Carbona’s novel air-blown bubbling fluidised bed gasification technology was originally developed by the Gas Technology Institute (GTI) in the US and was licensed to Carbona when the company was formed in 1996 to take over the gasification business of Tampella Power.

Since then, Carbona, which is owned by its founders Kari Salo and Jim Patel, has continued the development of the gasification system, especially with different biomass fuels, in a 20 MWt capacity plant in Tampere, Finland (see panel, right).

The effort has now borne fruit with the signing of a first contract – for a CHP plant at Skive in Denmark.

The Skive CHP project

This first CHP plant represents a further innovation in that it combines the gasification/gas cleanup technology developed by Carbona with gas engines – one gasification unit supplying three gas engines.

This biomass gasification gas engine (BGGE) power plant will be in operation in late 2005 and will be the first commercial application of Carbona’s technology.

The BGGE concept was selected for Skive because in small scale decentralised CHP power generation, the electrical efficiency must be maximised to make the plant economically feasible. The installed capacity of the CHP plant is 5.5 MWe/11.5 MWt.

With an overall efficiency of 87% and electrical efficiency of 28%, the Carbona technology enables the BGGE plant to produce about 50% more electricity than a conventional steam process from the same amount of biomass, with much lower emissions.

The project is financed on a commercial basis. However, since the plant will be a first of a kind demonstration plant, subsidies are being provided by the US Department of Energy, the European Union and the Danish Energy Agency. The project is also a model of international public and private sector co-operation, helping towards the meeting of Kyoto commitments. Assuming successful completion of the project Carbona plans to offer BGGE technology fully commercially.

Skive Fjernvarme is the owner of the plant and will act as the main contractor, having responsibility for integration of the various parts of the CHP plant. The new BGGE CHP plant will replace an existing 2×10 MWt district heating plant at the same site.

In supplying the gasification plant Carbona Inc’s scope will include fuel feeding, gasification, gas cleaning (tar reforming catalyst, filter and scrubber), gas cooling and distribution. The Technical Research Center of Finland (VTT) acting as subcontractor to Carbona will participate in the tar reformer design and testing of the gasification plant. GE Jenbacher AG/Austria will provide three JMS620GS engines, which have been developed for low calorific gas combustion. Other main subcontractors are Bruun & Sørensen A/S for district heat and power generation, Rambøll Group for the building and plant erection and Aaen Consulting Engineers as owner’s engineer.

Fuel – about 100 t/day of wood pellets – is supplied from the existing indoor wood pellet storage next to the gasification plant and is fed by feeding screws into the gasifier, to the lower section of the fluidised bed. As already noted, air is used as the gasification medium. The product gas generated in the gasifier contains CO, H2 and CH4 as the main combustible components. The gas also contains a small amount of heavy hydrocarbons – tar – which has to be removed by a novel catalytic cracker tar reforming system. This has been developed and tested together with VTT in pilot plants and at a commercial biomass gasification plant in Finland.

After catalytic tar reforming the product gas is partly cooled in a fire tube type gas cooler. The cooling medium is district heating water. The gas cooler is followed by a bag filter unit, which separates the remaining fine dust passing through the cyclone of the gasifier. In normal operation the filtered product gas is directed to the gas engines. Gas can also be combusted in two gas boilers or in the flare.

The heat from the gas engine cooling (lubrication oil and jacket cooling) and from the gas engine exhaust gas is recovered producing district heat in separate heat exchangers.

The plant can be operated with a large turn-down margin, with load determined by the heat demand of the district heating network.

RR Bio project

The high pressure version of the technology is applied when the product gas is burnt in a gas turbine, where the fuel gas has to be supplied at a pressure higher than that dictated by the pressure ratio of the gas turbine. The high pressure gasification plant, which includes the high pressure fluidised bed gasifier and gas cleanup, is typically integrated with the combined cycle process gas turbine, HRSG, and steam turbine.

The RR Bio project will be an IGCC plant with a total installed capacity of 14 MW. The plant consists of a Carbona high pressure fluidised bed gasifier that will convert solid biomass fuel into syngas, which will be cleaned using Pall filters, and a combined cycle power plant that will include two Typhoon gas turbine generators, one heat recovery steam generator and one condensing steam turbine generator.

The syngas produced from the gasifier will be cooled, and then cleaned using a metallic candle element hot gas filter with pulse cleaning system, and fired in the modified combustors of the gas turbines to generate power. The exhaust gases from the gas turbines will be passed through the HRSG to generate steam, which in turn will drive the steam turbine to produce additional electrical power. The heat rejection system for the steam cycle will be by means of cooling tower.

The plant will be equipped with a demineralisation system to treat raw water, water treatment to treat wastewater, biomass fuel handling and storage systems, and a fire protection system. The project electrical system will interconnect the power output from the plant to the existing substation and the transmission grid.

The biomass fuel sources will be located in the 12 upland regions of the West Godavari district, which are about 35 to 40 km from the plant. A system of three main depots and 12 sub-depots will be used to collect, prepare, and store the biomass. Various contracted local agents will collect the biomass and solar dry and store it in sub-depots. The biomass will then be transported by truck from the sub-depots to the main depots by the fuel supply contractor.

The project will require about 80 000 tons per year of biomass fuel. The Non-Conventional Energy Development Corporation of Andhra Pradesh Ltd (NEDCAP) has granted the project an exclusivity which prohibits other biomass projects from being built within a 50 km radius. Studies by NEDCAP and by TERI (for the client) indicate the adequate availability of biomass in the West Godavari District.

The nominal capacity of the IGCC plant is 12.5 MW of net exportable power. The overall plant performance at nominal capacity is as follows:

• Fuel flow as received (t/h) 8.8

• As received fuel moisture (%) 20

• Net power generation (kW) 12500

• Net electric efficiency (LHV) (%) 37

Le Calorie Srl is the owner of the RR Bio project. Le Calorie is a management and investment company of Bio Development International, Inc (BDI) and Anthony DiNapoli, who is the founder and chairman of BDI, is the project director and owns 100 percent of LeCalorie.

APTRANSCO, which was formed as a result of the Andhra Pradesh Electricity Reform Act of 1998 has entered into a 20-year renewable energy power purchase agreement with RR Bio to purchase electricity from the project.

Toyo Engineering India Limited (TEIL) will be the engineering, procurement, and construction (EPC) contractor and, the operation and maintenance (O&M) contractor for the Project. TEIL is an EPC company, headquartered in Mumbai, India. It was established in 1976 by Toyo Engineering Corporation of Japan (TEC), which owns about 74% stake in the company.

Carbona will be contracted by TEIL for the project to provide the biomass gasifier, basic and partial detailed engineering for the gasification plant, and assistance in the basic engineering of other parts of the power plant. In addition, Carbona will also provide support for construction supervision, commissioning, and start-up supervision and training for the gasification plant of the project.

The Demag Delaval Industrial Turbomachinery division of Siemens, formerly owned by Alstom, will be contracted by TEIL to supply the gas turbines for the project. DDIT’s UK division will provide Typhoon gas turbine technology. Pall will be contracted by TEIL to supply the regenerable blowback filter for use in cleaning the synthesis gas of biomass fines prior to combustion in the gas turbine.

Project financing is being provided by Rabo India Finance Private Limited a subsidiary of Rabo Bank of Netherlands. The project is being financed on a debt/equity ratio of 65/35 and has been through a rigorous due diligence conducted by an international engineering firm.

The financial closure took place at the end of April, 2004 and engineering was expected to start in July, with the plant due to be ready for commercial operation in April 2006.

The Carbona gasifier

Carbona’s bubbling fluidised bed gasifier is of simple design with wide biomass fuel flexibility. The gasifier has been developed by Carbona for biomass gasification based on licences from Gas Technology Institute (U-Gas and Renugas processes) and its own R&D work.

The gasifier is being developed in a low pressure and a high pressure version. The low pressure fluidised bed gasifier is suitable for connection to a reciprocating engine/CHP system and is the system being used at Skive. The low pressure gasifier is also applicable where the product gas is directly burnt as biomass derived substitute fuel, for example in existing coal fired boilers.

The gas cleaning system of the low pressure version, particularly the use of the catalytic tar reforming and gas scrubbing, is different from that employed in the high pressure application.

After fuel preparation (chipping and drying) the biomass feedstock is supplied to the gasifier through lock hopper systems. The fuel is pressurised in the feeding system by using inert gas and fed into the lower section of the bubbling fluidised bed by screw conveyors. The fluidised bed is separately fed inert material which also acts as a catalyst for tar reforming.

The fluidising and gasifying medium is air and steam, and is introduced into the reactor through a special gas distributor. The gasifier is at 1-5 bar in the low pressure, BGGE, case, and 20-30 bar for the IGCC application. The operating temperature is determined by the properties of the biomass fuel (typically 750-850 °C).

The cross section area of the fluidised bed is dimensioned for typical bubbling fluidisation conditions at the full load. The freeboard cross section area is larger than the bed area to decrease gas velocity and entrainment of fine particles and increase solids and gas residence time in the gasifier. The gasifier is a cylindrical vessel with a double layer refractory lining. The bulk of the fine particles (fuel ash and circulating bed material) elutriated from the gasifier is separated from the product gas in a single external cyclone. The fines are returned to the fluidised bed, maintaining the external circulation. The cyclone is also refractory lined.

The quality of product gas generated in the gasifier depends mainly on the characteristics (LHV, moisture, ash properties) of the biomass fuel. The gas primarily consists of hydrogen, carbon monoxide, carbon dioxide, methane, lower hydrocarbons, water vapor, nitrogen and some tars. The biomass derived product gas generated in Carbona’s fluidised bed gasifier contains low quantities of heavy tars because of the vigorous mixing in the fluidised bed, the use of dolomite as bed material and large feedstock particle size.

The fuel flexibility of the gasifier is high and is able to handle (alone or mixed) the following: forest residue; wood chips; agricultural waste; saw/paper mill residues; straw (compacted); waste derived fuels; and waste sludge

The range of components in the gas producec by varies within the following limits (Vol %): CO, 15-25%; H2, 15-20%; CH4, 2-7%; CO2 9-14%; N2, 45-55.

The lower heating value of the fuel gas is 4-6 MJ/Nm3.

A difficult path

The journey from pilot plant to demonstration plant is a difficult one – long, arduous, cumbersome and full of challenges. The challenges are of three main types: technical; economic; and institutional.

The technical challenges are those associated with: scale up from pilot plant size; lack of long term data; gaps in design information; long commissioning period requiring extensive measurements and sampling; and provision of performance and reliability guarantees.

The economic challenges are: relatively high investment cost for the first of a kind plant and high commissioning costs due to lack of previous experience; the need to meet lending banks’ due diligence and liquidated damages requirements; obtaining government grants and subsidies; overall risk management; and formulating the right type of supply contract (eg turnkey or owner as general contractor).

The institutional challenges include: biomass fuel availability and supply arrangements; working with government carbon emission reduction targets and renewable support mechanisms; and resolving conflicting stake holder requirements in the demonstration project.

Perhaps the most critical issue for a successful demonstration project is to select the right customer and the plant location.

Pilot plant at Tampere

Development of Carbona’s gasification technology has made extensive use of a pilot plant located in Tampere, Finland (still owned by Enviropower, formerly a Tampella Power company, now belonging to Kvaerner). The pilot plant (pictured below) includes all essential modules for research, component testing and the completion of the development of the gasification process for different applications.

The pilot plant converts 20 MJ/s thermal input of fuel to a product gas, which is suitable for combustion in gas turbines, gas engines and boilers. The gasifier can be operated at pressures from atmospheric up to 30 bar and at a temperature of up to 1100°C.

The pilot plant was built in 1990-1991. Since then, testing has been done on different types of coals and biomass to get experience in all aspects of gasification and its application to power generation. The pilot plant has been operated over a wide range of operating conditions to provide information for process design, mechanical design, gas cleanup, construction materials, instrumentation and control, and scale up. Total accumulated test time is over 5000 hours including several long duration tests. Enough experience has been obtained for Carbona to be in a position to supply complete design of commercial size gasification plants for most type of biomass fuels.