A method for calculation of the composition of Estonian oil shale and its combustion products is based on the various correlations governing the composition of this oil shale, and relations between its components. To understand better the properties of oil shale and the influence of mining conditions on oil shale quality, a general characterisation of Estonian oil shale is first presented. This unique fuel is utilized for power production, by direct firing in boilers. This paper presents calculations useful for the boiler design engineer for estimating the average characteristics of received oil shale as well as the possible deviations from these mean values. The calculations in this part allow finding the composition of oil shale and the composition of combustion products of oil shale based only upon its heating value. Various considerations in performing oil shale combustion calculation are presented. A calculational example is presented in the appendix of this part. Part 2 of this study extends the method, given the availability of additional fuel analysis data. (C) 2003 Elsevier Ltd. All rights reserved.