR-001, Sept Problems solved and Problems not solved in UCG, Gunn Robert D., University of Wyoming and the Energy Research and Development Administration Laramie Energy Research Centre, P.O.Box 3395, University station, Laramie, Wyoming Industry Review and an Assessment of the Potential of UCG and UCG Value Added Products, Price water house Coopers Report for Link Energy Ltd, May Best Practices in Underground Coal Gasification, Burton, E., Friedmann, J., Upadhye, R., LLNL,