Influence of pressure and CO2 in fluidized bed gasification of waste biomasses

Abstract

An autothermal fluidized bed reactor was used to research the influence of pressure (0–2 barg) on the gasification process of different types of biomasses. The tested feedstocks were bark and lignin while softwood pellet was used as a reference fuel. A mixture of O2/CO2/H2O was used as a gasification agent. The impact of the application of CO2 on the yield of H2 in product gas was determined. Resulting product gas was characterized by a high content of CO which makes its use for applications based on chemical synthesis very difficult without extensive upgrading or supply of H2 from external sources. CO2 proved to improve carbon conversion efficiency (CCE) of the gasification process and to be an option for its chemical sequestration (negative carbon footprint). A slight modification of conventional indices used to evaluate efficiencies of gasification systems (CCE and water/carbon ratio) was proposed, to take into account the impact of the additional source of carbon fed into the reactor. The increase of system pressure led to changes in the composition of the product gas in line with predictions of Le Chatelier’s principle. The influence was predominantly visible in higher yields of CH4 and lower overall production of product gas. For higher hydrocarbons (CxHy), the trend was unclear. A set of stable gasification parameters were achieved for each pressure level and a standard gasification temperature of 850 °C, except for gasification of lignin performed at 2 barg. A proposed explanation for the problem is the combined effect of the increasing concentration of ash in the fluidized bed and its low characteristic melting temperatures. Due to the obtained experimental findings, a new ash agglomeration index was formulated.

Use of CO2 in Pressurized, Fluidized Bed Gasification of Waste Biomasses

“…The results presented hereafter should be viewed as taking into account the previous small-scale gasification experiments [27]. The lab-scale tests indicated the following issues as potentially most problematic:…”

“…Here, the bark was blended with straw (85%/15% w/w) to assess the limits of the above-described idea. In previous publications of the Heat-to-Fuel consortium, it is possible to find more details behind the rationale of the selection, thermodynamic deliberations, as well as small-scale laboratory and fluidized bed experiments [27,32,34].…”

“…Hence at the bench scale, agglomeration of the bed was observed only after two conditions were fulfilled simultaneously; the temperature of the process exceeded the melting temperature of ash and a large amount of ash was collected in the bed material. Therefore, the concepts of time before defluidization or critical concentration of ash in bed material were analysed against models and theories that can be found in the literature [26,27]. Interestingly, during the PDU-scale experiments, no signs of bed agglomeration could be detected for the same gasification conditions.…”

Novel Process to Convert Wet and Dry Organic Feedstocks into 2nd Generation Biofuels: A Scientific Review of the Heat-to-Fuel Project

“…An autothermal fluidized bed reactor was used to carry out research on the influence of pressure on the gasification process of different types of biomasses [10]. The gasified feedstocks were bark, lignin and softwood pellets.…”

Conversion of CO2 during the DFB biomass gasification process

“…Other research groups like CEA in France [11], Jeremias et al [12][13][14] in the Czech Republic, Stec et al [15] in Poland, Cheng et al [16] in Singapore, and Szul et al [17] from the Institute of Chemical Processing of Coal (IChPW) in Poland also already examined the use of CO 2 as gasification in fluidized bed reactor systems. The main outcomes of their works comply with the findings of the first experimental test runs in the DFB reactor system.…”