Thermal plasma gasification of sewage sludge: Optimisation of operating parameters and economic evaluation

The article aims to define the optimal operating parameters of the gasifier such as temperature, equivalence ratio, steam-to-fuel ratio and selection of gasification medium for the process of thermal plasma gasification of sewage sludge. A comprehensive parametric study is carried out with the developed one-dimensional mathematical thermodynamic equilibrium model for plasma gasification. The model is based on the principle of the minimum of the Gibbs function and uses only the ultimate and proximate analysis data as input to the model. The output of the model is the equilibrium composition of the thermal plasma gasification products as a function of temperature in the plasma reactor. The parametric analysis has shown that the optimum conditions for the gasification process of sewage sludge are a temperature of 1200 K and an equivalence ratio of 0.2 for the air gasification case, i.e. 1200 K and the steam-to-fuel ratio of 0.1 for the case of steam gasification. When comparing the different gasification media, gasification with steam leads to a higher yield of H2 in the produced syngas. For the selected optimal conditions, the economic analysis of the process shows that the price of gasification per ton of sewage sludge is lower in the case of steam gasification and amounts to 72.30 <euro>/t, compared to 88.12 <euro>/t in the case of air gasification. The analysis was carried out using the example of a plant with a capacity of 360 kg/h. The sensitivity analysis was carried out taking into account the influence of the electricity price, labor costs, the price of CO2 emissions and the ash disposal price. The analysis shows that labor costs have the greatest influence on the price of thermal plasma gasification of sewage sludge.