HOT|COOL NO.3/2021 - "Don't waste it!"

Key figures for a 20 MW SkyClean system Input on an annual basis • 45,000 tons of rawmaterials (straw, deep litter, manure, fiber residue from biogas, wood chips, and the like). Output on an annual basis • 48,000 MWh heat, corresponding to 3,000 household annual heat consumption. • 6,100 tons of bio-oil, equivalent to 4,000 Danes' annual con- sumption of liquid fuels • 36,000 tons of CO 2 captured and stored in the formof biochar • 19,000 tons of CO 2 saved emissions from the fuels Promising potential for integration with district heating Due to the inherently local and regional aspect of a large-scale implementation of SkyClean where plants ideally are distrib- uted close to local feedstocks, there are promising synergies between a commercialized SkyClean deployment and local DH networks. The world is transitioning from centralized fossil fuel-based power sources towards decen-tralized renewable energy sources, which is also reflected in DH infrastructure develop- ment. Presently, in Denmark, legislators have decided to clear the way for local heat energy producers to offer their excess heat to the local DH network commercially. This has prompted Danish cooperative retail giant Coop to announce that they plan to provide excess heat from local stores to local DH networks. This type of development opens up many new possibilities for sustainable heat production, and with technologies like SkyClean, the prospect of connectivity with DH is very attrac- tive. A SkyClean plant could be made to fit into an existing DH infra- structure either as a new DH plant or as a source of heat energy to existing DH networks. The competencies required to oper- ate a SkyClean plant resemble those of conventional biomass- based plants already inte-grated with DH in some places.

Dry plant material typically contains approx. 50% carbon that the plants have extracted from the atmosphere in the form of CO 2 . In the pyrolysis process, half of the carbon in the residue is converted to biochar, while the other half becomes oil and gas. Biochar is a stable material that only decomposes very slowly, and half of the carbon that becomes biochar is thus effectively removed from the atmosphere. Half of the carbon that does not turn into biochar comes out of the pyrolysis process as carbon-neutral fuel in the form of gas and oil, and heat energy. Due to the nature of the SkyClean process, where half of the carbon in agricultural residue is stored as biochar in each cycle, SkyClean achieves the paradoxical but very beneficial effect that the more fuel the plant produces, the more CO 2 is removed from the atmosphere. The roadmap towards commercialization SkyClean is being developed by the Stiesdal subsidiary Stiesdal Fuel Technologies. In August 2021, the company introduced a fully automated 200 kW SkyClean test facility capable of treat- ing 500 tons of agricultural residue annually, which gives a to- tal CO 2 emission reduction of approximately 600 tons. The test plant is part of the preparations for the ten times larger 2 MW SkyClean plant, which Stiesdal Fuel Technologies will finance and build later this autumn, expected to be inau- gurat-ed in early 2022. The preparation of the SkyClean technology towards actual commercialization will continue in 2022 when another 2 MW pilot plant is established further to develop processes and further optimization for commercial production. This plant has received a state grant of DKK 23 million and is being built in collaboration with Haldor Topsoe, Arla Foods, Ørsted, and the Technical University of Denmark. Stiesdal Fuel Technologies expects to begin construction of the first actual prototype of a commercial 10-20 MW SkyClean plant in the autumn of 2022. The company is aiming to make its technology scalable from the outset and to reduce costs through standardization. This means standardized modules for feedstock storage and preparation, standardized pyrolysis units, and streamlined post-processing of oil, gas, and biochar.

A visualization of how a standardized 20 MW plant would look.

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