Production of biocoal from wastewater sludge-biomass feedstock

Abstract

The increasing volume of wastewater sludge and sugarcane bagasse from wastewater treatment and sugarcane facilities is becoming a prominent concern globally. The disposal of sludge is particularly challenging and poses severe environmental hazards due to the high content of organic, toxic, and heavy metal pollutants among its constituents. The emissions from burning sugarcane bagasse are known to have an impact he respiratory health. At the same time, the availability of energy supply is in demand. The reliance on fossil fuels in the 21st century is unsustainable as the world's reserves are limited and are continually depleting. This depletion of reserves demonstrates the need for alternative energy sources. To minimize the reliance on fossil-based energy sources, a renewable resource such as biomass can be optimized as an energy source. Wastewater sludge and bagasse have the energy potential to produce high calorific value biocoal, this will contribute to the supply of energy in South Africa. South Africa is a major consumer of coal, to produce electricity therefore the development of renewable energy is essential to reduce fuel shortage as concerns for clean and sustainable energy grows. The synthesis of biocoal from wastewater sludge and bagasse through an artificial synthetic coal production process, i.e., hydrothermal carbonization (HTC) is preferred over other thermal conversion techniques as HTC is capable of handling feed having a high (75-90%) moisture content. This study focuses on the production of biocoal from wastewater sludge and sugarcane bagasse as an alternative to sustainable bioenergy supply and is one of the potential solutions for reducing net CO2 greenhouse gas (GHG) emissions from fossil-fuel power plants. This study followed the application of the HTC process, with the purpose to convert wastewater sludge and sugarcane bagasse into valuable biocoal. The wastewater sludge and sugarcane bagasse were subjected to hydrothermal carbonization in stainless steel batch reactors, where the effect of temperature, solid loading (solid-liquid wt. %), and biomass type were investigated, while other process parameters were kept constant. The effect temperature was explored at 180 ℃, 210 ℃, 240 ℃, and 260 ℃. A ratio variation of 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100 dry sludge to bagasse(SB), and the composition of solid to liquid (solid loading wt. %) of 1:10, 2:10, 23:10, 2and 4:10 (SB:H2O) corresponding to 9.09%, 16.67%, 23.08% and 28.57% loading were investigated. The process yielded gaseous, solid, and aqueous phases. Calcium hydroxide was used as a binding medium of the produced biocoal. The results obtained in this study revealed that solid loading, temperature, biomass type, and ratio variation had a substantial impact on the yield and calorific value of biocoal produced. The highest biocoal yield of 23.36 wt. % was achieved at 210°C and derived from sludge/bagasse (S/B) with a sludge content of 20%. Across all runs, the highest calorific value of 20.21MJ/kg was achieved at 260ºC when pure bagasse was employed (0% sludge content)