Fast pyrolysis of lignin pre-treated with magnesium formate and magnesium hydroxide

Authors: PATEL, M - University Of Maine; Mullen, Charles; GUNUKULA, SAMPATH - University Of Maine; DESISTO, WILLIAM - University Of Maine

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2020
Publication Date: 9/23/2020
Citation: Patel, M., Mullen, C.A., Gunukula, S., Desisto, W.J. 2020. Fast pyrolysis of lignin pre-treated with magnesium formate and magnesium hydroxide. Energies. 13:4995.

Interpretive Summary: Lignocellulosic biomass such as wood, grasses, and crop residues is comprised of cellulose, hemicellulose and lignin. While the carbohydrate fractions (cellulose, hemicellulose) of biomass is used in many applications including pulp, paper, fiber and cellulosic bio-ethanol industries, lignin is often left as a low value byproduct in these industries and is typically burned for energy. Production of value added co-products from lignin has long been a goal of researchers in these industries; however, lignin has been proven difficult to convert. One method considered is thermal depolymerization of the lignin by pyrolysis, the heating in absence of air to produce liquid bio-oil that can be refined to biofuels and renewable chemicals. However, pyrolysis of lignin has proved difficult resulting in only partial depolymerization, low yields and often rapid fouling of pyrolysis reactors. In this work lignin was pretreated with the additives including magnesium hydroxide and formic acid. This improved the properties of the lignin so that they could be pyrolyzed with reduced fouling of the equipment. A pre-run laboratory test was developed to predict whether a particular formulation could be successfully continuously pyrolyzed. Pyrolysis of the pretreated lignin produced a bio-oil with high concentration of phenolics, potentially valuable renewable chemical products. Pretreatement with both magnesium hydroxide and formic acid (generating magnesium formate) were more successful in terms of yield and concentration of phenolics than pretreatment with magnesium hydroxide alone. The results of this work will allow for the further development of thermochemical lignin conversion methodologies and will be of interest to those looking to utilize residual lignin to produce renewable industrial chemical feedstocks from the aforementioned sources.

Technical Abstract: Kraft lignin (Indulin AT), pretreated with magnesium formate and magnesium hydroxide was fast-pyrolyzed in a continuously-fed, bench-scale system. To avoid fouling issues typically associated with lignin pyrolysis, a simple laboratory test was used to determine suitable ranges of magnesium hydroxide and formic acid to lignin for feeding without plugging problems. Various feedstock formulations of lignin pretreated with magnesium hydroxide and formic acid, were pyrolyzed. For comparison, calcium formate pretreated lignin was also tested. Organic oil yield ranged from 9 to 17% wt on a lignin basis. Carbon yields in the oil ranged from 10 to 18% wt on a lignin basis. Magnesium formate pretreatment increased oil yield and carbon yield in the oil up to 35% relative to the higher a 1:1 g magnesium hydroxide/g lignin pretreatment. However, a lower magnesium hydroxide pretreatment (0.5:1 g magnesium hydroxide/g lignin) resulted in oil yields and carbon yields in the oils similar to the magnesium formate pretreatments. Magnesium formate pretreatment produced more oil but with a higher oxygen content than calcium formate under the same conditions. GC-MS analysis of product oils indicated that phenols and aromatic hydrocarbons were more prevalent in pyrolyzed magnesium formate pretreated lignin.