Microwave Heat Treatment of Black Liquor for the Degradation of Hemicellulose

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 proces
Popular Science Summary: Production of biofuel – processing of paper mill by-products to fuel components A new process is being developed to produce fuel from the paper mill by-product black liquor. In this Thesis, a new heat treatment method was investigated to degrade large molecules in the black liquor to smaller ones, using microwaves. The production of fuel is the largest use of fossil petroleum. Therefore, finding sustainable alternatives is an important step to reduce the emissions of greenhouse gases that are causing climate change. One such alternative is to use biomass, for example waste products from the agriculture and the forest industry. In Sweden, the forest industry is an important part of the national economy. The pulp and paper mills produce excessive amounts of a by-product called black liquor, which today is used to produce steam for energy production in the mills. However, the black liquor has a high content of the macromolecule lignin, which has the potential to be used for more refined and valuable products. Lignin is one of the main components of wood, providing the wood with its firm structure. The lignin molecules consist of various rings of carbon atoms linked together in complex irregular networks. Breaking down these networks into smaller fragments, the resulting components are similar to molecules found in petrol. An idea is to use black liquor to produce biofuel similar to gasoline or diesel. SunCarbon is developing a process that can be integrated with pulp mills with the intention to extract the lignin from the black liquor and recycle the other chemicals to the mill. The lignin is separated from the black liquor by a series of processes starting with membrane filtration, after which follows a heat treatment that breaks down lignin and other large molecules into smaller components. It is especially important to degrade hemicellulose, which is another molecule found in wood. Otherwise, the hemicellulose may cause problems downstream i