Biofuel targets and biomass
The demand for biofuels produced from lignocellulosic feedstock is projected to increase significantly in the future, as part of reaching the targets for renewable energy in the transport sector, especially in forest endowed countries like Sweden. However, the potential for increased sustainable biomass usage is still limited, making it important to use the resource efficiently. An opportunity that the forest industry, including sawmills and pulp mills, provides is the availability of by-products that could be used for biofuel production. Three types of by-products are of interest: forest residues, bark and sawdust.
Estimations of the future potential for forest biomass indicate significant prospects for increased biomass supply, compared to the just over 10 TWh per year currently used. The potential for logging residues in 2030 is estimated to 25 to 31 TWh per year. In addition, 13-16 TWh stumps could potentially be harvested, giving a total potential of over four times the current use. The deployment of stumps as biorefinery feedstock is however highly uncertain, due primarily to environmental concerns, but also to technical reasons.
At pulp mills, the bark is removed from the pulpwood before the further processing. Most market kraft pulp mills do not need the bark to satisfy the internal process steam demand. The steam production from the recovery boiler is enough to satisfy the mill process steam demand. At a large Swedish kraft pulp mill, producing 2000 ADt of pulp per day, approximately 70 MW is available.
Sawdust, woodchips and bark are by-products from sawmills. Approximately 15 MW of bark, 20 MW of sawdust and 50 MW of woodchips are produced at sawmills with an annual capacity of 250,000 m3 sawn wood. Part of the by-products (just over 10%), primarily bark, are used to satisfy the internal heat demand.
Synthetic natural gas
Different value chains based on domestic forest biomass for the production of bio‑SNG were evaluated. The results show that the total cost for SNG is dominated by capital cost and the cost for raw materials and is therefore found to be sensitive to the investment cost, as well as the price of raw materials.
A higher SNG production rate will result in significantly lower total cost because the decrease in specific capital cost is greater than the increase in transportation costs. The lowest total cost was found for value chains in which falling bark was dried at pulp mills and transported to the SNG plant. Similar total costs are found for value chains in which forest residues were transported directly to the SNG plant and for value chains in which forest residues were first transported to a pulp mill for drying.
The use of falling bark from kraft pulp mills is interesting from an economic point of view because the first transportation step can be avoided and no additional investment for biomass handling at the mill will be required. However, there is uncertainty about how much bark can be used in the SNG process. No additional costs related to the O&M of an SNG plant were included when bark was used, which could be the case in reality.
The value chain using pellets indicated lower transportation costs, but the total costs were the highest for this value chain due to the relatively high energy use for pellet production, i.e., more pre-treatment than was required for the SNG process to lower transport costs was found unprofitable. Value chains with intermediate products based on forest residues had higher total transportation costs than direct transport of forest residues to the biofuel plant. However, if bark were used the transportation costs became lower. Using the available pulp mill excess heat for drying bark or forest residues is a way to “move” excess heat to another site (the SNG plant) where it could be used for district heating, which increases the revenue for the integrated SNG plant.