While our modern world primarily seems to be out of plastic and metals, wood still plays a crucial role in existing products, especially in our living rooms. Increasingly people also think of wood as a carbon negative construction material which solidifies the role, wood and its production plays now and in the future. In the United States, the wood-product industry consumed 388 trillion Btu, which is equivalent to 113,7 TWh, while the sawmill industry consumed 161 trillion Btu, corresponding to approximately 47,2 TWh (EIA, 2021). This may not be the most energy intensive industry, however there is a need to reduce emissions from the energy used.

The energy used is mostly thermal. After trees are cut, they undergo a series of processes to prepare and transform the wood into usable and stable material. This involves both mechanical and thermal treatments to shape, dry or maybe also improve the properties of the wood, which are important for the usage. In the woodworking industry a large sector of the energy needed is thermal energy, around 60-80% of the total energy used (Todd A. Morgan et al., 2019; Jingge Li et al.). This thermal energy is primarily used to heat air or steam. Usually, fossil fuels are used to generate heat, smudging the sustainable character of wood as a carbon sink. Going forwards, the wood industry needs to become more sustainable, not only regarding monocultures and land protection but also in processing the material.

After trees are felled, they are transported to a sawmill, where the logs are cut into many different products such as boards, planks, or veneers. Right after these mechanical steps, the wood must be treated thermally, involving several processes which each require specific temperature ranges, energy inputs and heat transfer media.

One of the most used thermal processes in the wood industry is kiln drying. Here, the moisture content of wood is reduced to 6-15%, depending on the intended use later on (Von Wood, Glossary). The removal of the moisture is crucial, because it prevents the product from undesired side effects like cracking. There is a wide selection of kilns, such as solar kilns, vacuum and radio frequency kilns, but the most frequently used kiln is the drying kiln using fossil fuels (Walker et al., 1993; Desch and Dinwoodie, 1996).

Drying kilns use special chambers designed to remove the moisture from lumber under controlled conditions. Large kilns may reach lengths up to 30 meters and more. Smaller kilns are mostly used inlower-capacity operations like specialized wood processing.

Mostly the kilns are made of durable, heat-resistant materials such as brick masonry or hollow cement-concrete slabs and coated inside with lime, mortar plaster and impermeable coatings. In modern kilns there are often insulation materials used such as glass wool or PUR-foams (xiandry.com). The temperature range for kiln drying operates between 60-130°C (140-266°F), also depending on aspects such as species and thickness of the wood used (Boone.S.C., C. Bois., and E. Wengert. 1988).

The heat media used for the kiln drying typically is hot air. The machinery responsible for heating the air for the drying kilns includes heat exchangers, burners, fans and pipes of many sorts. The heated air iscirculated to transfer the heat to the wood and can be generated through burning fossil fuels. Kraftblock thus can supply the hot air in existing kilns with systems that use renewable electricity rather than fossil or limited bio sources.

In direct-fired systems, the hot gases mix with air entering the kiln. Depending on the interface, fossil gas can be replaced by hot air from clean sources. This is even more unproblematic in indirect-fired systems, where metal surfaces, which act as a heat exchanger, are heated to transfer heat without direct contact of the hot gas.

Clean steam for the wood industry

Another step in the woodworking is steam bending. Bent wood is used in furniture, boats or in architectural woodwork. The end temperature for the product is around 100°C (212°F). The steam penetrates and softens the woodfibers, in cooling out the new form is kept. Steam bending requires specific wood that responds well to this process (internationaltimber.com). For industrial produced bent wood components, steam bending is done in industrial scale and thus requires a shift to steam from green sources like renewable electricity.

Wood can also be thermally modified. Thermal modification describes heating the wood to high temperatures to alter the chemical and also physical properties, such as improving the resistance to decay, its durability and stability. The temperatures in this process range between 160 - 240°C (320 - 464°F), out of the range of heat pumps. These high temperatures break down the hemicellulosein the wood, which can therefore no longer absorb water and being more resistant to factors like biological degradation. The used heat media is once again steam, but in this case superheated steam. The superheated steam is often used in combination with inert gases, such as nitrogen, so the process occurs in an oxygen free environment, reducing the risk of combustion of the wood (Munir, Muhammad, Tanveer et al., 2020). The heat must be supplied by long durations as shown in the scheme of Percin and Altinok:

Generating steam on different temperatures or pressures is a standardized solution of Kraftblock: Electricity is charged when cheap and converted to high-temperature heat to be stored efficiently. When needed, also 24/7, steam is produced by hot air coming out of the storage units. Thus, fossil-free steam replaces the fossil boilers used in the processes. For many countries, the cheap electricity from sun and wind allows to compete with natural gas, and in the future to result in lower costs..

In conclusion the thermal processes are essential for the wood industry. Electrification paves the way to free the industry from fossil fuels. With flexible electrification the cost reduction of electrification the industry can transition smoothly without cost disruptions. In case of controlled hot air and steam applications, no equipment needs to be replaced, but can be supplied from a Kraftblock system.

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