Clean coal technology breakthroughs are expected to keep coal a viable option
Awash in a sea of initializations, clean coal has gone from oxymoron to fact of life in the global power generation market
By H. Tod Kennedy
Not so long ago the term clean coal was considered an oxymoron, and a power plant which could produce electricity and offer a combination of reduced capital and operating costs, higher efficiencies and lower emissions seemed like a research and development pipe dream. However, current technology–including coal gasification, fluidized bed boilers, combined-cycle innovations and advances in solid-state instrumentation and control–is changing all this, no doubt accelerated by global deregulation, stiff competition and environmental imperatives.
According to World Bank estimates, if the expected growth in fossil fuel-fired power plants continued without widely deployed pollution abatement technologies, harmful emissions would increase more than four times within the next 20 years and tenfold in the next 40 years, based on present records. Clean coal technologies are needed in three areas:
1. reduction of harmful contaminants of the raw coal,
2. more efficient conversion of coal to electricity and
3. enhanced methods of reducing harmful emissions and waste products.
These approaches include precleaning technologies, advanced boiler systems, particulate removal and flue gas desulfurization equipment, fluidized bed combustion (FBC) systems and integrated coal gasification combined-cycle (IGCC) technologies. When these systems are applied, according to a World Bank analysis, the reduction of particulates, SOx and NOx would be in the order of 90 percent, while also improving the thermal efficiencies of power plants.
These ever-improving technologies, developed in industrialized countries to achieve the cleanest performance results out of a variety of coals, are now impinging on neo-industrializing nations from Asia through to eastern Europe, where demand for electricity far outpaces supply, and often large fuel resources, such as low-rank coals, are readily available but need efficient, non-polluting power plants for generating electricity now and in the future.
According to Shell representatives, “The most practical and economical method known today for reducing CO2 emissions from coal is to increase the energy efficiency of coal utilization technologies.” This is particularly true for power generation where each one percentage point increase in absolute power generating efficiency results in a 3 to 4 percent reduction in CO2 emissions. All this is highly relevant in view of the world`s projected coal-fired generation future. New generating capacity is expected to be about 629,000 MW up to the year 2005, and according to some forecasters more than 50 percent of this added capacity is going to be in Asia, where about 50 percent (160,000 MW) is expected to be coal-based.
Case histories of new installations and upgrading and retrofitting success stories are plentiful. For example, when the Tampa Electric Co. (TEC) of Florida decided on the most efficient and cleanest technology for its 250 MW Polk Power Station, it chose IGCC technology. Charles Black, TEC vice president said, “All together the new Polk plant will be 10 to 12 percent more efficient than conventional direct coal-fired power plants and have far less emissions.”
In Poland, a large modernization/retrofit of a 40-year-old, EC-Zeran district heating and power plant in Warsaw was required to meet emission demands at minimal cost, and the authority decided to install circulating fluidized bed (CFB) boiler technology. Now two units replace three old ones in flow and heating capacity, while turbine efficiencies increased 13 percent due to new blade profiles.
In China it is reported that an estimated (US)$54.2 billion will be spent within the next five years to prevent pollution. About 70 percent of smoke and dust in the air and 90 percent of the country`s sulfur dioxide emissions come from burning coal used for industry and residential heating. However, China consumes only 30 percent of its coal in electric power plants. There is abundant coal in China, with low quality, high ash content, low ash fusion temperature and high sulfur content which cannot be burnt successfully in pulverized coal (PC)-fired boilers but is suitable for CFB boilers. The Chinese are currently building world-class 300 to 600 MW PC power plants but will need to use more advanced technologies.
In Australia coal bed methane gas extraction is becoming an energy source with an important future, since it shares many of the same qualities as natural gas and produces about half the CO2 emissions per ton compared with average power plant coals. In addition, ultra-clean coal technology is being developed by the government-funded CSIRO in a process where ash is chemically removed from raw coal, thus providing a new class of fuel. Underground coal gasification has been developed in which, air and oxygen are introduced and coal is partially burnt in situ. The heat generated by combustion decomposes and gasifies more coal in the seam to produce a combustible gas which is then extracted at the surface.
Fluidized bed boilers
FBC boilers have the ability to burn a range of variable quality coals and other fuels. They can be satisfactorily fueled with low-rank coals, high-sulfur coals and even coal washery waste, sewage sludge and scrap tires–the latter providing an added value in economic disposal of wastes in an environmentally acceptable manner.
The fluidized bed material, usually ash and limestone, is up to 99 percent non-combustible, and the limestone removes up to 95 percent sulfur in the fuel which would otherwise be oxidized into sulfur dioxide–a major pollutant.
Since the FBC operates at temperatures up to 850 C, the amount of NOx formation in the bed is greatly reduced. This provides the means for reliable and economic burning of a wide range of coal types with potential efficiencies approaching 37 percent, even when firing high-sulfur coals. FBC technologies also reduce emissions of SOx and NOx. SOx is captured by the use of limestone in the combusting bed, and NOx is reduced through low-temperature combustion.
The next technological advance is in the CFB boiler technology, in which the coal dust is forced through under pressure, collected in a cyclone and circulated back into the bed. However, this requires a large heat mass in the fluidizer to be maintained to keep it in operation. It also provides more heat recovery in the exhaust gases, thus making it more efficient. The popularity of CFB is increasing due to its ability to burn low-grade fuels, while reducing NOx, SO2, CO, VOC and particulate emissions.
Babcock and Wilcox has developed an advanced design CFB with internal solids recirculation. The IR-CFB compact design is said to have lower capital and operating costs than former types. Provided the fuel stock is of low to medium sulfur content (less than 3 percent), CFB plants offer some of the lowest generating costs when compared with PC, IGCC systems and natural gas-fired, combined-cycle plants.
Pressurized fluidized bed combustion (PFBC) systems have all combustion gases under pressure, and large amounts of heat is available for recovery in combined-cycle operations. PFBC is favored by many since it is based on a conventional coal combustion system (operationally) and is therefore most attractive to the repowering market using existing steam cycle equipment and auxiliary support systems which could save utilities 20 to 40 percent of the capital cost of new capacity.
According to the Electric Power Research Institute (EPRI), “Although local or special circumstances could give IGCC the economic advantage over PFBC for new power plants, using coals less than 3 percent sulfur, the EPRI studies suggest that IGCC is likely to be the better choice for high-sulfur coals, and PFBC to be more economical for burning low- to medium-sulfur coals.” To avoid sintering the ash and releasing alkali metals that could foul or corrode gas turbines, PFBC cycle efficiency is limited to less than 42 percent to keep the combustor temperature at the required 900 C. The inlet temperature is far below that of gas/oil turbines (1,290 C), but advanced gas turbines of the future might allow higher firing temperatures.
One simple means of boosting flue gas temperature is to fire a topping fuel, such as natural gas, ahead of the gas turbine, or perhaps gas from the coal could alternatively be used as a topping fuel. Such systems are being developed by British Coal and PowerGen, the Anglo French consortium GEC Alsthom and by Foster Wheeler via the US Department of Energy (DOE) under its Clean Coal Technology program.
ABB Carbon is a leader in PFBC technology and has licensed its bubbling bed design to companies in the US, Japan, Spain and the Czech Republic. The Karita Unit of Kyushu Electric in Japan is a 350 MW plant, while another 350 MW ABB unit is planned for Taiwan.
These units have rugged gas turbines with special blade coatings to withstand any dust at the gas inlet. However, partially cleaned high-temperature gas for higher efficiencies in advanced PFBC systems will require special hot gas cleanup technology in which gas is directed through large ceramic filters prior to entering the turbine.
Advanced pressurized fluidized bed combustion (APFBC) is being developed by the US DOE and Foster Wheeler. This is part of a combined-cycle coal-fired plant which initially ran at 32 percent efficiency using pulverized coal but now runs at 40 to 46 percent efficiency with APFBC repowering. Very low production costs have been reported. This provides the opportunity to upgrade a unit of 20 percent capacity factor in start-stop duty to baseload service. Advantages of this system include high energy efficiency, low generation production costs, excellent environmental characteristics, a compact site footprint and competitive cost.
The system has a pressurized bubbling bed carbonizer which converts part of the coal into fuel gas and the rest into char. The PFB combustor burns the char to produce steam and heat combustion air for the gas turbine. The fuel gas burns in the topping combustor, heating the gases to the combustion turbine`s rated firing temperature, while the gas turbine heat recovery steam generator (HRSG) produces more steam, giving high combined-cycle efficiencies.
Some APFBC power comes from the gas turbine while the balance comes from the steam cycle. This allows combined-cycle efficiency to occur using coal as the only fuel. Coal consumption per kW is said to be 25 percent lower than for an atmospheric fluidized bed plant. This high-efficiency APFBC combined cycle boasts much lower emissions per kW of CO2 and pollutants and a 95 percent level of sulfur capture which exceeds the removal criterion for best available technology.
The IGCC system is the process of producing combustion gas from coal in a gasifier to fuel a gas turbine for electric power generation. The technology is based on a process which has been around since the early 19th century to produce mainly town gas, now replaced by natural gas in advanced countries. It has also been used extensively by the chemical industry to make product gases such as hydrogen, CO and CO2. This provides added-value options in an IGCC operation which is in effect a self-contained chemical plant.
IGCC is a complex process of coal gasification which in essence produces gas from pulverized coal in a separate pressure vessel to fuel a combustion gas turbine while generated exhaust gases are used to raise steam to supply a steam turbine in a combined-cycle setup. Presently, although gasification equipment is expensive to install, power producers around the world, especially in Europe, realize that they will be forced to use this technology more and more in the future to comply with global emission restrictions.
In a typical IGCC plant, steam and oxygen are added to the fuel mixture to partially oxidize and produce a synthetic gas. The gas, composed of hydrogen and CO, exits the gasifier at about 1,400 C and is subsequently cooled to about 230 C thus solidifying the fuel ash and generating more steam. The gas is separated by a cyclone and scrubbing with water. Once free of ash the gas is sent to a reactor in which the sulfurous components are changed into H2S, which is absorbed in a second gas treatment by a dissolvent.
Gasification offers not only big potential for clean coal usage but has the potential to use 25 percent less fuel compared to conventional plants. The resulting fuel gas, usually of low calorific value, can be burnt efficiently in a gas turbine combustor. There are three main gasifier technologies:
1. The moving bed gasifier which produces fuel gas with an exit temperature of 400 to 500 C.
2. The fluidized/bubbling bed gasifier which improves conversion efficiency through constant mixing and ash removal, rapid heat transfer and sulfur removal with addition of sorbent. The fuel gas exit temperature is more than 900 C.
3. The entrained flow gasifier produces fuel gas at an exit temperature of 1,000 to 1,500 C, which is higher than the ash melting point. Coal conversion is high and the residue (solids) is slag.
IGCC technology has been identified by a growing number of utilities as the potentially dominant coal-based power generation technology for the late 1990s and beyond. It is presently in its commercial demonstration phase, with plants from 250 MW to 450 MW in the US and Europe.
“Advances in gas turbine design, proven in operation above 200 MW, are establishing new levels of combined net plant efficiencies, up to 55 percent, and providing the potential for a significant shift to gas turbine solid fuel power technology,” said Douglas Todd of GE Power Systems. “These new efficiencies can mitigate the losses involved in gasifying coal and other solid fuels and economically provide the superior environmental performance required today.”
Although low-rank coals such as the lignite used in Australia`s Latrobe Valley for electric power generation are low-cost and have low sulfur and nitrogen content, they typically have high moisture content and variable ash compositions which can lead to severe fouling problems.
Conventional power stations designed for these coals are usually larger and more expensive than those designed for higher rank coals. In addition, control of CO2 emissions has weighed against the use of brown coals despite improved technology such as fluidized bed drying, hydrothermal dewatering, PF combustion, IGCC and direct coal-fired turbine systems.
An Australian company, HRL Ltd., based in Melbourne, has made headway with its newly designed integrated drying gasification combined-cycle (IDGCC) clean coal technology which is expected to reduce the cost of power generation and CO2 emissions by 30 percent using low-rank fuels, according to the company`s managing director Graeme Pleasance.
Late last year the company, formerly the Victorian government-owned Herman Research Laboratories, opened a (A)$100 million, 10 MW scale demonstration plant at Morwell using the IDGCC process. This reveals potential both in Australia and overseas, especially in Asia, since low-rank coals are more widely distributed than bituminous coals, accounting for about 30 percent of the world`s total economically recoverable coal energy reserves, yet presently satisfying only about 3 percent of demand.
The IDGCC process closely integrates the pressurized drying and gasification system for conversion of low-rank coal with a gas turbine combined-cycle system for highly efficient and cost-effective results of an environmentally friendly nature. The major stages in the process include coal feeding, drying, gasification, gas cleaning, gas turbine cycle and steam cycle.
The water evaporated from the coal is used to increase the power output of the gas turbine. The dryer operates at the same pressure as the gasifier so that the dried coal can be fed directly into the gasifier without intermediate storage.
The Morwell facility has a throughput of up to 10 tons per hour of raw coal at the full process pressure of about 25 bar, representing a process capacity of 10 MW output. The coal is then fired in an EGT Typhoon gas turbine (nominal 5 MW capacity) to produce electricity which is fed to the local grid.
Records show that the IDGCC power plant is technically feasible and economically attractive against a range of alternatives for power generation in southeast Australia and is likely to have even better potential in countries with high fuel costs.
Perhaps in the future, combustion turbines will be fired directly with coal combustion gases, but efficient cleaning at high temperature and in large columns will be necessary since high performance gas turbines are very sensitive to chemical and particulate contamination.
In the case of PFBC, gases may be cleaned mechanically to an acceptable degree to power a ruggedized gas turbine. However, the temperature limits overall heat recovery in the combined cycle. Better results could be achieved in a hybrid gasifier/combustor process or topping cycle in which some of the coal is gasified and the remaining char combusted.
Still in the advanced developmental stage, PFBC should be commercially available before the year 2000, according to EPRI. Costs are being lowered as more plants, both demonstration and commercial, are established. To date there are about 22 installations, one of which is a 350 MW unit.