Emission Calculation Methodologies for CHP Plants

The paper discusses different calculation methods to correctly express the emission level of air pollutants produced by CHP (Combined Heat and Power) systems, in order to take into account the environmental benefit due to cogeneration. New and established methods to estimate the emission saving due to cogeneration of heat and power are reviewed and compared, with reference to small scale CHP systems; the paper clarifies that a proper emission assessment of CHP systems should consider both the global and the local scales of environmental impact. In particular, the method of the “avoided heat generator” is proposed in this study, for a local-scale environmental impact evaluation. This approach calculates the reduction of emission due to CHP operation, in comparison with the non-CHP operation of the same machine, taking into account the amount of pollutant emitted by a heat generator which provides the same thermal power of the cogenerator. Moreover, it is shown in the paper that another method, based on the PSI (Pollutant Saving Index) value, is suitable to estimate the global-scale environmental impact. The problem of the most representative unit of measure is also discussed, highlighting the advantages of an “output based” approach and providing easy-to-use formulas and graphs for the conversion of concentration values into output-based emission factors. Finally, a numerical evaluation of the CHP environmental benefit is provided for small electric power size CHP machines.