Thermochemical Heat Recovery Based on External Heat Engine

Abstract

This work discusses the issues of thermochemical recovery as a method to improve efficiency of fuel utilization in external heat engines (EHE).Key words: Catalyst, Synthesis gas, Thermochemical recovery, External heat engine, mathematical model.Practical application of thermodynamic cycles of thermal machines is related with implementation of heat transfer from heating source to working fluid (WF) or heat withdrawal from it after working cycle. With regard to thermal engines, conversion of chemical energy of fuel into work is implemented in two stages: at the first stage energy is converted into heat, and at the second stage the obtained heat is converted into work. In this relation intensification of heat transfer is one of the conditions of improvement of the observed efficiency of thermodynamic cycles of thermal machines. Theoretical efficiency of reversible thermodynamic cycle is determined by the ratio of difference between average maximum temperature and average minimum temperature of heat withdrawal and does not exceed the efficiency of the Carnot cycle. Increase in the average maximum temperature is limited with thermal resistance of materials and increase with heat losses of engine into external environment. In this regard selection of WF for EHE is highly important. Decrease in average temperature of heat withdrawal is limited with environmental and engineering considerations. Due to existence of such limitations the only way to increase overall efficiency of engine is related with heat recovery of exhaust gases (EG) by means of TCR either of cogeneration.It 3027-3039 (2015) with EG into environment equals to 35 %; heat transferred to cooling system: 30 %; portion of losses due to incomplete fuel combustion: about 2 %, and unaccounted heat losses reach 3 %. Theoretically, it is possible to recover heat losses related with heat withdrawal into environment and into cooling system, and thus to achieve increase in thermal efficiency. The essence of TCR is that a portion of EG heat and heat, transferred into engine cooling system, is used for endothermic conversion of initial fuel into another type of fuel possessing higher enthalpy of products. Due to this there is an increase in total efficiency of fuel utilization [2][3][4]. In terms of energy the most profitable solution is conversion of fuels into SG, which is a mixture of CO and Í 2 . The first scientific background of this method to improve EHE efficiency was given by Nosach [5], who specified it as TCR, and its practical implementation was applied to stationary solid fuel facilities.The use of TCR makes it possible to achieve two important purposes simultaneously: 1) to increase the lower heating value of reformates with regard to initial fuel. Theoretical maximum increase can reach 20-22 % upon steam reforming of propane to CO and H 2 (without ÑÎ 2 and especially of ÑÍ 4 in the composition) [2]. 2) to increase stability of catalytic burning in nonadiabatic operation mode of catalytic burner (due to heat withdrawal into engine or just heat losse...