The days of mega-sized coal-fired projects are over, at least for this country. The use of coal as a fuel for power generation will nonetheless continue for many years in the future, with the difference that small-scale (clean) coal-fired plant (SSCFP) is seen to be the technology of choice. Future plant could be of a much smaller scale than the existing mega-projects under construction, and the plant is likely to be radically different in design. This article takes a look at the potential for small-scale coal generation in a future energy mix.
The South African Department of Energy (DoE) has included two SSCFP in the REIPPPP programme . The plant size is not specified, although SSCF is implied. The proposed plant will be of a much smaller scale than the existing mega-projects under construction and the plant is likely to be radically different in design. In addition, the DoE of a major western country has embarked on a SSCFP support programme , which is aimed at investigating the feasibility of developing SSCFP meeting the following requirements:
World-wide there are still a large number of SSCF plants in operation, of older design an operating at low efficiency. The move toward larger plant was motivated by economy of scale and the higher efficiency of larger plant. The problems experienced recently in constructing large plant, including cost overruns, long construction periods, and delays, have considerably reduced the projected economy of scale advantages at many projects, and technology to improve efficiency is being applied economically in smaller plant to a large degree.
Problems facing mega coal projects are long construction periods, especially for first-of-a- kind projects. Economy of scale may apply to large projects but the inability to get into service in as short a period as possible affects the lifetime costs. Building a large-scale unique plant takes years from planning to final commercial operation, and if the final capacity does not match demand then costs of stranded plant could even be higher. Nonetheless, there are numerous latest-technology coal-fired plants which have been constructed on time and within budget.
The arguments in favour of SSCF generation are numerous:
Requirements of SSCF plants
To meet the requirements of a future grid, these SSCFP will need to be :
The driving force behind any SSCFP programme will be the procurement of capacity and energy at highly competitive levels. Consequently, price is of paramount consideration in the programme, and plant will have to operate at high efficiencies. Achieving this when operating plant below the supercritical regions achieved with larger plant will require the latest technologies and techniques.
Size of plant
A potential benefit to building small and modular designs is the ability to build more quickly, but the question that this approach poses is simply “What size plant is optimum?”. The range of plant size covered by the term “small-scale” is not defined but it appears to be generally accepted as plant in the range 50 to 350 MW with a maximum size of 500 MW. The optimum size of plant will depend on technological developments to meet the requirements given above, and this is in the hands of plant developers and manufacturers. Coal-fired power-plants in the range of 50 to 500 MW make up a large percentage of the installed base world-wide, so proven standard designs are readily available. Nevertheless, the requirements raise some questions as to what combinations of boiler plant and turbine would provide the optimum solution.
Although much has been said about small-scale distributed generation as the ideal path of future growth, very little research has been done on the optimum sizing of such distributed generation and how such a network should grow. Items such as localised demand growth patterns could play a huge role in the choice of technology, but eventually the technology capable of meeting the above list of requirements will determine minimum and maximum sizes of plant at least, and also possibly optimum modular sizes that allow modular expansion and growth.
One of the problems facing the “modular” design concept is the fact that different types of coal require different types of boiler, which may mean that each plant has to be designed individually, increasing cost. Taking a modular approach could actually be a more difficult way to build a coal unit. Coal is not homogeneous, so a plant that burns one type of coal may not be efficient burning another type of coal, complicating modular construction. This could possibly be overcome by the use of fluidised bed boilers, which can handle a wide range of different fuels without individual factors, and coal gasification.
Even small-scale coal-fired power stations have a significant coal supply requirement, and the availability of coal may limit the number of participants in the programme to organisations which are already in the coal supply business. A power station of 500 MWe capacity running in baseload mode would require between 1,8 and 2 Mt of coal per annum, a figure which is greater than the annual production of several of the smaller coal mines in this country. A 100 MWe power station would still require 360 to 400 kt of coal per annum, a substantial portion of the output of many mines. The problem here would be securing a supply for smaller power stations, as the cost of developing a mine solely to supply that amount of coal may be prohibitive, and the options would be limited to expanding the capacity of existing mines or finding a mine with that amount of surplus capacity.
The DoE however estimates that the coal mining industry produces about 60 Mt/a of discard coal with an estimated accumulated total of 1-billion t . Assuming a figure of 50% usage, this offers a huge opportunity which does not require any additional mining and could possibly result in 7,5 GW (15 x 500 MW) of baseload generation without using accumulated stocks. Even 25% utilisation would give 3,75 GW using scrap coal. This could be the option available to operators developing smaller sized stations, and could provide a significant price advantage, although this will depend on where the coal is located, and how much discard coal is available at each site.
The strict requirements placed on SSCFP virtually dictate the use of advanced technologies, especially on the combustion side. The discard coal option and the size of plant anticipated determines the use of fluidised bed or integrated gasification combined cycle (IGCC) technologies, and small-sized plant using these technologies is becoming increasingly available.
Fluidised bed combustion systems
Fluidised bed boilers are seen as the optimum technology for direct combustion SSFCPs . The plant is required to be flexible and capable of high ramp rates so sliding pressure operation, and a variable steam supply is required. Fluidised bed boilers have reached an advanced stage of development in the market with units ranging up to 600 MWth and working under super- and ultra-critical conditions.
Circulating fluidised bed combustion (CFB) is a proven and well-established technology which can use all types of coal. Choosing a CFB boiler makes also sense for captive power plants located near to coal mines and operated with residues of low grade coals which have no market value. Coal is not pulverised but ground into particles, the size of which depends on the plant and the fuel grade. One of the viable options available for discard coal, the CFB boiler has long been viewed and accepted in the industry as viable technology in the 20 to 350 MWe sub-critical class. Sub-critical units obtain the same efficiencies as pulverised coal. Circulating fluidised bed (CFB) boilers operating at super-critical range in the 400 to 500 MW range are in operation at several sites.
In order to take advantage of the economies of scale, most existing units are rated at over 300 MWe, but this minimum economic barrier is dropping with development of new technology. There are relatively few units with outputs from a single boiler/turbine combination of over 700 MWe. This is to reduce the effect on the generation capacity if the unit is taken out of service for any reason. Multiple smaller units are installed for security of supply and maintenance reasons.
For the same reason most applications envisaged in the SSCFP sector would probably use multiple boiler/turbine combinations. Units have been developed in a modular configuration allowing efficient sizing of plant and prefabrication of major components, giving rapid installation times. Modules down to 34 MW in size have been developed, which can be configured to operate with a wide variety of coal grades and types.
The fluidised bed boiler has been traditionally operated with drum type water tube boilers but several Benson type boilers using fluidised bed combustion technology have been installed in the market. These tend to be in the range >400 MWe, but serve to illustrate the wide range of options open to parties wishing to enter the power generation field.
CFB units also offer the possibility of in-furnace desulfurisation, by the inclusion of limestone in the fluidised bed, obviating the need for external desulfurisation where this is a requirement. The technology of choice is the circulating fluidised bed system, which offers higher efficiency than older bubbling fluidised bed types.
Integrated gasification combined cycle (IGCC) systems
IGCC is seen as the most efficient of all coal combustion systems and offers the greatest possibility of meeting the efficiency targets set for SCFFP. IGCC power plants combining gasification with a gas and steam cycle are being developed to provide environmentally clean and efficient power generation from coal, and smaller economical plants in the SSCF range are becoming available.
In the IGCC cycle, coal in any form, including pulverised, small pellets, fines and even slurry is fed into a gasifier where it is converted to syngas, consisting primarily of carbon monoxide and hydrogen. After a cleaning process the syngas is combusted and used to drive a gas turbine, which in turn drives a generator. The hot gases from the turbine exhaust are passed through heat exchangers to generate steam which is used to drive a steam turbine which drives a second generator. The gas turbine runs independently of the steam turbine and can be used to provide flexibility and ramping while the steam turbine contributes to the overall efficiency of the plant. In more recent designs, storage of the syngas is included which allows flexible operation of the turbine while maintaining the gasification process at a constant efficient level.
Existing IGCC plants tend to be large and complex in structure, but gasifiers based on a small-scale, modular design that are simple to deploy at site-specific locations, are becoming available. Units down to 1 MW in capacity are under development and units in the range of tens of MW are already available. A study done in Indonesia  showed that coal gasification plants of the order of 10 MW are economically viable and the Indonesian government plans to build ten coal gasification power plants with the capacity of 8 MW each in ten locations across Indonesia, especially in remote areas where electricity demand is low.
An advanced gas combustion system has been developed using the Allam cycle  which allows CO2 capture at near zero energy cost. This plant, originally designed for natural gas, has been adapted to include a coal gasification stage, with the same results.
The final consideration facing SSCFPs is that of compliance with environmental standards, and the question here is whether small generating plant will have to meet the same stringent requirements imposed on the larger installations. Requirements that would have to be met comprise:
Recent developments in the field of flue gas handling, using activated carbon, have resulted in systems which can remove all of the above substances economically at the size applicable to SSFCP, and have made the possibility of SSCFP meeting environmental requirements economically far more viable . Current commercially available systems of this type are operating on industrial plant of comparable size to SSCFPs.
 DoE: “About the coal baseload IPP procurement programme”, www.ipp-coal.co.za/Home/About
 D Proctor: “DOE Set to Support Small Modular Coal Units”, Powermag, May 2018.
 US Dept of Energy: “Small-scale modular coal-fired plants of the future”, US office of fossil energy, May 2018.
 P Zuldian: “Economic analysis of coal gasification plant for electricity and thermal energy supplies in Indonesia”, JOCET, 2017.
 R Allam, et al: “Demonstration of the Allam Cycle: An update on the development status of a high efficiency supercritical carbon dioxide power process employing full carbon capture”, https://bit.ly/2wZb0o3
 M Rycroft: “Activated carbon provides multi pollutant flue gas treatment”, Energize, July 2018.
 DoE: “Coal resources: discards”, www.energy.gov.za/files/esources/coal/coal_discards.html
 A Hotta: “Circulating fluidised bed technology”, Fourth EU-South Africa clean coal working group meeting, Kempton Park, 5 – 6 November, 2012.
 V Asthana and P Panigrahi: “Performance of power plants with high temperature conditions at sub critical pressures”, Fifthth European thermal-sciences conference, The Netherlands, 2008.
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