Introduction of a ternary diagram for comprehensive evaluation of gasification processes for ash-rich coal

Abstract

The present paper addresses the development of a comprehensive thermodynamic approach for the evaluation of gasification processes. A ternary diagram is introduced for a South African coal with an elevated ash content of 25.3wt.%(wf). The ternary diagram allows the evaluation of most of the commercially applied gasification technologies depending on the three variables O2, H2O and coal mass flow. Cold gas efficiency, dry CH4 yield, specific syngas production, H2/CO ratio, CO/C and CH4/C selectivity as well as temperature and carbon conversion were selected as performance measures. Based on literature data, generic models of the commercial Shell, Siemens, ConocoPhillips, HTW and GE coal gasifications systems were developed enabling an integration into the ternary diagram at standardized boundary conditions. The graphical approach indicates the existence of optimum configurations for the specific gasifier types and leads to an individual potential assessment. At a typical gasification pressure of 30bar, a theoretical maximum cold gas efficiency of 87.4% was identified at a temperature of 980°C for the above mentioned coal, whereas the maximum syngas yield of 2.09m3(H2+CO STP)/kg(waf) was located at 1135°C. It is shown that only fluid-bed or two-stage processes have the potential to achieve these global maxima. The sensitivity of these maxima to varying ash contents from 5 to 45wt.% and to coal rank is investigated as well. The study is concluded by the introduction of a simplified user diagram which was derived in order to drive a process towards the identified maxima.