Investigation of hydrothermal gasification products of biomass digested by anaerobic method
Project no. 1337 · Status: Kapanmış · 2017
Summary
Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel. Biogas is produced by anaerobic digestion (AD) or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and energy crops. This type of biogas comprises primarily methane and carbon dioxide. There are two optimum temperature ranges for methanogens (methane producing bacteria), one at 30–40°C and one at 50–60°C. These are known as the mesophilic and thermophilic ranges respectively. To operate at the higher temperature, range a heating system must be employed. This makes the system more complex and expensive. The rate of methane generation approximately doubles for every 10°C rise. Temperature fluctuations should be minimized since they severely inhibit the methanogens. There are some parameters such as C/N (carbon/ nitrogen), temperature, pH ratio etc. that affect biogas production from the fuel stock materials. After production of fuel gas (CH4) during anaerobic digestion, remaining solution (digestate) still has high energetic value. Usually remaining solid is evaluated as fertilizer, but pathogenic content and transportation difficulties restrict this application. Digestate can be evaluated in supercritical water gasification (or hydrothermal gasification, SCWG) to obtain clean energy carrier gas, H2. Supercritical water gasification emerges as an advantageous method that allowing conversion of biomass to hydrogen and/or methane without any pre-drying process. While valuable gas mixture produced during SCWG of biomass, undesired coke and tar production substantially reduced. Hybrid process of AD and SCWG is applied in this project for high conversion of organic content in selected biomass. In this study, cauliflower residue (80 g/L) was digested in anaerobic fermentation at 35 and 45°C by using sheep manure (40 g/L). Biomass with high organic matter content remaining at the end of digestion cannot be evaluated very effectively by using it as fertilizer. In addition, the water content of the remaining balance is quite high. In hydrothermal gasification method, it reacts with organic structure under conditions above the critical point of water (374°C, 221 atm) and fuel turns into gas products, a small amount of tar + coke with liquid products that can be used as chemical raw materials. In this study, hydrothermal gasification experiments with digested biomass were carried out in batch type reactor assembly with an internal volume of 100 cm3. Experiments were carried out under different (500 and 600°C) temperatures without catalyst and as catalysts with KOH, NaOH and K2CO3. Depending on the reaction conditions and catalyst type, changes in gas and liquid product composition and tar and coke formation; GC was determined by analysis with GC-MS and TOC. The usability of gas products as fuel gas and the evaluation of liquid products as platform chemicals were examined and these chemicals can be evaluated as platform chemicals.