Australia’s proposed carbon emissions trading scheme⁽¹⁾ will impose cost burdens on the electricity generation sector due to Australia’s high proportion of coal fired power stations, many of which still have a substantial remanent technical life.

There are approximately 21 GW of black coal and 7.5 GW of brown coal fired power plants in Australia. These produce some 184 million MWh or just over 80% of Australia’s electricity. In doing so approximately 190 million tonnes of Greenhouse gases are emitted; this represents about 30% of Australia’s Greenhouse gas inventory⁽²⁾ .

Reducing the greenhouse gas emissions⁽³⁾ of existing coal fired power stations by significant efficiency increases can only be achieved at great cost because of the need to change the power stations’ design conditions, material selection and equipment configuration.

It is for this reason that typical, existing sub critical boiler turbine set cannot be readily upgraded into a substantially higher efficiency configuration. The cost of changing the materials and equipment to facilitate increased operating parameters often makes it more economical to build a new power station.

Nevertheless power station owners will be exposed to carbon emission costs and when contemplating the range of strategies that could be applied, it is reasonable for them to take a closer look at what improvements are available and that may be applicable.

So what are some of the options?

Higher efficiency steam turbine blade design

Research and development has improved the performance of steam turbine blades by the use of a three dimensional profile. These more recently designed turbine blades can be (and in some Australian plants already have been) retrofitted into existing steam turbines with an increase in the turbine efficiency in the order of 3% in some cases. These improvements together with up to a further 1% available by improving the turbine seals and the recovery of some efficiency deterioration would give a notable reduction in CO2 emissions. Although some of these gains would be lost over time to efficiency degradation as the turbine ages, overall there would still be some long term reduction in CO2 emissions.

Boiler efficiency improvement

Boiler efficiency can be improved by increasing the boiler heat transfer surface area to remove more heat from the flue gas before discharging it to atmosphere.

The design of boilers has been optimised to the steam turbine system and to an economic flue gas temperature as assessed at the time of the design. Although additional energy can be extracted it requires additional equipment and the associated capital outlay. The extent of this extra heat recovery is also limited by the need to have exit temperature of the flue gas sufficiently high for environmental and technical reasons (principally to provide plume buoyancy and to minimise ductwork corrosion).

Improved efficiency of auxiliary drives

For plant that is subject to varying demand there is a trend towards variable speed drives from the traditional fixed speed type because advances in technology have delivered better economy and reliability. The use of variable speed drives enables the driven machine to be controlled to an optimum output negating the additional losses associated with less efficient throttle valves, dampers or control vanes that have been traditionally employed.

The extent of power saving is dependent on the efficiency of the driven device at the prevailing operating point. The maximum gain in power saving will be achieved where the driven device is often operating away from its normal design point, such as when the power station operates at less than full output or with changed fuel conditions.

Improved pumps and fans can also be fitted in many instances to obtain power savings such as in the use of higher efficiency fans that have variable pitch blades.

Pre-drying brown coal

Brown coal has significantly more moisture within it than does black coal.

Pre-drying of brown coal removes some of the moisture in the coal before it is burnt to avoid the latent heat loss that would have occurred if it remained in the fuel.

Technologies for pre-drying brown coal have been developed in Germany and Victoria, including technology based on the steam fluidised dryer invented and patented by Professor Owen Potter of Monash University in the 1970s.

The latest German design uses fine grain coal with the heat for drying provided either from bled steam from the turbine or by recompression of the vapour released in the drying process. This latter method is more energy efficient with little if any of the moisture leaving the process in the vapour state phase.

Mechanical drying technologies that substantially avoid the evaporation process are also available, although these are not as advanced compared to steam fluidised bed drying and have not yet been used in full scale applications.

Pre-drying brown coal reduces the greenhouse gas emissions closer to a level that is achieved by black coal fired power stations, but it is unlikely that a brown coal fired power station could match the efficiencies achievable by otherwise comparable black coal fired power stations. This is because some energy penalty is invariably imposed by the drying plant.

Biomass co-firing

Biomass fuels release about the same amount of carbon dioxide as fossil fuels, however, fossil fuels release carbon dioxide captured by photosynthesis millions of years ago and is essentially a "new" greenhouse gas. Biomass releases carbon dioxide that is largely balanced by the carbon dioxide recently captured the biomass’ growth and is therefore generally considered (near⁽⁴⁾) Greenhouse gas emissions free. The reduction in greenhouse gas is therefore approximately proportional to the proportion of biomass used.

Biomass used for co-firing is generally wood waste because coal fired boilers can usually co-fire a small amount of wood waste without major modification to the existing equipment. Given the limited supply of this material and the need to collect and transport it from a number of locations, it is unlikely for most geographies that the biomass available to co-fire would represent more than 1% of the fuel input to a large power station on an energy basis.

Conversion to natural gas

The conversion of coal fired boilers to full or part natural gas firing will reduce the greenhouse gas emissions because natural gas has a lower carbon intensity but at the expense of higher fuel costs which flows through to the production cost of electricity. Natural gas of up to 25% of the fuel energy can be can be co-fired in black coal boilers without extensive modification to the heat transfer surfaces although some flue gas recirculation equipment may be required.

Solar heating

The use of solar heating for power generation produces no greenhouse gases in operation and can be used with a stand-alone generator or integrated with an existing power station.

The integration of solar heating into an existing power station that has been optimised as a coal fired generator would result in a less efficient solution than using a stand-alone power generator optimised for the solar collectors.

The solar heating system will shut down overnight and in cloudy conditions and without a means of storing heat, the heat input from the solar collectors is therefore limited to only a portion of the power plant’s daily operating period. The warming time required by the system each time it is brought back into service would also impact on operating efficiency.

Carbon capture and storage

Carbon capture and storage (CCS) can significantly reduce Greenhouse gas emissions by storing them indefinitely. CCS is energy intensive and requires a significant amount of plant and equipment to remove the CO2 from the flue gas and to compress and transport it to the storage site.

The energy requirements for CCS with existing technologies result in the electricity generated being reduced by the order of 30 % so that, although the greenhouse emissions are typically reduced by 90%, the greenhouse intensity in kg/MW is reduced by approximately 85%. Additional generating capacity will need to be constructed to replace the energy used by the CCS equipment and the cost of this additional generation also needs to be included in the cost of using CCS.

With current technologies the flue gas must have very low levels of sulphur oxides (SOx) and nitrogen oxides (NOx) for the CO2 to be effectively and practically removed. This requires SOx scrubbers and Selective Catalytic Reduction (SCR) equipment to be installed on Australian power stations which in turn increase the cost of using CCS.

Unlike the Australian market the European and North American markets tend to have SOx scrubbers and SCR equipment already installed on existing coal fired power stations⁽⁵⁾ and the CCS costs quoted in these geographies are therefore lower than would be applicable in Australia.

Indicative abatement costs

The following figures are based on an in-house study of the cost of greenhouse gas abatement on a typical mature sub-critical black coal unit rated at 500 MW. As some of the technologies are still being developed and the abatement costs are a function of the power station design and its operating parameters, these values should be considered as indicative only. The cost of abatement for the options can be sensitive to the fuel and pool prices: in this case a coal price of $1.30/GJ and a natural gas price of $4.00/GJ has been used with a average electrical energy price of $55/MWh (includes $10/MWh attributed to the carbon emissions trading scheme).

Figure 1: Abatement costs without CCS

Figure 1 shows the abatement costs, without CCS for a range of abatement options.

Figure 2 shows the abatement costs with CCS. The reduction in greenhouse emissions due to the inclusion of the CCS option results in less greenhouse emissions being available for abatement by other options. That is, the abatement potential of the various options is not necessarily additive. For example, a reduction in the greenhouse emissions by 90% due to the CCS reduces the abatement provided by other options to 10% of their stand-alone value therefore resulting in a ten-fold increase in their costs when expressed as $/ tonne CO2-e.

Figure 2: Abatement costs with CCS

It should be noted that the above cost curves are indicative and to establish the costs and savings for a particular installation a techno-economic assessment needs to be undertaken on a case by case basis.

Conclusion

While a modest reduction in greenhouse gas emissions can usually be achieved on existing coal fired power stations at a ‘reasonable’ cost, major and expensive redesign and rebuilding is generally necessary if any major reductions are to be achieved.

The use of biomass as a fuel to replace coal is limited by its availability compared to coal and switching to natural gas has a price disadvantage on existing black coal.

The technologies used for CCS are still being developed and at present the additional plant required and the energy used give a high cost of abatement. If CCS is to be included as an option, then the economics of the other options will be significantly impacted.

It would appear that in the short term the economic options to reduce greenhouse gas emissions from black coal stations are limited. The following link provides access to the ‘GES Abatement Cost Calculator’ which gives an indication of abatement costs for existing power stations, both coal and gas fired.

Footnote:

⁽¹⁾ Formally known as the Carbon Pollution Reduction Scheme
⁽²⁾ ESAA ‘Electricity Gas Australia 2008 and Australia’s 2006 GHG Inventory (Kyoto Account)
⁽³⁾ Primarily carbon dioxide (CO2)
⁽⁴⁾ Emissions from collecting and transporting biomass should be recognised.
⁽⁵⁾ Because of the different coals used and the different environmental impacts of emissions.

© Sinclair Knight Merz
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