Implications of feedstock diversity on forest-based ethanol production

The utilization of lignocellulosic biomass to produce biofuels, such as bioethanol, has the potential to provide a sustainable alternative to fossil fuels, and thus mitigate greenhouse gas emissions from the transportation sector. Forest biomass is expected to be a significant source of such biomass, as it can serve as an abundant and sustainable feedstock for bioethanol production. It is unlikely that white wood chips will be used as a sole commercial feedstock for the production of bioethanol, due to increasing feedstock competition and requirements to meet large scale. The high demand for biomass means that other forestry assortments, not traditionally utilized by the forest industry, such as harvesting residues, will have to be exploited. However, the presence of bark in these forest residues is expected to pose a challenge in the traditional wood-to-ethanol process and adversely affect the conversion efficiency.
Ethanol production from softwoods was investigated with the main objective of
assessing the potential of expanding the feedstock base of an ethanol plant to include not only white wood, but also other forestry residues from a process perspective. Bark was found to be significantly more difficult to hydrolyze to monomeric sugars than white wood. This could mainly be attributed to the condensation reactions of bark extractives during acid-catalyzed steam pretreatment, which rendered the otherwise water-soluble extractives insoluble, and altered the structure of the solid fraction, resulting in impaired enzymatic hydrolysis. Techno-economic evaluations showed decreasing profitability of ethanol production with increasing bark content in the feedstock. Thus, the utilization of bark-containing forestry residues will not lead to significant cost reductions compared to higher-value pulpwood at current market prices, unless the conversion of cellulose and hemicellulose to monomeric sugars is improved.
Another alternative to increase the future biomass supply for large-scale bioethanol production is the use of fast-growing trees such as willow and poplar. Although the production of ethanol from these hardwood species is well documented, the inclusion of biomass from fast-growing tree species in a softwood feedstock base for bioethanol production has not previously been investigated. The structural differences between hardwood and softwood could be expected to reduce the pretreatment efficacy when treating a mixture of the two. However, it was found that the use of a mixture of poplar and spruce would presumably be constrained more by the performance of the fermenting microorganism, than the efficacy of steam pretreatment, and that the
ethanol production process could be sufficiently robust to allow small amounts of hardwood in a softwood-to-ethanol process.

Carbon dioxide and other greenhouse gases released into the atmosphere from a range of human activities are causing warming of the global climate. In order to limit the increase in temperature to well below 2°C above pre-industrial levels, a goal pledged in the Paris Agreement by nearly 200 countries, the emission of these gases worldwide need to approach zero in the long term. Today, the largest contribution to climate change, in Sweden and around the world, is from the burning of fossil fuels such as oil, coal and natural gas to provide us and our industries with heat and electricity, and to run our vehicles.
Ethanol, a plant-derived renewable fuel, has been identified as an alternative to fossil fuels, with the aim of decreasing the carbon emissions associated with the transport sector. Ethanol, or ethyl alcohol, has the same chemical formula regardless of whether it is in alcoholic beverages or in fuel. It is a colorless, volatile and flammable liquid that can be produced from biomass (and is thus often called bioethanol), and can replace gasoline in our cars. Ethanol has become a price-competitive fuel due to rising global oil prices, however, it is currently mainly produced from edible feedstocks, such as corn, wheat, sugarcane and sugar beet. Research suggests that a greater reduction in
greenhouse gas emissions could be achieved by utilizing the residual biomass from industrial, agricultural and forestry activities. While well-established technology can be used to produce ethanol from grains and other sugar-containing crops, the technology required for the production of ethanol from these so-called lignocellulosic biomass feedstocks is still being developed.
Sweden is a country dominated by forests, and sustainable forestry is vitally important for its national economy. With its access to raw materials, the forest industry is well-positioned to diversify its products through wood-to-ethanol production. This would contribute significantly to reaching the goal of zero net emissions of greenhouse gases, which Sweden has pledged to achieve by 2045 at the latest. Increased environmental concerns and technological advances in the production of ethanol from wood biomass make forest-based ethanol an increasingly attractive option, but large-scale implementation requires the efficient utilization of low-cost residues from forest or silvicultural harvesting (e.g., thinnings, branches, low-value decayed trees).
The aim of the work presented in thesis was to assess the feasibility of utilizing various forest-based feedstocks potentially available as raw materials for future ethanol production, and its implications on the wood-to-ethanol conversion process. Different types of forest biomass have different properties (e.g., energy content, moisture content, particle size), and different degrees of heterogeneity, which can affect the conversion process. Moreover, the presence of bark in these feedstocks can also place extra demands on the process and influence conversion efficiencies.
Acid-catalyzed steam pretreatment, one of the pretreatment strategies commonly used for processing wood biomass, was not found to be effective for the pretreatment of bark, and techno-economic evaluations showed decreasing profitability with increasing bark content in the raw material. It was shown that several key aspects of the process need to be further developed and optimized before forest harvest residues can be used to produce ethanol. For instance, fine-tuning of the pretreatment process and the pretreatment conditions based on the feedstock composition is needed to ensure maximum sugar recovery from bark-containing forest residues. This would provide significant cost improvements, and facilitate the implementation of large-scale ethanol production from wood.