by Linda Bertelsen
The idea to combine the production of district heating (DH) with carbon-negative technologies is still in its infancy, with few operational concepts implemented. This reflects the overall picture regarding carbon-negative technologies; they are urgently needed to reach the goals of the Paris Agreement, but no technology-based large-scale carbon capture and storage methods are in place yet.

By Jesper Ahrenfeldt, Chief Engineer at Stiesdal Fuel Technologies, Denmark.

The Danish climate technology firm Stiesdal intends to change that with their SkyClean technology, which is currently scaling up towards commercialization. Stiesdal relies on industrial-scale biochar production for carbon capture and storage combined with green fuel production. The company’s approach happens to be a great fit for local and regional DH.

How SkyClean works

The core of SkyClean is a pyrolysis process in which organic waste from agriculture and forestry is converted into biochar, gas, and oil by heating to a high temperature without oxygen.

Dry plant material typically contains approx. 50% of carbon that the plants have extracted from the atmosphere in the form of CO2.

In the pyrolysis process, half of the carbon in the waste 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, oil, and heat energy.

How SkyClean works

Due to the nature of the SkyClean process, where half of the carbon in agricultural waste is stored as biochar in each cycle, SkyClean achieves the paradoxical but very beneficial effect that the more fuel the plant produces, the more CO2 is removed from the atmosphere.

The roadmap toward 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 treating 500 tons of agricultural waste annually, which gives a total CO2 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 inaugurated in early 2022

A visualization of the soon-to-be 2 MW SkyClean facility in GreenLab, Skive, Denmark.

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 aims 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.

Key figures for a 20 MW SkyClean system

Input on an annual basis

  • 45,000 tons of raw materials (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 consumption of liquid fuels
  • 36,000 tons of CO2 captured and stored in the form of biochar
  • 19,000 tons of CO2 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 distributed 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 decentralized renewable energy sources, which is also reflected in DH infrastructure development. 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 its 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 attractive.
A SkyClean plant could be made to fit into an existing DH infrastructure either as a new DH plant or as a source of heat energy to existing DH networks. The competencies required to operate a SkyClean plant resemble those of conventional biomass-based plants already integrated with DH in some places.

Carbon-negative heat production can accelerate climate goals.

Keeping global warming within 1.5 – 2 degrees C is not possible with reduced CO2 emissions only. It also requires the large-scale application of technologies that can remove CO2 from the atmosphere. The minimum amount of CO2 that will need to be removed from the atmosphere to breach the maximum allowance of atmospheric CO2 specified by IPCC will be ten gigatons per year from 2055 onwards.
Few CCS technologies on the horizon promise to be able to deliver at such a scale. Biochar production, however, promises to be one of the few pathways towards large-scale sustainable sequestration of CO2 at low cost since almost any type of biological waste can serve as feedstock for pyrolysis processes like the one SkyClean uses.
Experts from the Technical University of Denmark and Aarhus University have estimated that Danish agriculture can reduce greenhouse gas emissions by 50 % using the SkyClean technology.
It is not too much of a stretch to see the existing synergy between DH and biofuel expand to include biochar-based heat production. Fun fact: to remove ten gigatons of CO2 from the atmosphere annually, 200,000 SkyClean plants with 20 MW capacity could do the job.

Biochar is posed to enter the big league of large-scale CCS technologies

Stiesdal’s SkyClean technology joins the growing efforts to elevate biochar from a soil-improving by-product of pyrolyzed feedstock to a center-stage role among CCS technologies.
Biochar is a stable material that does not decompose for hundreds of years. The biochar pellet output from the SkyClean process is easily handled and spread on farmland with existing farm tools and machinery. It will integrate with farming at any stage of technological development.

Among the wide variety of current and proposed carbon capture and storage (CCS) technologies, biochar production from sustainable biomass has several advantages:

  • It is by far the cheapest CCS technology
  • It actively integrates with and boosts local economies
  • It supports sustainable growth in the farming sector

Biochar has multiple benefits besides carbon sequestration. It delivers long-term carbon sequestration, and biochar also improves soil and increases food production – it has been known for millennia.
The relatively low temperature (600 degrees C) in the SkyClean pyrolysis process leaves the nutrients in biochar readily available for absorption by new crops. It makes biochar a desirable soil additive.
In recent years researchers have found biochar to possess a wide range of additional benefits for agriculture:

  • Biochar significantly improves water retention in sandy and other dry soil types.
  • Conversely, biochar improves water flow in heavy clay soil, which is attractive in regions suffering from increased precipitation due to climate change.
  • Remains of antibiotics, pesticides, hormone-like substances, and microplastics occur in slurry and manure and tend to be concentrated in the soil after fertilization. The pyrolysis process breaks down these residues into harmless constituent molecules.
  • Biochar reduces the leaching of nitrogen. In addition to the benefits of biochar, pyrolysis will also help agriculture lower emissions from methane as the pyrolysis process eliminates methane from the feedstock and incorporates the constituent elements into the oil and gas fractions.

The technology has caught the attention of the Danish authorities and is listed as one of the essential contributing technologies that will help Denmark reach its 2030 CO2 emission reduction target.

For further information, please contact: Jesper Ahrenfeldt, at jah@stiesdal.com

A 20MW SkyClean plant processes 5 tons of biomass per hour and can supply 4 MW of heat output to the district heating system while the rest of the energy in biomass ends up in the biochar and bio-oil.

Meet the author

Jesper Ahrenfeldt
Chief Engineer at Stiesdal Fuel Technologies, Denmark.
SkyClean, a carbon-negative technology ideally matched for district heating” was published in Hot Cool, edition no. 3/2021