What are the comparative environmental benefits of CHP?

Answer by Dale Blundell (Atkins)

Carbon Dioxide Emissions

Burning a carbon-bearing fuel produces carbon dioxide by combining atmospheric oxygen (O2) with the carbon in the fuel.  The amount of carbon dioxide produced depends on the carbon content of the fuel concerned and the quantity of the fuel that is used. For natural gas, the CO2 content is equivalent to about 185Kg/MWh, whereas coal might be 285kg/MWh.

CO2 is a traded commodity as part of a European Union’s Emissions Trading Scheme (EUETS) Cap & Trade arrangement. Carbon Credits are traded to meet EUETS emissions obligations creating a supply and demand market. Thus it can be in the interests of participating organisations to reduce CO2 emissions in order to avoid paying for credits needed to meet their obligations to such schemes.

Carbon Footprint

Reducing carbon emissions will in turn reduce the full lifecycle carbon footprint of goods and services that are in any way handled or processed on the site.  Reducing an Organisation’s Carbon Footprint can be useful in demonstrating to investors and clients that the organisation has a positive environmental attitude.

Carbon Monoxide

Carbon monoxide is produced by the incomplete combustion of carbon, which can be caused by a number of different factors in a combustion process. The rate of production (parts of carbon monoxide per million parts of exhaust gas) is usually small, but is significantly increased if there is poor combustion. Carbon monoxide is a toxic gas in high concentrations, and quantities of any significance in the atmosphere can contribute to local smog and respiratory problems. Its production can be limited by properly maintaining and controlling combustion equipment.

Other Emissions

Combustion of such fuels as fuel oil or coal in conventional systems produces oxides of nitrogen and of sulphur (NOx and SOx) that can form acid rain, causing damage to buildings and ecology. Particulate matter & NOx emissions contribute to smog that can be harmful to respiratory systems.

SOx  Sulphur Oxides

SOx is an overall term for the mixture of oxides of sulphur, SO and SO2 gases.  These are produced by the combustion of sulphur containing fuels.  Natural gas has almost no sulphur, whilst heavy fuel oil may contain 2% sulphur.  In order to control the emission to atmosphere, flue gas desulphurisation (FGD) is often required to extract the SOx from the combustion gases.

 

Sulphur content in fuels and typical SO2 emissions (UK DECC)

Fuel

% sulphur in fuel

S02 content (Kgs) / MWH thermal input

Coal

1.2

3.47

Heavy Fuel Oil

2.0

3.33

Gas Oil

0.15

0.24

Natural Gas

nil

nil

 Using natural gas fired CHP to displace heat generated by another fossil fuel such as heavy fuel oil or coal, can reduce SOx emissions.

NOx  Nitrogen Oxides

NOx is the term for the oxides of nitrogen, NO and NO2, and is formed through the high temperature reaction of oxygen with atmospheric nitrogen. Some CHP suppliers offer variants of engines that produce different amounts of NOx. For example, spark ignition engines will typically be quoted as having emission limits of 500mg/m3 NOx or 250mg/m3 NOx.  If very low NOx is required, a selective catalytic convertor can be installed to dramatically reduce the NOx still further.  Generally each stage of NOx reduction will come with a compromise of reduced electrical efficiency.

 Other pollutants

Combustion processes can produce small quantities of organic & chemical compounds which include unburned hydrocarbons (UHC), typically associated with poorly controlled combustion or poor maintenance. Combustion of by-product or waste fuels may emit other compounds such as dioxins, furans and heavy metals that require cleaning up before discharge to the environment.

The reduction of emissions to atmosphere is due to the reduction in electrical transmission and distribution losses, and the utilisation of waste heat that would otherwise have gone to waste.  Taking account of the typical efficiency of centralised electricity generation plant, and the additional losses in the transmission and distribution network, the net efficiency of grid supplied electricity will typically be 37-39%.  In addition, heat produced on site is typically provided by boilers with a conversion efficiency of 70-90%.  With a typical mix of electricity and heat, the overall provision of energy (power and heat) from conventional sources may therefore be around 50-60%.

In contrast, by generating electricity close to its point of use and capturing as much waste heat as technically possible, CHP can achieve efficiencies as high as 75%-90%, subject to scale, technology and energy demand profile. The average amount of CO2 emitted to atmosphere per unit of primary fuel will depend upon the mix of technology and primary fuel used, conversion efficiency and transmission & distribution of energy, with each Country having different specific emissions figures.  A common reference for carbon emissions from combustion processes (generating heat) is the EU Emissions Trading Scheme (EUETS), that has standard emissions factors on which to base emissions calculations for each of the participating Nations.

 

Emissions Reductions from a range of CHP Options (UK) (g/kWh)Source DECC, UK

Scenario

CO2

NOx

SOx

GT with heat recovery boiler replacing coal-fired boiler

560

1.9

10.4

GT with heat recovery boiler replacing HFO-fired boiler

436

1.9

8.9

GT with heat recovery boiler and back pressure steam turbine replacing HFO-fired boiler

370

1.7

6.8

Lean-burn SI engine with heat recovery boiler replacing HFO-fired boiler

546

nil

8.9

 

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