Efficiency improvement of coal-fired power stations



 

Coal-fired power stations (CFPS) are going to be around for many years to come. In addition to existing stations, new CFPS are being built at a steady rate. Much benefit can be gained from improving efficiency, such as using less coal per unit of energy, and emitting less CO2 into the atmosphere. There are several international programmes aimed at improving efficiency of CFPS.

Minimising heat losses is the greatest factor affecting the loss of CFPS efficiency, and there are many areas of potential heat losses in a power plant. Efficiency of older CFPS becomes degraded over time, and lower power plant efficiency results in more coal being used and more CO2 being emitted per unit of electricity generated. The options most often considered for increasing the efficiency of CFPS include equipment refurbishment, plant upgrades, and improved operations and maintenance schedules [1].

Moving the current average global efficiency rate of CFPS from 33% to 40% by deploying more advanced off-the-shelf technology could cut 2 Gt of CO2 emissions, while allowing affordable energy for economic development and poverty reduction [1]. According to the International Energy Agency, only about 29% of the existing coal-fired power fleet can be retrofitted for carbon capture, utilisation, and storage. Thus, efficiency improvements are the first step on the road to major emissions reductions [1].

Fig. 1: Effect of efficiency factors of various coal-fired technologies [4].

An effective and sustainable strategy for CFPS in the future must integrate environmental imperatives with the legitimate aims of energy security and economic development, including poverty alleviation. This means that there must be a role for cleaner coal technologies, including high-efficiency low-emission (HELE) coal-fired power generation.

HELE technologies are commercially available and, if deployed, can reduce CO2 emissions from the entire power sector by around 20%. They are the low-hanging fruit in global CO2 mitigation which should be given more attention. It is imperative that these technologies are deployed as widely and as quickly as possible. Deploying HELE coal-fired power plants CFPP is a key first step along a pathway to near-zero emissions from coal with carbon capture, use and storage (CCUS).

The average efficiency of CFPP around the world today is 33%. This is well below the state-of-the-art rate of 45% and even “off-the-shelf” rates of around 40%. Increasing the efficiency of CFPP by 1% reduces CO2 emissions by between 2 and 3% [1].

In addition to CO2 abatement, and probably more important for economic expansion, comes the reduction in use of coal/kWh and hence reduction in running costs, plus overall reduction in the use of coal and extension of the lifetime of coal reserves. Reduction of 20% in CO2 emissions implies a reduction of 20% in coal usage.

Performance measurement

Performance is measured by the heat rate. The heat rate of a CFPP represents the amount of heat, typically in MegaJoules (MJ), needed to generate 1 kWh of electricity. Accordingly, typical units for heat rate are MJ/kWh. Heat rate is the heat energy input per unit of electrical energy output, or fuel consumption rate for specific levels of power plant output. Heat rate is also the inverse of plant efficiency. CFPS efficiency is measured in terms of the energy produced per unit of heat consumed, i.e kWh/MJ.

In its simplest form, a plant’s heat rate (for a particular period) can be defined as follows:

HR = F / E               (1)

where:

HR = Heat rate (MJ/kWh)

F = Heat energy input supplied by fuel to the power plant for a period (MJ)
E = Energy output from the power plant in a period (kWh)

Since a single kWh of electricity is equivalent to 3,6 MJ, thermal efficiency can be calculated as:

TE = 100×3,6/HR               (2)

where:

TE = Thermal efficiency (%)

Increasing efficiency by a few percentage points may seem insignificant, but can lead to significant savings in cost and reduction in emissions.

As an example, using an average heat rate of 12 MJ/kWh for a 1000 MW, 80% availability coal-fired power plant using coal rated at 20 MJ/kg, the efficiency is 30% and the annual coal consumption will be 3,85 Mt. Increasing the efficiency to 35% drops the annual consumption to 3,6 Mt, an annual savings of 250 000 t of coal, which at R300/t amounts to savings of R75-million/annum.

Improvement upgrade or replacement?

The cost of the improvements is often compared to the expected return in increased efficiency as a primary determinant of whether to go forward with a programme. At the low to medium end of cost expenditures are combustion, steam cycle, and operations and maintenance improvements. Efficiency and operational improvements are seen as a possible alternatives to replacement when considering a range of equipment upgrades and refurbishment options.

In an analysis carries out by NETL [1], it was concluded that retirement of lower efficiency units combined with increased generation from higher efficiency refurbished units, and advanced refurbishments with improved operation and maintenance, would be necessary to achieve an improved overall fleet efficiency level.

Fig. 2: Impact of different steam-cycle conditions [2].

These improvements would generally be considered low to medium cost upgrades.

However, at the higher cost end are major plant retrofits and upgrades (i.e. conversion of sub-critical CFPP units to super or ultra-supercritical CFPP units), which would raise efficiencies more substantially.

The general conclusion is that where life extension is possible, this would also result in an improvement in efficiency which would justify the extension of life of power plant. The option of upgrading some existing CFPS to super or ultra-supercritical operation is also a possibility rather than total replacement.

Retirement and change in technology (HELE)

Retirement of an old power station does not necessarily mean abandonment of the site and infrastructure, with existing networks and transmission lines in place. New technology could be erected on the same site and much of the existing plant could be re-used. Particularly where the coal supply is still adequate. The most effective efficiency improvement comes about in changing technology from subcritical to super-critical (SC) and ultra-critical (UC) technologies. This is not always possible with existing plant and may require complete replacement.

Fig. 1 shows the effect of representative efficiency gains for the different technologies. Reduction in CO2 is directly related to reduction in coal usage. There are many plants today operating as super-critical mode as well as several ultra-critical plants. The availability of both boiler and turbine materials capable of withstanding the high temperatures and pressures encountered in these systems has made their wide adoption possible. In addition there are also supercritical fluidised bed combustion systems in operation.

Fig. 2 shows the impact of the various steam cycles on efficiency.

To illustrate the potential of HELE technologies, Fig. 2 summarises the impact of different steam-cycle conditions on an 800 MW power station boiler burning hard coal and operating at an 80% capacity factor. Such a unit would generate 6 TWh of electricity annually and emit the quantities of CO2 shown in the figure, depending on its steam-cycle conditions. The coal usage would also decrease correspondingly.

Improvements to existing plant

Not all plant will be retired though, and there are many ways of improving the efficiency of existing plant through life extension, without major replacement of plant or reconfiguration.
Several studies [2] have shown that up to 5% improvement in efficiency can be achieved by modifications and maintenance. This is equivalent to the change from subcritical to super-critical operation.

Fig. 3: Areas of a pulverised coal plant where efficiency loss can occur [4].

Areas where efficiency losses occur

Several studies have been done to identify areas where efficiency losses occur. The following section summarises the most common areas identified and possible remedies. Fig. 3 shows areas where improvement can be achieved [3]. Much of the energy loss is associated with auxiliary equipment such as pumps, mills and fans, and attention to these items can bring about great improvement.

Improved maintenance

The first area that can give efficiency gains with the least amount of disruption is improved maintenance, which may also be combined with improved monitoring and control. Improvements related to improved maintenance are in the range of 1 to 3%.

Flexible operation control

The increasing amount of renewable energy in the network means an increasing requirement for plant to operate in cyclical or flexible mode. Flexible operation refers to the ability of a plant to operate at part load and in load-following and cycling (on and off) modes. Operating conditions under flexible operation can result in reductions in plant efficiency and increased degradation and maintenance requirements on components due to constant swings in operating temperature and pressure. Various studies have identified cost-effective capital modifications and adjustments to plant operating procedures to improve heat rate during cycling operation [5].

Other plant improvements

Fig. 4 shows efficiency gains that are possible with improvements to various items of plant. It would appear that gains are possible in every section of the power train, from the boilers to the flue gas cleaners.

Control system upgrades

Changing the control system is perhaps one of the least disruptive efficiency moves to consider, and can give significant gains in performance. Newer systems allow more monitoring and control of the various stages of combustion, steam and turbine operation, allowing each to be optimised.

Remote monitoring centers (RMCs) have been used for many years to track and improve equipment reliability, and in many cases, these same RMCs have thermal performance software installed for monitoring heat rate. The value of finding and fixing reliability issues can often be quantified, but placing a value on heat rate monitoring is not so easy. An EPRI study evaluated the use of remote monitoring systems as it relates specifically to heat rate improvement. All of the companies visited were able to verify heat rate improvements based on the activities of the monitoring centers in addition to improvement in equipment reliability.

In many cases, the heat rate improvements were significant and well surpassed the incremental costs for monitoring heat rate in addition to reliability. Heat rate improvements in the range of 2,5 to 4% have been reportedly attributed to the actions resulting from these RMCs [5].

Table 1: Efficiency gains possible [4].
Potential efficiencies from plant improvements in APEC countries
Category Area of improvement Net efficiency gain (%)
Combustion systems Pulveriser and feeder upgrades 0,3
Air heater repair or upgrade 0,25
Sootblower improvements 0,35
Excess air instrumentation and control 0,2
Steam cycle Feedwater heater repairs 0,4
Heat transfer tube upgrades 0,6
Steam turbine blades 0,5
Cycle isolation 0,4
Condensor repairs 0,5
O&M O&M training Included in combustion and steam cycle gains. Efficient operation realised over the long term
Computerised maintenance and management systems and reliability centred maintenance
Distributed control systems
Combined total 3,5

Using renewables to improve CFPS efficiency

One of the more promising and less complicated methods of increasing efficiency of older CFPS is the use of solar energy to supplement the energy generated by the plant. Heat rate improvement could potentially be achieved using solar technologies to either provide heat at various points in the steam cycle, or to provide power to the equipment used to run the plant, thus curbing on-site equipment electricity use. Two possibilities have been explored: solar PV and solar thermal.

In the solar PV solution, electricity from the PV system is used to supplement the energy used to drive the plant. A CFPS can use up to 15% of its generation to drive the various items of machinery necessary to keep the plant running [3].

Solar PV plant needs a substantial area and this can limit the use of such plant.

The solar thermal-coal hybrid plant is a promising technology that could upgrade existing power stations to reduce fuel costs, improve heat rate, and minimise environmental impact.

Solar energy has been proposed with various utilisations in conventional steam power plants, such as feed water heating, superheating/reheating of steam and air pre-heating. In the solar thermal solution solar heat is used to replace heat drawn from the steam train for various purposes, allowing more energy to be obtained from the steam.

A typical application would be solar heat at around 300°C, used to replace the steam extracted from the high pressure turbine, to pre-heat the feed water before the economiser stage of the boiler. In this way, the replaced high-temperature and pressure steam can be further expanded in the turbine, boosting output of the plant. In addition, studies have shown that the conversion rate of solar heat to electricity in this application can be higher than that of a solar heat to electricity unit operating on its own.

Solar heat is these cases is typically provided by trough-type or compact linear Fresnel CSP units. There are several units in operation worldwide. Of the options available, linear Fresnel seems to be the most suitable. In India a 15 MW Fresnel CSP system is being installed at the Dadri plant complex and will be integrated into a 210 MW wet-cooled unit. The CSP is expected to produce 14 GWh/a, increasing the efficiency of the plant [6].

References

[1] R J Campbell: “Increasing the Efficiency of Existing Coal-Fired Power Plants”, Congressional Research Service report 7-5700.
[2] I Barnes: “Upgrading of the efficiency of the world’s coal fleet”, IEA clean coal centre, July 2014.
[3] C Gellings: “Program on technology innovation: Electricity use in the electric sector”, EPRI 2011 technical report 1024651.
[4] International Energy Agency (IEA) Clean Coal Centre: “Upgrading and Efficiency Improvement in Coal fired Power Plants”.
[5] S Korellis: “Coal-Fired Power Plant Heat Rate Improvement Options, Part 1”, Mypower, 11 January 2104.
[6] H Hashem: “Fresnel developer builds India supply base to serve 1,7 GW CSP-coal market”, CSP today, 25 Jan 2017.

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