Higher power efficiencies

Paul Boughton

Combined cycle power plants are particularly effective in cogeneration and district heating applications. Cogeneration or Combined Heat and Power (CHP) is an extremely efficient way to deliver the maximum benefits of heatpower and cooling to industry and commerceas well as the public sector and private homes.

CHPthe simultaneous production of heat and poweris an important path for economies with a high growth rate to reduce emissions and at the same time meet the energy needs for the foreseeable future. It is also the favourable solution in terms of economical and environmental efficiency.

A CHP plant generates usable heat and power in a single process. It consists basically of one or more prime movers (usually gas and/or steam turbine) frequently driving an electrical generatorwhere the heat generated in the process is utilised via suitable heat recovery equipment for such purposes as industrial processescommunity heating and space heating. Indeedthe cost-effective adaptation to district heating and industrial heat customers is the primary design focus of such plants.

In terms of total efficiency (the sum of electricity and useful heat divided by fuel input) all cogeneration technologies can reach very high per centages of up to 90 per cent and moreprovided the boundary conditions like heat levelreturn condensate temperature and heat load curve are appropriate for the respective technology.

Generally it is boundary conditions such as fuel availability (eg coalnatural gaswaste gasesoil)fuel pricespower and heat demand curve which define the power generation technology used.

If natural gas is available for power generationthe most efficient CHP solution in terms of environmental efficiency is a combined cycle power plantwhich is very flexible in terms of the ratio of power and heat output. The heat to power ratiowhich describes how much electrical power a plant produces in relation to the heat outputis one of the main differentiators for environmental efficiency of a plant.

The benefit of CHP plants can be seen if the CO2 emission of a CHP plant is compared to separate generation of electricity and heat via best available technology. If natural gas is availablethe best available technology for power generation is a large-scale combined cycle power plant with a net efficiency of 57.5 per cent. The best available technology for heat generation is a gas-fired boiler with 90 per cent efficiency. In order to produce the same amount of electricity and heata typical modern combined cycle CHP plant (50 per cent electrical efficiency and 85 per cent total efficiency) uses 20 per cent less fuel and consequently emits 20 per cent less CO2 than separate power and heat generation plants.

Siemens offers turnkey solutions for combined cycle power plants in all power ranges. If used as CHP plantthey can generate heat levels from 50 up to 500¢ªCwith a high flexibility of power and heat generation. The gas turbine exhaust is packed with thermal energywhich is recovered in a heat exchanger generating either high quality steam or hot water. These sources of heat are then used to meet local domestic and industrial needs or to generate additional electricity through a steam turbine.

The smaller combined cycle power plants are very often designed as CHP plants to provide heat for district heating or industrial purposes. The main customer groups here are the oil and gas industryand the chemicalpetrochemical and process industriesas well as the power generation industry. Other smaller or specialised industriessuch as food and beveragepulp and paperceramicstextilesbuilding complexesautomotiveare also discovering the advantages of a grid-independent captive CHP plant to furnish all their power and process needs.

This was recently done at the whisky distillery of William Grant & Sonsin GirvanScotlandrepresenting one of their largest and most important investments in new technology in recent years. External supply had begun to form a major and rapidly-increasing percentage of the company’s total production costs. The integration into the facility of a SGT-100 gas-turbine driven CHP solution from Siemens now gives Grant complete control over their process and their power costs. In the four years since the CHP plant was installed the initial capital investment has been completely recovered through savings in fuel and operating costs.

No less than a quarter of the generating capacity owned and operated by Essent Energiethe Dutch power utilityis centred on CHP installations. One of these is a recent Siemens project in AntwerpBelgiumbased on a SCC-800 combined cycle power plant. Essent opted for a Siemens CHP (a ‘combicogen’a combined cycle plant used in cogeneration application) for both environmental and business reasonsas it is company policy to utilise high efficiency energy technologies.

The site hosts eight third-party chemical manufacturersaround half of which are supplied with steam by INEOS. The plant will supply these industrial customers with powersteam of different pressure levels and hot water; plant operation will be optimised according to the power prices on the local power exchange market.

It is a prerequisite for CHP plants that there are users for the waste heatas heat cannot be stored or transported over distanceswhich makes this type of concentrated facility ideal for the technology.

Lynne Anderson is with Siemens Industrial Turbomachinery ABFinspongSweden. www.siemens.com/powergeneration

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