Introduction
Combined heat and power (CHP) production via gasification is the most developed application regarding gasification of biomass and waste, with more than 1,700 operational CHP facilities in Europe alone; the number worldwide is much higher.
Most are small-scale units (output 10–500 kWe/30–800 kWt) utilizing local biogenic waste feedstock such as wood chips or agricultural residues, and are predominantly located in Germany, Italy, and Austria. There are also larger gasification-based CHP plants in the UK, Denmark, Sweden, and Finland (output 6–50 MWe/13–90 MWt).
Production Pathways
To produce heat and power, the feedstock is converted via gasification into combustible gas, called producer gas.
The maximum temperatures of the conversion process vary between 800° and 950°C; air, steam, or their mixtures can be used as an oxidizing agent, and its equivalent ratio varies between Λ = 0.2 and 0.5 depending on the type of reactor and feedstock used. In the case of CHP, fixed bed or fluidized bed gasifiers are mostly employed.
The producer gas can be used in several ways:
- After the gasification process, the producer gas is cleaned to remove undesirable compounds (dust and ash) using filters and cooled down by wet scrubbers and/or absorption materials to be used in internal combustion engines for power and heat
Figure 1. Production pathway of power and heat using an internal combustion engine
- The producer gas could be directly combusted in a boiler without cleaning to produce high-temperature heat or steam or to be combined with a steam turbine to produce
Figure 2. Production pathway of high-temperature heat using gas boiler
- Another method is integrated gasification combined cycle (IGCC). In a biomass IGCC plant, biomass is gasified, and after cleaning the producer gas is burned in a gas
Figure 3. Production pathway of power and heat using gas turbine
However, IGCC systems are not as widely commercialized and thus may require additional development and demonstration prior to widespread application.
The pioneering biomass IGCC project in Varnamo, Sweden (6 MWe + 9 MWt), completed its demonstration program in early 2000. Additional biomass IGCC projects have also been developed in the UK.
Opportunities
Bioenergy in general can be seen as a dispatchable energy source. As opposed to solar photovoltaics, wind power, or hydropower, which are dependent on weather or season of the year, it can deliver power and heat 24/7 all year.
Furthermore, by using biogenic feedstock, biochar is produced as a by-product of the process. The biochar can be used in many different ways (e.g., soil amendment, animal feed, water treatment, green road, green concrete), it is a medium for capturing CO2, and it can be seen as an additional income for the gasification operator/owner.
Current Commercial Developments
Thousands of small-scale CHP facilities are spread over the world. Most of them employ lignocellulosic feedstock (wood chips or pellets) for the production of heat and power. More information can be found in the Task 33 database.
Several facilities additionally produce biochar or use agricultural residues or other waste materials. In addition, several large-scale and micro-scale facilities for the production of power and heat should also be mentioned.
CHP and Biochar Production
One example of a successful gasification application for the production of heat, power, and biochar is the Austrian company SynCraft.
The SynCraft technology is based on two-stage gasification. First the biomass is pyrolyzed, and then the fixed material (char) comes to the gasification zone. The gasifier is a floating fixed bed. This unique technology enables the production of high-quality producer gas with very low amounts of tar. In addition, the valuable by-product, biochar, could be used in many ways because of its purity and high carbon content. SynCraft produces facilities with output of 200–500 kWel and 320–770 kWth.
The technology flow sheet can be seen in Figure 4.
Figure 4. SynCraft technology overview (source: https://en.syncraft.at/wood-power-plants/overview)
Nowadays, the company works to further develop and utilize their products and by- products and investigate lower-quality feedstock such as waste wood. Figure 5 demonstrates their possible product pathways.
Figure 5. Possible utilization pathways of SynCraft’s producer gas
Biochar is produced as a by-product of each process, and 1 kg biochar is equivalent to 3 kg CO2. There are several possibilities of biochar utilization, which includes soil enrichment (terra preta), animal feed, and water treatment. Biochar can play an important role as an additive in asphalt, where it is stored for long time, but the main mission is negative- emissions technology.
Employing biochar is currently in the early stages of deployment, but there are many other ways to use biochar practically. For example, biochar can be employed as an additive to building materials, such as concrete, as SynCraft is pursuing (www.syncraft.at).
RESET SyngaSmart technology is based on multiple Imbert design, downdraft fixed bed pyrogasification reactors (Figure 6). It operates a four-stage process at about 900°C peak temperature and releasing syngas and biochar from separated outlets, thus ensuring the highest biochar purity for further uses.
Figure 6. CHP200 model from RESET
Figure 7 summarizes the whole biomass-to-energy process.
Figure 7. Biomass-to-energy process
SyngaSmart technology can apply different feedstocks besides lignocellulosic material, including waste streams such as municipal solid waste (MSW) (organic fraction), sewage sludge, and digestate from biogas plants.
In 2022/2023, RESET installed a plant with 250-kg/h input capacity at an MSW treatment and sorting facility, located near Rome, Italy. The plant consists of an integrated solution for waste transformation into a solid biofuel, to be used in the facility itself for on-site energy and heat production. The installation consists of a pretreatment module, gasification facility (Gas Unit 520), and a power unit with output 200 kWel/292 kWth.
Figure 8. Organic fraction of MSW, as-is and briquetted
Figure 9. Side view of RESET installation
Large-Scale CHP
A gasification facility in Lahti, Finland, should be mentioned as a representative example of a large-scale CHP facility. Lahti Energy’s Kymijärvi II power plant runs on solid recovered fuel that is gasified, and the producer gas is cooled and cleaned before combustion. The plant produces 50 MW of electricity and 90 MW of district heat for the city of Lahti. The fuel consists of various types of industrial, commercial, and building site waste, and some treated household waste is also used. This waste is first converted into producer gas, which is then combusted in the ordinary gas boiler. When gas is cleaned before combustion, it is possible to produce high-pressure and high-temperature steam for the turbine without risk of boiler corrosion. In the Lahti case, the electrical efficiency is over 30%, made possible by the extraordinary steam parameters of the waste boiler (540°C and 120 bar).
Figure 10. Lahti gasification plant setup
Additional large-scale CHP facilities such as in Skive, Denmark, or Joutseno mill, Finland, can be found in the Task 33 database: https://www.task33.ieabioenergy.com/menus/show_database.
Micro-Scale CHP
The gasification systems designed and produced by CMD are based on downdraft gasifier. This architecture is applied in the ECO20x micro-CHP system manufactured by CMD, already commercialized for use with a biomass rate of 20–25 kg/h of woodchips or different biomass compositions.
Figure 11. Circular economy until biochar utilization for growing biomass feedstocks
In addition to green biomass, ECO20x can process specific residual materials such as textile waste and exhausted sewage sludge. CMD is now engaged in developing a new system for hydrogen from syngas (https://eco20cmd.com/en/).
