CLEAN COAL TECHNOLOGIES
For sustainable development of energy sector in Pakistan
By Engr. ABDUL WAHEED BHUTTO
May 16 - 22, 2005
Pakistan has 18 gigawatts (GW) of electric generating capacity. The total power generating capacity has increased rapidly in recent years, due largely to foreign investment, leading to a partial alleviation of the power shortages Pakistan often faces in peak seasons. Rotating blackouts are, however, still necessary in some areas. The rules of the game for generating electricity are changing rapidly. Country's remaining recoverable reserves of crude oil is estimated at 310 million barrels, which indicate that there is no prospect for Pakistan to reach self-sufficiency in oil. Country has 26.8 trillion cubic feet (Tcf) of proven gas reserves, and currently produces around 0.8 Tcf of natural gas per year. While the energy demand is surging in Pakistan, at the same time pressure is building worldwide to reduce greenhouse-gas emissions. Pakistan is looking forward to find ways to overcome coal's present disadvantages due to presence of relatively higher moisture, sulphur and ash contents, so to use this readily-available, indigenous resource to generate clean, reasonably-priced electricity. The one way to overcome this problem is to convert coal from solid to a clean gas, which can then be burned like natural gas. When linked with modern combined-cycle turbines, gasification is one of the most efficient and environmentally sound ways of producing electricity coal. Coal IGCC offers numerous benefits for the environment, power producers and consumers. This technology can help diversify the fuel supply and help balance the Pakistan's future dependence on foreign sources of energy.
The Pakistan government has announced that a new coal gasification plant to be set up in Bhakkar, Punjab, will produce 3 million cubic feet of gas per day, consume 0.8 metric tonne per day of coal and produce around 0.35 metric tonne per day of coke. Sui Northern Gas Pipelines Ltd (SNGPL) is responsible for the construction of the gas plant, which will use locally available coal. The process involves a low cost plant for supply of gas to small towns situated in the vicinity from Makarwal and Salt range coal mines. The plant would be the first one in the chain of coal gasification plants foreseen for future energy use in Pakistan. The use of indigenous coal as a source of energy would help create new jobs in coal mines and plant operation. It would improve the quality of life and stimulate the economic growth. The estimated cost of the coal gasification plant was around Rs250 million and would produce gas by adopting the "vertical retort destructive distillation process".
The Sindh provincial government is also shown interest in development of coal deposits in the Thar desert using underground gasification technology to develop a 1200MW power station and coal washing plant at the adjacent Lakhara coal deposits, enabling the cement industry to use local coal.
Based on common practice elsewhere, the most likely technologies to be appropriate power generation from Thar lignite are either pulverised coal combustion with flue gas recirculation, or circulating fluidised bed combustion, probably with limestone addition to the bed.
The possible uses for local coal in addition to power generation are seen to be: as fuel in the processing industry, including cement, sugar and fertilisers; making smokeless briquettes, replacing kerosene oil for domestic use and providing an alternative heat source for high altitude areas; making towns gas as an alternative to syngas gas and liquefied petroleum gas (LPG); gasifying the coal in-situ to supplement natural gas resources; extracting and using coal bed methane; and, producing substitute natural gas on the surface.
One of the goal of the country is the development of the necessary technology to provide secure, reliable, affordable and environmentally-sound source of energy. A secure sources is important to ensure the economical stability and growth, and reduce future environmental impacts associated with the power generation in the country. Potentially expanded use of abundant coal reserve is one key to a secure and affordable source of energy in the country.
Coal has been used as an energy source for hundreds of years, fueled the Industrial Revolution of the 19th Century and also was the main fuel for the speedy development of electricity generation in 20th century. Until the 1960s, coal was the most important source of the world's primary energy. In the late 1960s, it was overtaken by oil, natural gas and nuclear. However, since then coal's share of primary energy supply has declined. Nevertheless, coal demand has grown steadily over the past thirty years and it is still essential to global economic and social progress. Currently, coal accounts for about 25 percent of commercial energy demand worldwide and some 38 percent of the world's electricity generation.
State-of-the-art developments in coal fired power plant have the capacity to significantly reduce the emissions to air from coal combustion and reduce the environmental impacts from waste disposal. There are many technological options that have been developed to reduce emissions from combustion, many of which have been deployed and are proving highly economic.
The selection of environmentally-friendly technologies requires evaluation and optimization of several technical, environmental and economic factors, including quality of coal, waste disposal requirement, yearly operating time and operating lifetime of the plant. It will be necessary to develop and commercialize technologies capable of producing electricity at significant higher overall system efficiencies. To achieve the overall efficiencies of above 40-45 percent in an environmental acceptable manner, development and demonstration of the advance technology is paramount.
Use of washed-coal in power production is one of the most cost-effective mean to reduce environmental impact. Coal cleaning reduces the ash content of coal and of substances such as inorganic sulfur and sodium associated with corrosion and deposition in boilers. Besides the use of washed coal offers several other advantages to the plant owner, such as increase efficiency and availability, less wear and lower maintenance cost, and reduced waste generation at the plant.
Coal quality is a major influencing factor in the design of the power plant, especially if high ash coals have to be used. Coal cleaning increases the generation efficiency, mainly due to the reduction in energy loss as less inert material passes through the combustion process, reduction of load on the particulate removal equipment, and reduction in the amount of solid waste that has to be taken care of, at the plant.
Fluidized-bed combustion (FBC) technology allows the bed temperature to be held significantly lower than the combustion temperature in a conventional coal-fired power plant. The lower operating temperature reduces Nitrogen Oxides and Sulfur Oxides formation. The Sulfur Oxides emission is further reduced by the presence of calcium based limestone or dolomite. The fuel flexibility of FBC boilers is extremely wide, probably the widest of any power generation technology. One single boiler can be designed for a wide range of fuels such as biomass, peat, lignite, and hard coal together or separately. Sulfur Oxide emissions are low since sulfur is captured in the bed by the injection of limestone.
The Pressurized Fluidized Bed Technology (PFBC) plant permit the use of deeper combustion bed and slower fluidization velocities, which results longer residence time through the bed . This in turn gives rise to greater combustion efficiency, more effective sulfur capture, lower temperature and high pressure result lower Nitrogrn Oxide emission.
Gasification of coal is an old, and well proven technology that has attracted renewed interest and is now undergoing modernization. In the early part of this century a large number of coal gasifiers were commercially operated in several countries. With the availability of abundant and inexpensive natural gas and crude oil, most of the coal gasification units were abandoned by mid 50's. The oil crises situation and together with the potential capability of integrated gasification combined cycle power plants to compete favorably with direct coal combustion for electric power generation have significantly increased the attractiveness of coal gasification technology and reduced its economic risk.
The major advantage of gasification is that coal is converted into a gaseous fuel which is easy to handle and is a clean form of energy. In the gaseous form, it enables to substitute petroleum products and natural gas. The synthesis gas has wide range of applications. It can be used in a combined cycle system for efficient and clean generation of electric power. It is suitable for the manufacturing of hydrogen and chemicals such as ammonia, methanol, acetic acid; as substitute natural gas, and as a reducing gas for metallurgical purposes etc. It can be used in multipurpose plants for the simultaneous production of electric power, chemicals/fertilizers and fuels which also improve the economics of coal gasification. In the gasification process sulfur present in the coal is converted to hydrogen sulfide (H2S). These sulfur compounds can be easily and economically removed from gas streams by a wide variety of commercially available processes. The particulate content in the fuel gas after gasification is negligible since the gas cleaning steps capture almost all the particulate. A broad range of coals with varying ash content, coal fines can be successfully gasified.
In a gasification process, electricity is produced in a gas turbine fueled by a synthetic gas produced by the partial oxidation of coal in a gasifier. Steam, produced by synthetic gas cooling, drives a steam turbine. Coal ash is removed as slag from the gasifier. The major drawbacks are that the process is very complex, it requires a large surface area and there is very little commercial experience of operation. The investment cost is also very high. The construction time is expected to be four years.
Integrated Gasification Combine Cycle (IGCC) power production technology is designed to meet the higher efficiency and stringent environmental regulations required in the 21st century. IGCC systems have the potential to compete economically with conventional coal-fired steam plants and have lowest possible level of pollution. As environmental control requirements increase, the economic advantages of IGCC would correspondingly increase. Similarly, with further developments in coal gasification and gas turbine technologies taking place, the economic and performance benefits of IGCC would increase significantly. The efficiency of IGCC which is now around 40-45% is likely to increase to 55-60%.
Throughout the world, coal play an expanded role in the production of the affordable energy necessary to meet the demand of the economic development and growth. The negative effects of coal on global environment, eco-systems and public health are well documented, its use must be balanced between the development needs of a country and the welfare of its people and land. Greater efficiencies will reduce emissions and prevent waste generation, and must be implemented in the short-term period. Planning should strive for increased utilization of by products and waste. Also, the production of liquid and gaseous fuel from coal will be required to effectively meet the broad spectrum of the future energy needs.
Coal is expected to remain the dominant fuel for electricity generation worldwide. Coal is already a cheaper source than any other fossil fuels. As a result of continually improving clean coal technologies, coal will be used more and more efficiently. New technologies will bring the significant contribution to the goal for environmental emission control to which all countries are committed through the Kyoto agreement. To achieve the global application of clean coal technologies, the transfer of new technologies to developing countries is pre-requisite. World reserves of coal are abundant compared with oil and natural gas, are widely dispersed and are estimated at close to one trillion tonnes.
The coal resources which country has substantial reserves, their utilization technologies will be vital factors in shaping economic and social progress. At present, our total coal reserves are estimated around 184.5 billion. The Thar region is said to contain 250 years' worth of coal reserves. The overall use of energy in Pakistan is around 70 million tonnes of coal equivalent (Mtce) per annum, and it is estimated that it will grow to over 90 Mtce by 2008. Advanced technologies when applied to Pakistan's coal resources, can improve the efficiency and minimize environmental impacts of coal utilization.
Since reserves of oil and natural gas are meager, they need to be substituted with coal to the extent feasible. At the same time all the three fuels, specially coal needs to be conserved for the future generations. The energy sector, requires efficient, clean and dependable energy supplies. Hence coal has to be utilized with multi-pronged strategy i.e. higher efficiency, environmental acceptance, prolonging its availability and as replacement for oil etc. which is possible only through sustainable development and gasification is the best option to achieve it.
In the country the only coal-fired power plant, 150MW FBC power plant Lakhra is facing serious operating problems. Development of the Thar coal field has not been initiated to date. No further coal-based power generation capacity has been added in the power sector after FBC Lakhra
Using the Thar lignite directly in the cement, fertiliser or sugar industries would involve either transporting the fuel to existing and remote industrial sites, or building new factories and infrastructure near the mine. Thar lignite is likely to be used at or near the mine-mouth, both because of its high inserts content and its probable tendency to spontaneous combustion.
The coal industry in Pakistan is small, and its knowledge base and troubleshooting resources are extremely limited. There is no body of people equivalent to the 'coal communities' that have arisen in the major coal producing/using countries, who have built up the necessary experience to address the various problems that inevitably arise with coal use. This is one of the issues that needs to be addressed if local coal reserves are to be successfully exploited using a combination of indigenous skills and the appropriate purchase of foreign consultancy.
Pakistan has rich coal resource, it also has huge potential in wind, solar and other new energies. It's our inevitable choice to vigorously develop clean coal generation to make best use of power generated by affordable new energy and to realize a scientific and rational power structure in which multi types of energy supplement each other.
The author holds Engg. (Environment), B.E (Chemical) and Assistant Professor, Dawood College of Engineering and Technology, Karachi.