(582c) Hydrothermal Co-Carbonization (HTCC) of Coal-Biomass Blend

A 7.5 billion current population of the world is expected reach nearly 10.3 billion in 2040 while energy consumption for electric power is projected to reach 42 quadrillion BTU in 2040. Meeting these energy demands requires high output from our existing energy technologies as well as introduction of emerging technologies. In the US, coal-fired plants accounted for approximately 40% of electricity generation in 2015 and coal consumption has been expected to increase by 17.6% quadrillion BTU by 2040. To meet the projected energy demand without contributing additional greenhouse gas emission, co-combustion of biomass with coal is a viable solution. However, very low bulk density, energy content, and wet nature implements severe risks on co-firing biomass with coal.

Hydrothermal carbonization is one of the thermochemical processes, where biomass is treated with a hot compressed water. As the reaction medium is water, there is no need to dry the feedstock. Therefore, the expensive drying process can be eliminated by applying this technology. Now, sub-and supercritical water, unlike water at ambient condition, are very reactive due to its high ionic product, low density, and low dielectric constant. As a result, hot compressed water reacts with biomass components and release oxygen and ash from the biomass structures. The resulted solid product, often called hydrochar, is hydrophobic and energy dense, just as bituminous coal. Moreover, hydrochar can be further densified by palletization. Energy density via pelletization of hydrochar can be reached much higher than energy density of raw coal.

In this project, hydrothermal co-carbonization (HTCC) of coal-biomass blend was studied at various water temperatures (200, 230, and 260 °C) at water saturation pressures. For coal samples, Ohio’s Clarion #4a was used, where, Ohio grown miscanthus (miscanthus x gigantus), a perennial grass was used as biomass source. The main objective of this study was to produce a homogeneous hydrochar product in terms of density, fuel properties, and elemental composition. Resulted hydrochar produced from blend (50:50 wt %) was compared with control miscanthus and coal hydrochars. Furthermore, hydrochar pellets were produced using bench-scale single press pellet-press. Pellets mass and energy densities were measured and compared with control biomass or coal pellets.

Hydrochar produced from HTCC reveals that, coal’s elemental carbon content was not effected by water temperatures in the tested temperature range. However, sulfur and chloride content are reduced significantly. Hydrochar produced from biomass is located at the surface of the coal, which ensures the homogenization in the micro-level. The resulted hydrochar was hydrophobic, contains high energy (30-33 MJ kg^-1), lower ash, sulfur, chloride, and nitrogen than raw coal. The blend hydrochar pellets have very high mass and energy densities compared to their raw feedstocks. For instance, the blend hydrochar pellets can reach as high as 38 GJ m^-3 energy density, when biomass-coal blend was treated at 260 °C.

Therefore, HTCC technology can be applied as an upstream processing to feasible co-combustion of coal-biomass mixture. This technology produces homogeneous feedstock with superior fuel quality and low pollutants.