PVC2: Power and heat via gasification
Thermal gasification produces a syngas, which can be used for the production of power and heat, or further processed into transport fuels, see PVC1: Transport fuels via gasification.
For gasification, any lignocellulosic material is suitable as feedstock. What was mentioned before for PVC1 feedstock is also valid. The term lignocellulosic covers a range of plant molecules/biomass containing cellulose, with varying amounts of lignin, chain length, and degrees of polymerization. This includes wood from forestry, short rotation coppice (SRC), and lignocellulosic energy crops, such as energy grasses and reeds. Biomass from dedicated felling of forestry wood is also lignocellulosic but is not considered sustainable.
Gasification is a thermochemical conversion process at 800-1300°C using a sub-stoichiometric amount of oxygen (typically l = 0.2-0.5). Under these conditions the biomass is fragmented into raw gas consisting of rather simple molecules, such as: hydrogen, carbon monoxide, carbon dioxide, water, methane, H2S, NH3, HCl, etc. Solid by-products are: tar, char, and inorganic matter. After clean-up of the raw gas, the gaseous molecules are chemically re-synthesized to transport fuels..
After size reduction of the raw material, it is moved into the gasifier. Typical gasification agents are: oxygen and water/steam. The choice of the gasification agent depends on the desired raw gas composition. The combustible part of the raw gas consists of hydrogen (H2), carbon monoxide (CO), methane (CH4) and short chain hydrocarbons; the non-combustible components are inert gases. A higher process temperature or using steam as gasification agent leads to increased H2 content. High pressure, on the other hand, decreases the H2 and CO.
Entrained-flow gasifiers operate at high temperatures (1000-1300 °C) and are therefore suitable when low methane content is preferred, however feedstocks with very low particle sizes are required. Bubbling and circulating bed gasifiers in contrast are operated at lower temperatures (800-1000 °C) and have the advantage of being able to use highly heterogeneous materials.
The process heat can either come from an autothermal partial combustion of the processed material in the gasification stage or allothermally via heat exchangers or a heat transferring medium. In the latter case, the heat may be generated by the combustion of the processed material (i.e. combustion and gasification are physically separated) or from external sources.
Impurities of the raw gas depend on the gasification condition and used feedstock. They can cause corrosion, erosion, deposits and poisoning of catalysts during downstream processing. It is therefore necessary to clean the raw gas. Depending on technology, impurities such as dust, inorganic matter, bed material, tars and alkali compounds are removed through various cleaning steps.
The cleaned raw gas, now a clean fuel gas, must meet the quality requirement of the power (and heat) generating unit, which normally would be a gas engine or a gas turbine. In future it could also be low temperature (PEMFC, PAFC) or high temperature (MCFC, SOFC) fuel cells, where the latter ones – and there especially the molten carbonate fuel cell – would be preferable due to their higher impurities tolerance and their ability to directly use CO, methane and higher hydrocarbons as fuel directly in the cell.
Heat and power production
Use as engine or gas turbine fuel
The clean fuel gas can be used in spark-ignited (Otto) engines and in compression (Diesel) engines and for gas turbines for power and heat production. Engines are applied for power generation in the range of 15 kW up to the order of 10 MW. Biomass-to-electricity efficiency in practice lies between 25 and 30 %, and the overall performance between 80 and 85 %.
As heat is not always a desired output of a CHP plant, some plants use an organic Rankine cycle to further convert heat into electricity, increasing the electric output by about 10 percent points, i.e. to 30-35 %.
The clean fuel gas composition will vary slightly due to variations of the feedstock. This requires engine controls to adapt engine operations to maintain load unless a sufficiently large gas buffer tank is used.
Typically, engine emissions include CO, hydrocarbons and NOx, pre-dominantly from nitrogen compounds in the gas that originates from the fuel. Therefore, exhaust gas catalysts are typically used.
At larger scale, say 10 MWe up to 100 MWe, gas turbine combined cycles are more efficient, reaching from 35 up to over 50 % electric efficiency and a total efficiency of up to 90 %. As gas turbine combustion chambers operate under pressure, also the gasifier needs to be pressurized or the fuel gas compressed prior to its use. There have been such developments at the demonstration scale in the 1990’s and early 2000’s and also now there are some companies like Synova Power (Netherlands) and Phoenix Biopower (Sweden) developing such schemes as the high efficiency is attractive.
Stand-alone power production
In the case of waste fuels (MSW, RDF etc.) the conventional incinerator technologies have a fairly low efficiency (of the order of 25 %) compared to other thermal power plants due to that the steam superheat temperature is limited to the order of 400-450 °C due to corrosion.
One option that has been developed is to link a gasifier to a gas boiler via an intermediate gas cleaning to remove alkalis and chlorine, and then overcoming such limitations on the superheat temperature and thereby allowing a more efficient steam cycle reaching 30-35 % efficiency. One notable development is by Valmet at Lahti, where two circulating fluidised bed (CFB) gasifiers, 80 MW thermal each, feed cleaned gas into a gas boiler using 540 °C superheat to generate 50 MWe.
Use for co-firing
The idea of co-firing is to use existing large-scale power plants and replace part of the fossil fuels (coal, oil, natural gas) with renewable sources. The gasification of the feedstock before feeding gas to the boiler (aka indirect co-firing) offers a number of advantages compared to direct co-firing when solids are fed into the boiler:
- Higher fuel flexibility
- Gasification gases are easier to transport and manipulate.
- Combustion is more efficient and cleaner. No significant negative impact on the performance of the boiler from biomass ash and impurities.
- Less strict requirements in the producer gas quality as compared to other applications
- Possibility of keeping the gasifier ash separated from coal ash, which is used as a certified feedstock for building material.
More details are provided in section EVC 7: Biomass co-firing for heat and power.
Industrial high-temperature process heat
Gasifiers are also used to produce a fuel gas that wholly or partially substitutes fossil fuels in industrial high-temperature applications.
Over the years, in the Kraft pulp and paper industries, circulating fluidised bed (CFB) gasifiers have been used to gasify bark residues from the log feedstock for use of the gas in their lime kilns where limestone is calcined to quicklime that is used in the cooking liquor regeneration cycle and reverts to limestone in the process. The advantage of gasification is that ash constituents and impurities that would give operational problems in the liquor cycle can be separated from the gas before the burners, which is not possible if biomass is directly fired.
Another application is to provide secondary fuel to cement kilns. A 100 MW thermal CFB gasifier has been operated since 1996 at CEMEX Rüdersdorf, Germany. Several other installations in cement factories are also in operation in China.
The above installations all use air for gasification and generate a low calorific value (LCV) gas. For even higher process temperatures than the 1100 to 1200 °C in lime kiln applications, a medium calorific value (MCV) gas using either oxygen-blown gasification or indirect gasification is used. There have not been many installations of this nature so far. However, at Höganäs, Sweden, Cortus Energy is commissioning an indirect gasifier of 6 MW thermal where the gas will be used for firing a steel furnace.
Fundacion Cidaut: Spanish project producing 100-150 kWel.
Skive plant: Capacity of 6 MWel and 20 MWth; run by city of Skive (Denmark); operational since 2008.
Güssing plant: Capacity of 2 MWel and 4.5 MWth; was run by a public-private consortium (Austria) and in operation from 2002 to 2018, now closed as the green electricity preferential tariff has been discontinued. Further installations of the same technology have been realised in Oberwart, Senden and Heiligenkreuz.
Värnamo demonstration plant, 18 MW thermal pressurised gasification to run a Siemens ST-100 gas turbine combined cycle, 4+2 MWe. Operated between 1994 and 2000.
CFB Geertruidenberg: 600 MWel coal power plant, 30 MWel of co-firing product gas; run by the Dutch energy company Essent; operational since 2005 but now stopped.
Lahti plant: in stage 1, a fluidized bed gasifier with a capacity of 40 MW to 70 MW fuel input was installed at a pulverized coal boiler in Lahti with a capacity of 350 MWth and put into operation 1998.
Lateron: the power station was completely converted to gasification/syngas combustion for CHP on RDF with 2 gasifiers with a capacity of 80 MW fuel input each in 2011.
Vaaskiluoten Voima, Vaasa, Finland: 140 MW thermal CFB gasifier firing into a 560 MW thermal power plant, operational since 2012.
Fact Sheet: Biomass CHP facilities
 A description of this “Lahti II” is available in this document: https://www.ieabioenergy.com/wp-content/uploads/2019/01/IEA-Bioenergy-Task-33-Gasification-of-waste-for-energy-carriers-20181205-1.pdf
 Jorma Nieminen, Matti Kivelä: Biomass CFB gasifier connected to a 350 MWth steam boiler fired with coal and natural gas—THERMIE demonstration project in Lahti in Finland. Biomass & Bioenergy 15 (1198), 251-257. doi: https://doi.org/10.1016/S0961-9534(98)00022-1
 Markus Bolhàr-Nordenkampf, Juhani Isaksson: Refuse derived fuel gasification technologies for high efficient energy production. In Thomé-Kozmiensky: Waste Management 4, 2014 (https://www.vivis.de/wp-content/uploads/WM4/2014_wm_379_388_bolhar_nordenkampf)