Process Effluent Recycling in the Supercritical Water Gasification of Dry Biomass

Abstract

The influence of process water recycling during the Supercritical Water Gasification (SCWG) of dry biomasses was investigated. Dry biomass has to be diluted with water to a dry matter content of approximately 10 wt.% to gasify it in the process of supercritical water gasification. The treatment of wastewater in the SCWG process is cost intensive due to organic contaminants; therefore, the recycling of the process effluent is attractive. Salt separation is needed to avoid accumulation of salts in the effluents, since salts enhance corrosion rates and might cause blocking of the flow when the effluent is recycled. The grass Reed Canary Grass and grapevines were gasified. The recycling of the process effluent did not influence the composition of the product gas. In both cases the carbon efficiency decreased by 4% when wastewater was used to dilute the biomass. An increase in organic carbon and potassium in the reactor effluent was observed after gasification of the biomass with recycled process effluent. The addition of potassium hydroxide to the feed as a homogenous catalyst needs to be closely monitored and adjusted according to the potassium content of the reactor effluent. Insufficient salt separation proved to be an issue regarding formation of solid deposits in the reaction system.

Comparison of Experimental Results with Thermodynamic Equilibrium Simulations of Supercritical Water Gasification of Concentrated Ethanol Solutions with Focus on Water Splitting

“…The water consumption of the process describes how much water is converted into gaseous products when reacting with the organic compounds. In previous studies, it was shown that the reactor effluent of the SCWG process can be recycled. , The water consumption is thus important for the estimation of how much fresh water needs to be used to make up for the difference of water in feed and reactor effluent by distilled water and thus is important for cost estimation of the SCWG process. To calculate the generation of H 2 and CH 4 from water, the amount of C, H, and O in the gas phase is compared with the amount of C, H, and O that originate from the gasified ethanol n ̇ EtOH, gasified . ṅ EtOH , gasified = CE × ṅ EtOH , Feed where n ̇ i is the molar flow of species “ i ” (mol/h).…”

“…The LENA lab-plant is designed for the processing of real biomass. 30 For the gasification of ethanol solution, a simpler setup would also be sufficient.…”

Catalytic Supercritical Water Gasification of Canola Straw with Promoted and Supported Nickel-Based Catalysts

“…Hydrothermal gasification of biomass in supercritical water (SCW) overcomes this limitation for the production of H 2 without needing to pre-dry the feedstock [ 13 ]. Supercritical water gasification of biomass (SCWG) is conducted above the critical point of water, which exists at temperatures greater than 374 °C and pressure larger than 22.1 MPa [ 14 ]. Supercritical water exhibits unique properties such as non-polar nature, high diffusivity, mono-phase reaction mechanism, and high solubility of gases, which imparts low mass transfer limitation, easy product separation, and high thermal efficiency to the SCWG process [ 15 ].…”

Investigating Salt Precipitation in Continuous Supercritical Water Gasification of Biomass

“…An advantageous feature of SCWG is the absence of the energy-intensive drying step required in traditional biomass gasification [3]. This enables the utilization of wet biomass materials like microalgae, lignocellulosic biomass, sewage sludge, and animal manure [4][5][6][7].…”

“…The composition of the resulting gas and the efficiency of gasification in SCWG heavily depend on process parameters, such as temperature, pressure, and biomass concentration in the feed [8][9][10]. The primary constituents of the gas mixture are H 2 , CH 4 , and CO 2 , with lower amounts of C 2 and C 3 compounds and CO [4,11]. SCWG thus provides an option of producing hydrogen from biomass, which is in high demand as a clean fuel in the shift from fossil fuels to renewable energy sources.…”