The potential for IPP coal fired power generation in South Africa

January 9th, 2015, Published in Articles: Energize

 

The DoE has issued a request for qualification (RFQ) and proposals for new generation capacity under the coal base-load IPP procurement programme. The request does not mention plant size but stipulates both pulverised coal and fluidised bed systems. 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. This article takes a look at the potential for small and medium scale coal generation as well as the technical and other challenges facing potential operators.

The RFQ follows on the heels of the request for registration issued in July 2014. The coal baseload IPP procurement programme (CBIPPP) has been designed to procure capacity and energy derived from 2500 MW of coal-fired power generation. The new generation capacity to be provided in the first bid window, will be required to be connected to the grid as quickly as possible and by no later than December 2021. The main feature of the CBIPPP is to facilitate the participation of independent power producers in the baseload power generation industry in South Africa to supply increased energy security and contribute towards socio-economic and sustainable growth objectives [1].

It is envisaged that in the bid submission phases of the program, bidders will be able to submit responses in relation to:

  • Projects where the buyer shall be the sole purchaser of the capacity and energy generated by a project (single buyer)
  • Projects where the buyer is one) of  two) buyers of capacity and or energy generated by a project (multiple buyer)
  • Projects in which the facility is situated in a country other than the Republic of South Africa (cross border) [1]

This variety of options opens the door to a number of possibilities, the most innovative of which is the multiple buyer, a scenario where an organisation may set up a generating plant to supply its own or another customer’s electricity needs, as well as supply power to the South African grid. This may make the difference in viability of an own generation project, where own generation on its own cannot be justifed, but including the ability to supply to the grid at a bid competitive price may make the project viable.

The other interesting aspect is the cross border option. Proposals to build generating capacity in Botswana to exploit the extensive coalfields there were rejected in the past on the basis of price, a move which could have contributed to the current power crisis. If proposals are now being considered on bid price, the projects may well become a reality, the advantage being that all the original pre-project phase work and planning has been done. If this option is combined with the multiple buyer option, it opens the door to cross border power stations serving both the South African network and the local network in the country in which they are located.

The driving force behind the programme is the procurement of capacity and energy at highly competitive levels. Consequently, price is of paramount consideration in the programme. However, it is recognised that economic development is also a fundamental component of a successful procurement programme. Compliant bids will therefore be evaluated on a comparative basis, in relation to price and economic development.

No size limit has been stipulated in the RFQ but it is believed that a single-project capacity limit of about 600 MW may be stipulated [2]. This obviously would not limit the size of the station in a multiple buyer application. A major energy company has already stated the intention to build a 600 MW coal fired station co-located with one of its major coal mines [3], and this may be indicative of the size of projects to be expected. This, however, does not limit smaller operators from entering the bidding process. It is understood that no project will be specifically precluded from bidding where connection is either difficult or is associated with high costs. But it is likely that coal-fired projects situated near to existing connection capacity will have a better chance of being named as preferred bids [4].

Because price is of paramount importance, 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.

Baseload operation

The designation of the plant as baseload simplifies the requirements on both turbines and boilers. Baseload plant operates at its maximum all the time and is not required to do load following or operate at partial load states. This removes the need for complex control and operating mechanisms such as sliding pressure operation, which is commonly used on load following turbines, and which requires additional control valves as well as additional boiler functionality. Baseload may also allow the turbine and boiler to run at a specific load level constantly.

Size of plant

It is assumed that the bid would be limited to plant less than 600 MW in capacity and with multiple units installed, generator size may vary from less than 100 MW to several hundred MW. Coal fired power plants in the range 50 to 500 MW make up a large percentage of the installed base world-wide, so proven standard designs are readily available to potential bidders (39% of the thermal power generation plant in China is in the <100 MW range). Nevertheless the RFQ raises some questions as to what combinations of boiler plant and turbine would be used in this size range.

Coal requirements.

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 program to organisation who 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 600 MW proposed project mentioned above would involve development of a new mine to supply the power station. 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 tons. Assuming a figure of 50% usage, this offers a huge opportunity which does not require any additional mining and could possibly result in 15 x 500 MW = 7,5 GW of base load 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.

Turbines

Operation in this power range will normally be in the sub-critical steam temperature and pressure range and turbines in the power rating zone are available from numerous suppliers, with a wide range of operating options available. Small scale turbines differ significantly from mega-project super-critical turbines. The complexity of the turbine design will increase as the power output increases. Subcritical steam turbines operate at lower pressures and therefore casing strength and other requirements are lower.

Subcritical operation differs from supercritical in the temperature and pressure of steam used. Subcritical is normally characterised by lower steam pressure and lower temperature, although claims have been made that subcritical units can achieve the same efficiency gains as supercritical ones by operating at the same temperature as supercritical but at subcritical pressures [6].

Reheat vs non-reheat

One of the decisions faced with smaller installations is whether to use a reheat cycle or not. Not using reheat can reduce efficiency but also reduces cost and simplifies boiler design. Reheat involves extracting the steam after the high pressure stage, and raising the temperature in the boiler reheater stage before returning it to the intermediate pressure or low pressure stages . Reheat cycles improve the overall efficiency of the plant, achieving efficiency gains in the range 3 to 5%. For large plant this would well be justifiable, but for smaller plant the cost of a more complicated boiler structure would need to be weighed against the efficiency gained.

Smaller turbines in single casing non-reheat configuration are available from many suppliers. Single casings mean that all turbine stages run at the same speed, which may be disadvantageous. Typical single casing turbines run at a speed of 3000 to 3600 rpm, and can be used for power generation without the need for a gearbox. Single casing turbines with reheat facilities are also available. Larger units would usually use multiple casings for LP and HP components, although turbines in the range of 150 MW are available with separate LP and HP casings, with the HP turbine running at a higher speed than the LP, and coupling done via a reduction gearbox .

Non-reheat seems to find its main application in CCGT installations or cogeneration where the reheat option is not available but is also used as an option of initial choice in smaller dedicated power generation systems.

Single stage steam turbines

Single stage turbines cover the range up to 10 MW and are unlikely to find application here.

Multi stage steam turbines

Multistage turbines could be two stage, consisting of a high pressure and low pressure stage, which may be housed in a single casing, or three stage, consisting of a high pressure, intermediate pressure and low pressure stage. Smaller three stage turbines are commonly housed in a single casing, with all three turbine stages running at the same speed. Two stage turbines in the power range considered here are available in double casing configuration, allowing the high pressure and low pressure stages to be operated at different speeds, and to be optimised for the different temperature and pressure conditions encountered.

Boilers

The RFQ calls for both pulverised coal and fluidised bed boiler plant. Plant is designated as baseload so sliding pressure operation, and variable steam supply are not required.

Sub-critical pulverised coal boilers (PCC)

PCC boilers in the range <800 MWe are usually in the subcritical application range. PCC boilers have been built to match steam turbines which have outputs between 50 and 1300 MWe. In order to take advantage of the economies of scale, most new units are rated at over 300 MWe, but 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 CBIPPP would probably use multiple boiler/turbine combinations [6].

There are two options available, with a range of equipment serving both technologies.

Drum type water tube boiler

This type is commonly found in older Eskom power stations and is still used in smaller modern stations and very commonly in industrial steam applications. The boiler is equipped with economiser and superheater stages and may be equipped with a reheater stage if required. This type of boiler has reached an advanced stage of development and is available from numerous sources.

Once-through Benson boiler.

Although mainly featuring in supercritical and ultra supercritical configurations, the Benson boiler can be used in subcritical applications, but seems to be limited in size to plant larger than 350 MWe. It is probably not economic at sizes below this. It is not clear what the minimum economic size is for the Benson boiler.

Fluidised bed boilers (CFB)

The only viable option available for discard coal, CFB has been specified as an allowable technology in the RFQ. The CFB boiler has long been viewed and accepted in the industry as viable technology in the 20 to 350 MWe subcritical class [7].

Fluidised bed boilers have reached an advanced stage of development in the market with units ranging up to 600 MWt and working under super- and ultracritical conditions. Subcritical units obtain the same efficiencies as pulverised coal units. Units have been developed in a modular configuration allowing efficient sizing of plant and prefabrication of major components, giving rapid installation times. Units from a size of 100 MW are available on the market. Modules in the size of 34 MW have been developed by Alstom, and can be configured to operate with a wide variety of coal grades . Coal is not pulverised but ground into particles, the size of which depends on the plant and the fuel grade.

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 >400 MWe, but serve to illustrate the wide range of options open to parties wishing to enter the power generation market.

CFB units also offer the possibility of in-furnace desulphurisation, by the inclusion of limestone in the fluidised bed, obviating the need for external desulphurisation 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.

Environmental considerations.

The final consideration facing potential CBIPPP participants 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 Eskom installations. Stringent requirements may encourage the option of cross border generation where less stringent standards are in force. Requirements that would have to be met comprise:

  • Particulate emissions: An essential must-meet requirement which can be achieved at reasonable cost using standard bag filters and electrostatic precipitators. The cost will depend on the actual standard imposed on the station.
  • Flue gas desulphurisation: This could be an expensive and complex process which could add significantly to the cost of establishing and operating a small power station, although the in-furnace capability of CFB offers a cheaper solution.
  • NOx reduction: Could also increase the cost involved, although low NOx combustion technologies are readily available for all ranges of boiler size.
  • Carbon capture and disposal: This is probably totally unaffordable for a small power generator, and as a requirement could make the whole programme non-viable.

References

[1] DoE: “About the coal baseload IPP procurement programme”, www.ipp-coal.co.za/Home/About

[2] Engineering News: “DoE gears up for baseload-coal IPP”, 4 July 2014, www.engineeringnews.co.za/article/doe-gears-up-for-baseload-coal-ipp-procurement-2014-07-04

[3] Exxaro: “Energy projects”, www.exxaro.com/index.php/sustainabilty/energy-projects

[4] DoE: “Coal resources: Discards”, www.energy.gov.za/files/esources/coal/coal_discards.html

[5] V Asthana and P Panigrahi: “Performance of power plants with high temperature conditions at sub critical pressures.” 5th European thermal-sciences conference, The Netherlands, 2008, www.eurotherm2008.tue.nl/Proceedings_Eurotherm2008/papers/Thermal_Processes/THP_8.pdf

[6] IEA clean coal centre: “Pulverised coal combustion PCC”, www.iea-coal.org.uk/site/2010/database-section/ccts/pulverised-coal-combustion-pcc

[7] A Hotta: “Circulating fluidized bed technology”, 4th EU South Africa clean coal working group meeting, Kempton Park, South Africa, 5 – 6 November, 2012, www.energy.gov.za/files/4thEUSouthAfricaCleanCoalWorkingGroup/EU_SA_Workshop_20121105.pdf

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