An improvement in the work generated by expanding hot gas from exothermic reaction processes featuring an excess of high temperature and carried out above atmospheric pressure is achieved by providing pressurised gas supplementary to that required or produced by the process operating under the full load condition, heating this supplementary gas with a portion of the available excess high temperature heat, and then expanding the gas to generate work. The amount of the supplementary gas is such that the total amount of gas expanded is greater than that amount of gas which would be available for expansion by operating the process at the full load condition without supplying the supplementary gas.