The benefits of the flexible and efficient combined-cycle gas turbine for power generation are manifest. But high gas prices are prompting renewed interest in alternative turbine fuels. One possibility is the direct combustion of coal.
As the world’s largest coal exporter, exporting more than A$13.5 billion ($9.6 billion) worth annually, Australia is at the forefront of research and development aimed at finding new ways of using coal and improving the efficiency of coal-fired power generation. Much of the research in this area has been initiated under a collaborative effort involving the former Co-operative Research Centre (CRC) for Black Coal Utilisation, the Commonwealth Scientific and Industrial Research Organization (CSIRO) Division of Energy Technology and the Australian Coal Association Research Programme. One widely publicised technology is gasification, in which coal is heated in oxygen to produce a gaseous fuel, mainly hydrogen and carbon monoxide, for use in late generation gas turbines; a less well known possibility is the direct combustion of coal in a reciprocating engine or turbine.
The ultra clean coal (UCC) development programme began some 20 years ago after interest in the process was first aroused during the energy crisis of the 1970s. Its development as a substitute for fuel oil began in the early 1980s and in its first formulation the ultra clean coal product was a coal/water slurry containing some 0.5 % ash. Subsequently the process has been honed to the extent that it now allows the injection of the product directly into gas turbines.
Towards a pilot plant
UCC is coal that has had virtually all of the mineral contamination removed from it using caustic chemical leaching techniques similar to the Bayer process used to refine bauxite into alumina. The UCC technology produces an ultra low ash, pulverised solid fuel for direct firing in gas turbines and is currently being tested at a pilot scale project at Cessnock in New South Wales, Australia. Although based on coal, the UCC product is not seen as a substitute for conventional coal in conventional generating systems. It is being developed as an alternative to heavy fuel oil and gas where coal cannot be used and it is claimed to be financially competitive with both oil and gas on an equal energy basis.
UCC technology is being developed by UCC Energy Pty Ltd, a wholly-owned subsidiary of White Mining Ltd, in co-operation with CSIRO. In early 1988 UCC Energy formed Auscoal Energy, which was an unincorporated joint venture with CSIRO and The Australian Coal Industry Research Laboratories (ACIRL), to develop laboratory results into a process development unit capable of operating in a continuous mode. The NSW Department of Energy joined Auscoal Energy in 1992.
The basic process development unit, including a continuous digester, acid soak and washing stage but no chemicals regeneration or by-products removal, was constructed at the Ulan Coal Mine in NSW. This plant operated between 1988 and 1991 before it was transferred to Maitland near Newcastle, also in NSW. In 1998, following initiatives by White Mining and the Australian Federal government, CSIRO entered into an agreement with the Center for Coal Utilisation Japan (CCUJ) to assess UCC as a direct feed into power generating turbines.
The Japanese commitment to the agreement involved Mitsubishi Heavy Industries in developing and testing a turbine, Idemitsu Kosan Co. Ltd performing laboratory assessment and analysis of the UCC fuel and Kyushu Electric Power Company in the assessment of economic and environmental analysis of power generation from the UCC product.
Funding for this effort, at the Japanese end, was predominantly from the Japanese government through the Ministry of Economy, Trade and Industry with the balance from the companies involved.
The co-operation agreement saw the development of a new fully-integrated continuous pilot plant located at Cessnock in Australia’s Hunter Valley region.
Financing has also come from both federal and state governments with A$14.1 million ($10 million) in loans and grant funding from the federal government under the START programme, A$0.86 million from the New South Wales state government, A$4.2 million from the CSIRO, A$0.85 million from the UCC Anode Carbon Consortium and the balance of A$25.05 million from the White Group/UCC. A total of A$45 million has been invested to date, including around A$15 million on the pilot plant.
Although the pilot plant has a relatively low throughput of just 350 kg per hour it operates in a fully integrated continuous mode and incorporates all the components of the process including coal feed preparation, high-pressure caustic digestion, acid wash, hydrothermal wash, caustic regeneration and recycle, and byproducts generation. The plant, shown in Figure 1, is intended to generate data to establish the design and the capital and operating costs of a commercial scale plant, and to generate bulk UCC samples for evaluation and utilisation tests.
By definition, ultra clean coals are coals with less than 1 % ash with low mineral content and a low percentage of sulphur and alkali metals such as sodium. The patented CSIRO/White process produces a solid powder fuel with ash levels between 0.1 % and 0.2 % and uses alkali/acid digestion to dissolve the minerals out of the coal under moderate temperature and pressure conditions, without the loss of coal properties. The process works through a series of steps that convert the minerals to soluble forms and then remove them. The dissolved minerals are then precipitated predominantly as calcium sulphates and calcium aluminium silicates. A simplified schematic of the process is shown in Figure 2.
The process starts with coal, sulphuric acid and lime and produces ultra clean coal, gypsum, and silicate materials that can be used in ceramics production. The plant operates in a closed loop system for sodium hydroxide and water and more closely resembles a chemical plant than a conventional coal washing facility.
One third of the process costs come from chemical feeds, an overhead which can be reduced by using higher quality coals with less silica and aluminium compounds.
The control systems are key to managing the process and considerable investment has been diverted into software. However, with the control system development now complete, the company aims to move towards a fully-fledged demonstration project in the coming months. A typical analysis of UCC and its parent coal are given in Table 1.
Turbine manufacturer MHI performed the combustion trials in a modified MHI 501G at its Takasago facility, shown in Figure 3. MHI stipulated an ash content of less than 0.2 % and a particle size of less than 5 microns in order to avoid abrasion, given that particles of larger than 5 microns can penetrate the boundary layer and impact on a blade surface. Depending on the quality of the coal used, ash content can be as low as 0.01 % and particle size is always below the 5 micron threshold using the UCC process.
Mitsubishi modified the design of the M501G gas turbine combustor basket and fuel injection system so that it would accept UCC as a fuel instead of natural gas and subsequent assessment of some 6 tonnes of UCC from the pilot plant has shown that the product can be fired into a turbine. The results were very encouraging, showing that stable and efficient combustion of UCC could be achieved, and, with initial combustion trials successful, the consortium is now moving to optimising NOx control in the next phase of development.
By the first quarter of 2005, UCC aims to have produced some 400 – 500 tonnes of UCC product that will be shipped to MHI in Japan for the company to conduct lifetime tests on its adapted 501G turbine. Lifetime trials will assess degradation of turbine blades by testing considerable quantities of fuel at high pressures of say 10 atmospheres and correspondingly high temperatures. The trials are aimed at establishing the role of materials such as compounds of sodium and titanium that do not exist in petroleum products and which remain in the product after the UCC process. Although only limited quantities remain, ash fusion temperatures may also be an issue in modern gas turbines where very high temperatures are reached in the first stage.
Once the lifetime trials are complete, a pilot-scale demonstration plant is expected to follow by 2008 with a capacity of around 6 MW but perhaps reaching 15 MW. This facility is expected to run for some 18 months and consume around 20 thousand tonnes of UCC, requiring an initial upgrade of the current pilot plant before it is scaled up to demonstration/commercial size. If the commercial demonstration project is successful, the fuel product may find itself being consumed in commercial scale facilities within three years, although UCC Pty Ltd’s technical development manager Keith Clark acknowledges that a more realistic time scale would be four or five years.
UCC development was aimed at capturing the high efficiencies available within the most modern combined cycle gas turbine systems and is intended to provide an environmentally acceptable but less costly and more stably priced alternative to natural gas. When UCC is directly fired in a turbine, thermal efficiency is increased from around 38 % for a conventional coal-fired power station to approximately 53 %. In addition, the use of gas turbines enables smaller generation units that can be placed closer to population centres than conventional coal-fired systems. Proponents of the technology suggest that, with minimal transmission losses, overall energy conversion efficiencies of close to 60 % are possible. The system also allows coal to supply energy as a peaking fuel, previously the exclusive domain of gas and liquid fuels.
Life cycle analysis has shown that the UCC process offers significant reduction in greenhouse gas emissions of up to 20 % from mine to consumer when used in advanced combined cycle systems compared to conventional coal-fired plant. Despite the additional energy associated with the UCC process, such as chemicals production, transport, water heating and the like, the increased efficiency of combustion gives a net saving in emissions over conventional coal-fired systems. For example, as Figure 4 shows, the UCC process produces some 185 kg of CO2 per MWh, while conventional coal mining alone produces 71 kg/MWh. However, with the conventional combustion process adding a further 794 kg/MWh, this compares poorly with the combined cycle system which generates just 607 kg/MWh. Furthermore, under the Kyoto Protocol arrangements, there is the possibility that end users will benefit as some of the CO2 production associated with the UCC product over its lifecycle is retained in the country of the coal’s origin. The UCC process is also suited to most bituminous coals and is therefore of strategic importance to developing countries, a key growth market in coal consumption over coming decades with India and China featuring strongly in coal use scenarios. The process is also likely to be of interest to other major coal producing countries, such as the US, and the size of the primary market is therefore huge. If UCC replaced some 2 % of the gas to electricity market, it would require the annual production of 19 million tonnes of UCC.
Fundamentally, gas prices are unlikely to be lower than those of coal at any time in the foreseeable future and even with the additional processing costs of the UCC product, it still easily out-competes gas in terms of cost alone. At $3.00 to $3.50 per GJ, UCC is significantly cheaper than natural gas which is currently running at around $4.95 per GJ in the US, or fuel oils at $3.96 to $6.44 per GJ. Similarly, delivered into Japan UCC costs $3.45 per GJ of energy, whereas natural gas comes in at more than $4.50 per GJ, although gas can reach as much as $18 at times. In addition, the price of UCC will be less susceptible to the variability and the general upward trend seen in oil and gas, as UCC is based on an abundant and widely available first source.
As an economic and carbon efficient process, UCC could figure largely in years to come and UCC Energy is currently interested in entering into discussions with a potential joint venture or equity partners in order to proceed into the small commercial or demonstration scale for both UCC production and power generation.