Pretreatments Processes to Control Product Selectivity and Yields of Residual Lignocellulosic Biomass Fast Pyrolysis

The past two decades have been ridden with global energy concerns. Industrialization throughout the world, in addition to the increase of world population, has caused a drastic increase in energy use while still having a reliance on a limited source of fossil fuels, which in turn has caused an increase in energy prices. Furthermore, environmental concerns have risen around the escalating greenhouse gas emissions. Overall, the depletion of fossil fuels, increase in energy costs, and environmental concerns have shifted the research focus towards renewable energy options.

One type of renewable energy being researched extensively is biomass energy, a source of energy derived from lignocellulosic biomass. The carbon-containing components of lignin, hemicellulose, and cellulose provide an adequate feedstock for conversion to liquid fuels via biochemical and thermochemical conversion technologies and their combinations.

In order to truly decrease the reliance on fossil fuels by optimizing the utilization of biomass, biomass feedstock must be readily available in large quantities and require minimal energy derived from fossil fuels for production, collection, transport, and storage. The production of biomass should also avoid using fertile land that would be more suitable for growing food crops. Residual biomass materials, which are commonly considered waste products in agriculture, forestry, household, and manufacturing activities have large potential to be used as feedstock for biofuel production. They are typically readily available in large quantities in centralized locations.

Two biomass materials are investigated in this study, i.e. paper mill sludge and phragmites Australis(also known as common reed grass). Paper mill sludge (PMS) is generated by a typical paper mill at a rate of 15 to 50 tons per day, adding up to approximately five million tons annually in the United States alone. With minimal practical uses of the sludge, it is mostly disposed in landfills, causing environmental concerns. In addition, the decline of available landfill space and increased transportation costs have increased the disposal cost to roughly 60% of the paper mill’s operating costs. The increased financial burden of disposal has increased efforts to find alternative exploitations of the sludge.

The second biomass being studied, phragmites Australis, is a common reed grass that is   prevalent in the wetland on Villanova University’s campus. It is an abundant invasive plant in many regions of all continents, except in Antarctica. The grassy biomass offers a valuable feedstock due to its high yield on the order of 18 to 28 tons of biomass per acre annually as well as the ability to survive during the winter months.

This study focuses on the utilization PMS and phragmites Australis for production of fuels via fast pyrolysis. Fast pyrolysis is a thermochemical conversion process, in which biomass is decomposed to produce a liquid (bio-oil), a solid (bio-char) and a non-condensable gas mixture (syngas) by exposing biomass to 500^oC heat in the absence of oxygen. Fast pyrolysis is an attractive biomass conversion process since it can be used for processing any type of biomass.

It has been known that the selectivity and yield of fast pyrolysis products, i.e. bio-oil and bio-char, and their quality/properties are dependent on the physicochemical properties of the biomass feedstock. It is desirable to have the ability to control the selectivity and yields of fast pyrolysis products, particularly the yields of bio-oil and bio-char products as well as the chemical product distribution of bio-oil to meet the final application demands for the products. This study investigates two biomass pretreatment options, hydrolysis and torrefaction processes, which will result in biomass feedstock that can be pyrolyzed to produce bio-oil and bio-char products with common properties regardless the origin of the feedstock . The significant differences in the two biomass materials studied display promise for future implementation across a broad range of residual biomass materials as fast pyrolysis feedstock.

It has been demonstrated in the study that the pretreatments achieved more selective product distribution in the bio-oil composition as well as an enhanced bio-char composition. Fast pyrolysis of PMS and phragmites Australis that have been treated with  specific combinations of acid hydrolysis and torrefaction pretreatment process selectively decomposed the pretreated biomass towards the production of bio-oil containing primarily sugar-derived products, such as levoglucosenone. Bio-char composition was largely unaffected by the different levels of acidity, but showed ideal characteristics with low volatile matter content and consisting of mainly fixed carbon, an ideal composition as a soil amendment.