The utilisation of Searsia lancea Hydrochar produced by hydrothermal carbonisation and as a binder for South African discard coal fines

Abstract

The provision of energy globally is becoming an absolute imperative in sustaining modern life due to rising population demographics as well as socioeconomic needs in the developing world. The role of waste resources will become imperative in an endeavour to satisfy these demands. Waste resources such as discard coal are ever increasing and growing at an alarming rate in the coal producing countries. Nevertheless, the continued use of coal hangs primarily on the advances in clean-coal technologies (CCT) as an alternative route for the reduction in environmental risk factors such as the emission of greenhouse gases (GHG), coal or refuse dump, acid mine drainage (AMD) etc. Biomass and waste resources are receiving attention for tackling environmental concerns, energy security and long-term fuel sustainability. Biomass is regarded as the largest and most abundant source of renewable energy and its greenhouse gas emission status is zero to net negative as plants uptake carbon dioxide during their growth. The use of biomass in its raw form is known to lead to different challenges such as high transportation, storage and handling cost, and low conversion ratio of the biomass to high value-based products. To improve on the physiochemical properties of the biomass, pre-treatment methods such as hydrothermal carbonisation (HTC) are useful in transforming it into a product consisting of mainly carbonaceous solid hydrochar with improved physicochemical properties. This research study focused on producing a renewable solid fuel blend of hydrochar from raw biomass obtained via HTC and South African discard coal fines at different weight ratios. The raw Searsia lancea biomass utilized in this study was found to possess a superior fixed carbon content with lower ash content across all plant sections, and higher calorific value than the raw Tamarix usneoides also utilized in this study. From the HTC experiments conducted: the blend of Searsia lancea S11 and S12, the hydrochar produced under 280 °C and residence time of 90 minutes was found with the highest energy content of 29.71 MJ/kg compared to 16.73 MJ/kg and 17.23MJ/kg obtained from the discard coal and raw Searsia lancea, respectively. Three properties of the hydrochar were selected and used in developing polynomial regression equations, using central composite design (CCD). The regression equations were designed with response surface methodology (RSM) – which plots 3D surface responses plots of the combined effect of temperature and time. Regression analysis was performed to fit the response functions of mass yield (%), ash content (%) and calorific value (MJ/kg) and to provide a mathematical framework for future analysis of these three responses. The regression equations developed indicated that the values obtained experimentally agree with the predicted values from the models via correlation coefficients (R2) which are given as 0.9537, 0.9525, 0.9789 for mass yield, calorific value, and ash content, respectively. The analysis of variance (ANOVA) was performed to demonstrate the suitability of the model developed for each response. It was found that mass yield had temperature (T) as the only significant model term. While, calorific value (T), and time(t), are the significant model terms, and t, Tt, T2, t2, T2t, along with Tt2 were all significant model terms for ash content. The combustion tests showed that the pellet produced from the 100 wt. % (by weight) hydrochar had the highest reactivity with the lowest ignition temperature and burnout temperature. All the pellets produced from the blend of hydrochar and coal at different weight ratios, and the 100 wt.% hydrochar displayed a similar burning profile to coal but burnt out at lower temperatures to the 100 wt.% discard coal. The concentration of trace elements from the ash generated from these samples were found to meet the disposal standards herein discussed. In addition, of all the samples tested for their mechanical properties, the pellet produced from the blend of 25 wt.% hydrochar + 75 wt.% coal had the highest compressive strength, impact resistance and absorbed the least amount of moisture. This study has shown that the Searsia lancea trees planted as phytoremediation agents for treating AMD from platinum and gold tailings storage facility (TSFs), as well as rehabilitating polluted soils, have demonstrated to be a suitable binder to produce different grades of biocoal pellets