Gasification technologies offer an opportunity to use biomass more efficiently, especially when used in CHP mode. The commissioning of a novel gasification demonstration project located in Skive, Denmark, gives Kari Salo and Andras Horvath the opportunity to explore the operational challenges and the lessons to be carried forward
The plant at Skive, Denmark
At the Skive gasification demonstration project in Denmark’s northern Jutland, a bubbling fluidized bed (BFB) gasifier is used to produce fuel gas from wood-based biomass. This gas is then used to supply a series of reciprocating gas engines in a combined heat and power (CHP) application.
Based on a low pressure BFB system and three gas engines, a single gasification plant will supply fuel to all the engines. The commissioning of the plant started in late 2007 and, using one gas engine, operations initially began in the early summer of 2008. The second and third gas engines were installed during summer 2008.
The project is financed on commercial basis. However, since the plant is a first-of-a-kind demonstration facility, subsidies are being provided by the European Union, the US Department of Energy (DOE) and the Danish Energy Agency (DEA).
The biomass gasification gas engine concept was selected by Skive in 2004 because in small-scale decentralized CHP power generation, the electrical efficiency must be maximized to make the plant economically feasible. The gasification technology enables the plant to produce about 30% more electricity than a conventional steam process, from the same amount of biomass.
The local district heating company, I/S Skive Fjernvarme, is the owner and also acted as the main contractor in the project, having responsibility for integrating the various component parts of the station, which is expected to produce net 5.4 MWe and 11.5 MWth once fully commissioned.
Besides providing 70% of the district heating production for Skive Fjernvarme, the facility will also produce 40 GWh of renewable electricity annually. The plant is not operated during the hot summer months when district heating demand is low and the small amount required is produced with an existing biomass boiler plant.
The gasification plant was supplied by Carbona, a subsidiary of pulp and paper technology firm Andritz Oy, which also possessed circulating fluidized bed gasification technology (CFB). The scope of Carbona’s supply contract includes fuel feeding, gasification equipment, gas cleaning (tar reforming catalyst and filter), gas cooling and distribution systems. Meanwhile, GE Jenbacher supplied three of its JMS620GS engines, specifically developed for low calorific gas combustion. The Technical Research Centre of Finland (VTT) also acted as a subcontractor and licensed tar reformer technology to Carbona. VTT further participated in the design of the tar reformer and testing of the plant.
The gasification plant is designed to utilize wood pellets and/or wood chips. The moisture content of wood pellets is typically below 10%, while the moisture content of wood chips can be up to 30%.
Figure 1. Principles of Carbona BFB-gasifier
Figures 1 and 2 show the operational principles of the bubbling fluidized bed gasifier and a schematic flow diagram of the Skive CHP plant. Fuel, which is initially wood pellets, is supplied from the existing indoor wood pellet storage site next to the gasification plant. The fuel is fed through two lock hopper systems by feeding screws into the lower section of the gasifier’s fluidized bed. The gasifier is operated at a maximum of 2 bar over pressure and 850ºC temperature. Air is used as gasification medium and dolomite is used as fluidized bed material. The product gas generated in the gasifier contains carbon monoxide (CO), hydrogen (H2) and methane (CH4) as the main combustible components. The gas produced contains some 22% CO, 20% H2 and 5% CH4 by volume, and has a heating value of about 5 MJ/kg. In normal operation, the first step in the gas clean-up process is a novel catalytic cracker that reforms tar compounds (generated during the gasification process) to hydrogen and carbon monoxide. Next, the fuel gas is cooled and passed through bag filters to remove dust. The filtered product gas is then scrubbed with water where it cools to 30ºC while the water content of the gas decreases. The reheated gas of 80% relative humidity is directed to the gas engines. The heat from the gas removed in the scrubber is also used to generate district heat.
Figure 2. Process flow diagram of the Skive CHP plant
The internal combustion gas engines have a 2 MW electrical capacity and were developed for low calorific value gas combustion. They drive the generators. Both the heat from gas engine cooling (lubrication oil and jacket cooling) and the exhaust gas is recovered for the production of district heat in separate heat exchangers.
Product gas may also be utilized in two gas boilers of 10 MJ/s capacity to heat water for district heating, or flared off in a possible emergency situation.
The plant is designed to operate at between 30% and 140% load, corresponding to 28 MW of fuel heat input. Operation with all three gas engines running at 13 bar engine pressure (BMEP) is considered as 100% nominal load. Initially, the engines are expected to operate at a pressure of 10 bar, corresponding to 80% load when all three engines are running. The 130% load also corresponds to the full load operation of the two 10 MJ/s boilers. The gas boilers therefore provide operation flexibility when the gas engines are not available due to regular maintenance.
The capacity of the CHP plant is controlled by the heat demand of the district heating network. The overall plant performance using wood pellets gives a maximum efficiency of 87%. More detailed performance figures are set out in Table 1.
Commissioning, start-up, and operational experience
Plant commissioning and cold testing started in the autumn of 2007 and the gasifier hot start-up and the first gasification were executed later that year.
The start-up occurred stepwise with the gasifier/gas cooler/filter and gas boiler process operated first — as an independent system to verify the operation of the fuel gas production part of the process. Once the system was supplying gas to the boilers and delivering district heating to the hot water network of the town, the gasifier/reformer/gas cooler/gas filter/gas scrubber/gas boiler process was operated to commission a new innovative gas clean-up line.
The performance of the gasification system was tested during the spring of 2008. The plant logged 1040 gasification hours to June.
Initially, plant sub-systems were optimized separately from each other. The integrated operation of the systems and the related optimization was conducted when the gasification plant, including the full gas clean-up train, first supplied gas to the boilers. During gas boiler tests the product gas quality (all impurities and contaminants as specified by the gas engine vendor) was measured in detail. Based on the measured results it was decided to launch the gas engine operation with a single unit. The engine achieved full load operation after a few days adjustment, while connected to the Danish national grid.
The results achieved so far show that the gasifier system design is highly suitable for this type of application. The gasifier is working as predicted and the raw gas quality, including the gas heating value, corresponds well the original design requirements.
The gas cleaning system has also been thoroughly tested, showing that the gas quality required by the gas engines has been achieved.
Demonstration plant challenges
It was, once again, learned that the path from a pilot plant to a demonstration plant is a difficult one — long, arduous, cumbersome and full of challenges. The technical challenges include: scale-up from pilot to commercial, a lack of long-term data, integrated plant control, missing detail from design information, and a long and costly commissioning period requiring extensive measurement and testing.
The economic challenges are those related to the high investment and operating cost due to a first-of-a-kind installation. The lending banks commit a thorough due diligence, and liquidated damages requirements are normally strict. Government grants and subsidies are currently required to overcome the risks. The type of plant supply contract is not that clear in an ambitious demonstration project — it can be a turnkey contract or the owner can act as the general contractor, or some model in between, depending on decisions to share the project risks.
The institutional challenges of a demonstration project are: biomass fuel availability and supply, renewable energy prices and governmental carbon emission reduction targets, and availability of subsidies such as investment and tax credit supports. Also, the goals of the stakeholders in the demonstration project are likely to be conflicting. The owner of the project wants to produce cheap electricity, and heat and the equipment vendor wants to demonstrate his new technology. The most critical aspect for a successful demonstration project, however, is to have the right project owner and the right location.
The Skive project has managed to overcome these challenges and at the moment is operating and demonstrating the feasibility of this new technology.
Kari Salo is the managing director of Carbona Inc, Andras Horvath is the director of technology in Carbona.
Origins and future for biomass gasification
The gasification technology — which was first applied for coal gasification — was originally developed by the Gas Technology Institute (GTI) in the US and was licensed to Carbona when it was formed in 1996. The gasification technology can be applied at low or high pressure, depending on the nature of the product gas user. Air is used as gasification media when producing fuel gas for boilers, engines or gas turbines. However, Carbona is also working on a development to generate synthesis gas from biomass using oxygen gasification. The leading pulp and paper company UPM-Kymmene has announced co-operation with Andritz/Carbona to develop a process to produce transportation fuels from wood-based biomass — particularly second generation biodiesel.
Gasification and gas clean-up testing and development is also currently being conducted at GTI’s new pilot plant in Chicago, to come up with a process to produce synthesis gas suitable for the Fischer-Tropsch process — a commercial method using specific catalysts to produce liquid fuels like biodiesel from biomass-derived gas.