Connecting lignin gasification with syngas fermentation

Abstract

The development of lignin derived energy products is one way to increase the value of biorefinery residues, which is the scope of the EU project AMBITION. Gasification of (lignin-rich) biorefinery residues, followed by product gas cleaning and anaerobic fermentation, offers a potential to produce higher added-value products such as biofuels and chemicals. MILENA indirect gasification allows complete fuel conversion and produces a high value gas composed of CO, H2 and CO2, as well as compounds such as CH4, C2-C4 gases, benzene, toluene and xylene (BTX). The separation of the most valuable components of the product gas is a good way to maximize the value from the feedstock via co-production schemes. The product gas, after appropriate cleaning to remove impurities that can reduce the fermentability of syngas, can be applied in the gas fermentation process. Some anaerobic microorganisms, known as acetogens, can be used as a biocatalyst for the conversion of syngas into short-chain organic acids and alcohols, like acetate, ethanol, butanol, butan-2,3-diol and butyric acid. The ability of these microorganisms to withstand some of the impurities contained in the syngas and their flexibility to use different mixtures of CO and/or CO2 and H2 makes these bacteria an attractive alternative to the chemical catalytic processes. Despite these advantages, the integration of gasification with syngas fermentation is still in an early stage of development, where many questions exist concerning the syngas quality needed in the fermentation process. The challenge is to define the optimum gasification conditions for this type of feedstock that will provide a H2:CO:CO2 ratio at values suitable for syngas fermentation, as well as to identify and remove the compounds that can inhibit the performance of the microorganisms. In this work a first attempt to combine the two processes is presented.A lignin rich feedstock was gasified with steam at 780°C using MILENA indirect gasifier, at TNO. The product gas after removal of the main impurities, consisted of CO, H2, CO2, N2, CH4 and traces of other gaseous hydrocarbons, benzene and H2S. The influence of the obtained syngas quality and composition was evaluated in the fermentation process, at KIT. For comparison, product gas from beech wood gasification after cleaning was also evaluated in the fermentation process under the same conditions.The process involved growing cells in a batch system under continuous flow of biomass-derived gas. The strain used in this work is Clostridium ljungdahlii. The fermentation of both beech wood and lignin-derived syngas was successful, since no inhibition was observed. The carbon fixation onto products achieved for both cases was approximately 55%, while a slightly higher ethanol production was observed with the lignin-derived syngas. The total productivity (including both acetate and ethanol) at the end-point was 0.18 g/L/h for both fermentations.

Side-by-Side Comparison of Clean and Biomass-Derived, Impurity-Containing Syngas as Substrate for Acetogenic Fermentation with Clostridium ljungdahlii

“…Final Y P/S, used was 0.83 g product /g substrate , and final Y P/S, fixed , 0.97 g product /g substrate. Maximum gas usage was obtained between 17 and 38 h [31]. Regarding the lignin gas, the maximum CDW was 0.61 g/L at 66 h, similar to the beech wood syngas.…”

“…End-point Y P/S, used and Y P/S, fixed were, respectively, 0.79 g product /g substrate and 0.92 g product /g substrate . For this syngas, maximum gas usage was detected between 22 and 48 h [31].…”

“…These mixtures mimicked the composition of the syngas produced from the gasification of beech wood or lignin, respectively, at the MILENA indirect gasifier at TNO Energy Transition, Biomass and Energy Efficiency Unit, The Netherlands [13], but no CH 4 was included in the mixture. The composition of the two original biomass-derived syngases produced at the MILENA plant, its impurities and the outcome of its fermentation can be found in Liakakou, Infantes et al [31]. Set-ups 5 and 6 were performed under the same conditions as the beech wood and lignin biomass-derived syngas, respectively.…”

“…This means that the standard off-gas analysis, as described elsewhere [34], would not be possible, since N 2 is used in this fermentation system to calculate the total flow rate in the off-gas. The syngas from LNEG and ENEA contained CH 4 , and since it has been proven to be inert [31] and can be easily detected by GC analysis, CH 4 was included in the custom-made gas mixture and used in place of N 2 to calculate the off-gas flow rate in the same manner.…”

“…The results of the fermentation of two biomass-derived syngas, following the gasification of beech wood and lignin at the MILENA gasifier, TNO Energy Transition, Biomass and Energy Efficiency Unit (Netherlands), were shown in Liakakou, Infantes et al [31]. Some of the results obtained there were: for the beech wood syngas, the maximum CDW obtained was 0.62 g/L at 66.5 h; the final acetate and ethanol concentrations were 15.6 g/L and 1.6 g/L, respectively, with a total final productivity of 0.18 g/L•h.…”