The CO2 emission factor of peat fuel (original) (raw)

The CO2 emission factor of peat fuel

by John Couwenberg

CO2 emission factors (CEF) for fossil fuel combustion are expressed as tonnes of CO2 emitted per TJ of energy. As such they are determined by how much CO2 and how much energy are produced by combustion of 1 tonne of fuel.

The amount of CO2 produced by combustion depends on the carbon content of the fuel and on the so-called oxidation factor � how much of the carbon is oxidised during combustion. Typically, peat combustion results in 99-100% oxidation of the carbon in the peat.

The carbon content of a fuel is an inherent chemical property and does not depend upon the combustion process or conditions. In peat, the carbon content depends on degree of humification and varies from 45% to 60% of total dry weight.

The amount of energy produced by combustion of a fuel is referred to as its calorific value. Calorific value is also an inherent chemical property, dependent on the composition of chemical bonds in the fuel. A commonly used proxy for calorific value is the so called �fuel ratio� between fixed carbon and volatile matter[1] (figure 1). These two fractions show different combustion characteristics, influencing calorific value. The fuel ratio and calorific value may vary within fuel types. In peat, they increase with the degree of humification (figure 2).

Another factor is the moisture content. Moisture content varies from 15% for peat briquettes up to 55% for milled peat. Like mineral soil content (ash), moisture content influences combustion properties and negatively affects calorific values (figure 3).

The IPCC Guidelines (IPCC 2006) provide a default for peat calorific value of 9.76 GJ/t peat and an emission factor of 28.9 gC/MJ = 106 g CO2/MJ (compared to <100 g CO2/MJ for various types of coal). Countries may adjust these values to national circumstances.

There is not much room for adjustment, however, as the emission factor for peat is largely determined by chemical properties that � without substantial net energy losses � cannot be altered. Besides selecting more humified peat with a low ash fraction, moisture content can be lowered to reduce the emission factor and lower the climate impact of fuel peat combustion.

Anderson, A.R. & Broughm, W.A. 1988. Evaluation of Nova Scotia's Peatland Resources. Nova Scotia Department of Natural Resources - Mineral Resources Branch, Bulletin ME 6

Borman G.L. & Ragland K.W. 1998. Combustion Engineering. WCB McGrawhill, Boston

Ekono. 1981. Report on energy use of peat. Contribution to UN Conference on New and Renewable Sources of Energy, Nairobi.

IPCC. 2006. 2006 IPCC Guidelines for National Greenhouse Gas Inventories, prepared by the National Greenhouse Gas Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T., and Tanabe K. (eds). IGES, Japan.

Fig. 1 � Typical fixed carbon, volatile matter and calorific values (MJ/kg) for wood, peat and coal on a moisture and ash free basis (after Borland and Ragland, 1998)

Fig. 2 � Calorific value and fuel ratio of air dried Sphagnum peat at different degrees of humification. Data from Anderson & Broughm (1988)

Fig. 3 � Net calorific value of peat depends on moisture and ash content (after Ekono 1981)

[1] Volatile matter includes all products, other than moisture, given off as gas or vapour by a fuel (measured at 950�C). Fixed carbon is the non-volatile matter in fuels, other than ash. The ratio of fixed carbon to volatile matter is referred to as fuel ratio.