Microwave Heat Treatment of Black Liquor for the Degradation of Hemicellulose

Abstract: The largest use of fossil petroleum is the production of fuels. Hence, finding sustainable alter-natives is an important step to reduce the emissions of greenhouse gases. One such alternative is to produce fuel from biomass, such as waste products from agriculture and the forest indus-try. In Sweden, the forest industry is an important part of the economy. The pulp mills pro-duce an excessive amount of a by-product called black liquor, which has a high content of the biopolymer lignin. The company SunCarbon is developing a process to separate the lignin from the black liquor, purify it and recycle the remaining chemicals to the pulp mill. The purified lignin is to be mixed with a carrier oil to a lignin-rich oil that can be used in refineries to pro-duce fuel such as gasoline and diesel. One of the steps in the process is the heat treatment, which has the purpose to degrade espe-cially hemicellulose, but also lignin, into smaller molecule fractions. A problem with the heat treatment is the corrosion of steel in the reactor vessel, causing the release of metals that pol-lute the product oil and that are difficult to remove. To solve this problem, the idea to use mi-crowave heating has been introduced. Traditional heating methods require high temperature of the reactor vessel to heat up the medium by conduction and convection. Microwaves heat the material directly and thus the prior heating of the reactor vessel is not required. This means that the temperature of the reactor vessel should be lower and enable for the use of Teflon vessels that do not leach metals to the liquor. The direct heating of the material should also enable faster heating rate. In this Master Thesis, microwave heat treatment was investigated and compared to traditional heat treatment in a steel autoclave. Evaluation was made by ana-lysing the degradation of hemicellulose and lignin with chromatographical methods and lignin yield in the subsequent separation by acidulation in the SunCarbon process. The microwave treatment resulted in degradation of lignin of the same degree as the heating in the autoclave, at temperatures up to 200°C. The microwave had a faster heating rate than the autoclave. When varying the heating time and the time at the target working temperature, the results indicated equal degradation of lignin independent of treatment time. Analysis of sugar content after treating with acid hydrolysis indicates that the degradation of hemicellu-lose was greater in the autoclave for treatment at 200°C. This could be explained by the short-er heating rate or the instability of the pressure in the microwave vessels. The sugar concentra-tions were very similar in material treated with the two heat-treatment methods at lower tem-peratures, where no leakage had occurred, and the treatment time was equal. Neither of the two heat-treatment methods degraded hemicellulose as much as expected, based on previous studies by SunCarbon. This might be due to insufficient temperatures or treatment times, as temperatures above 200°C could not be investigated with the current laboratory microwave equipment. Conclusively, only a short heating time is sufficient to degrade lignin, but to de-grade hemicellulose longer treatment times or higher temperatures are required. The degrada-tion of both lignin and hemicellulose seem to be independent of heat treatment method, indi-cating that the use of microwaves can be as efficient as other heating methods. This was also confirmed by the nearly equal yields of lignin in the subsequent separation process. When treating black liquor retentate with acid hydrolysis in a pH range of pH 1-3, a substance is formed that precipitate and form crystals when dissolving the mixture with 50% acetoni-trile/water. Analysis indicates that the crystals are composed of oligomers of five monomers, each with the molecular weight 142 Da. Further investigation is required to identify the com-position of the crystals, for example using NMR analysis.