Mixing your fuels for best effect

Co-firing biomass in coal fired boilers increases the potential use of biomass. An innovative process known as BioCoComb has been employed at the Zeltweg coal fired power plant which has shown promising operating results.

Although an environmentally sound practice, burning biomass in small decentralized power plants has high investment costs compared to conventional large coal fired plants. However, if it could be co-fired in a coal fired boiler its use could be much more widespread.

Verbund, Austria`s largest power supplier, has developed a process known as BioCoComb for co-firing coal and biomass at a coal fired plant in Zeltweg.

The 10 MWth demonstration plant converts biomass by partial gasification (and attrition) in a CFB reactor. The biogas produced, which also contains fine wood char particles, is co-fired in the combustion chamber of the coal fired boiler. Unlike other gasification gases, the low calorific value (LCV) gas produced is sufficient for co-firing. This means the biomass does not have to be pre-dried. The co-firing process also provides for NOx reduction by using the LCV gas as a reburning fuel in the boiler.

The 10 MWth demonstration project involves six European partners: Verbund; Enel; Electrabel; ESB (Electricity Supply Board); EVS (Energieversorgung Schwa- ben) and Austrian Energy.

Co-firing problems

Most coal fired boilers burn pulverized coal. Biomass cannot be used in such boilers without pre-treatment since coal mills are unable to grind coarse biomass pieces such as bark, forest residues and chopped wood.

There are also a number of other characteristics of the resulting coal/biomass mix which have to be considered before the fuels can be burned in the same boiler:

– Residence time of the fuel in the combustion chamber: in large units, fuel has to be combusted quickly (2 to 3 seconds) with minimum emissions. Biomass (as pieces and chips) has a much longer combustion time, which also varies with particle size and water content. The nature of biomass also makes smooth combustion difficult

– Slagging of biomass ashes with a low softening point: temperatures in the furnace of conventional coal fired boilers range between 1000 degreesC and 1250 degreesC. However, the softening or melting points of many types of biomass are significantly lower, which can result in slagging on heat exchanger surfaces

– High temperature corrosion from co-firing high chlorine biomass: Co-firing large proportions of straw causes high temperature corrosion on heat exchangers with high surface temperatures >l400 degreesC.

– Damage of catalysts by alkaline substances: many power plants use catalysts for flue gas denitrification. Alkalies which are formed during biomass combustion could deactivate the catalyst

– Ash quality: ash from hard coal fired power plants is often used in the building industry. The effects of components of biomass ash in traces on the usability of the coal ash has to be checked for each application

– Changes in the boiler behaviour caused by the higher specific gas flow: This is caused by the higher water content of biomass compared to coal. Existing boilers are designed for a certain fuel quality and quantity and consequently for a certain flue gas flow. Changes in the gas flow, which exceed certain tolerable limits, will require adaptations of heat exchangers, recuperators or fans. This limits the maximum share of biomass for co-firing in existing plants.

The Zeltweg demonstration

The Zeltweg power plant has an installed capacity of 344 MWth (137 MWe). The plant was built in 1962, and in 1982 its firing system was changed to a tangential firing system for burning hard coal. Main steam data (HP/reheat) are 185 bar/44 bar at 535°C.

The flue gas cleaning systems for removing dust, NOx and SO2 were also replaced. An SNCR system with ammonia injection is now used to remove NOx. The use of a circulating fluidized bed (CFB) technology will keep SO2 to a minimum.

The plant is located in a rural region in Styria which has a lot of forestry industry. This made it an ideal site for a biomass project.

Verbund completed the design concept in 1993 and began the search for partners to help finance and build the project. Verbund was responsible for project coordination, general work, operating the unit and doing all plant related analysis.

Electrabel carried out software analysis of the gasification process and comparisons with test rig results.

Enel performed the characterization of reactor operation with measurements of fuel inlet, gas outlet and solid waste. This included activities at the plant and in the laboratory for measurements and sampling. Enel also performed thermodynamic performance tests of the gasifier and, through modelling, defined the optimal injection point of the biogas for NOx reduction (reburning).

Austrian Energy designed, constructed, and commissioned the CFB reactor.

ESB was responsible for the engineering of control and measurement equipment.

EVS, meanwhile, used the test results of the other partners to analyze long term effects of co-combustion on the SCR-deNOx plants.

Scientific advice was provided by the Technical University of Graz.

The total budget for the project was Ecu4.83 million ($5.61 million). In addition to financing from the partner companies, 42 per cent of the costs were covered by national and international funding including Ecu1.34 million from the EU Thermie programme.

BioCoComb process

The innovation in the BioCoComb process is how the CFB is operated and how it is combined with the coal fired boiler. BioCoComb is an acronym of Biofuel preparation for Co-Combustion, where preparation means reaching the minimum requirements for co-combustion with pulverized coal.

The energy from the biomass is transported from the gasifier into the coal boiler in three forms: as sensible heat, LCV gas and fine combustible char particles. In addition to reducing CO2 by burning less coal, the concept can also reduce NOx by using the LCV gas as a reburning fuel.

Although bark will be the main fuel, to guarantee flexibility the gasifier and its auxiliary components must be able to cope with a wide range of biomass such as old wood, saw dust or wood chips.

The project has a number of innovative features. These include:

– Using a CFB for preparation of biomass to reach minimum requirements for co-firing in a coal fired boiler. The CFB is flexible enough to handle a wide range of biofuels

– No milling, no predrying of the biomass (up to 70 per cent water content)

– Partial gasification of biomass is desired with maximum transport of fine char particles with the gas

– Partial gasification requires a shorter residence time leading to a smaller gasifier design

– Gas is led at high temperatures from the CFB to the furnace (there is no gas cooling). This means there is no condensation of hydrocarbons

– No hot gas cleanup equipment is necessary for the LCV gas since the char dust particles leaving the CFB are small enough for complete combustion

The possibility of using LCV gas for reburning on a bigger scale than ever before

– The efficiency of biomass conversion to electricity is almost as high as in the coal fired unit

– A simple technical concept and a small gasifier result in low investment and operating costs.

Design detail

Because the CFB works with a FD-fan, the gasifier is set at a slight over-pressure which requires gas tight components. A duplex rotary feeder with a plunging mechanism prevents gas escape at the fuel entrance. This feeder limits the particle size to 30 x 30 x 100 mm. A special separator picks out coarse, oversized particles, and a shredder cuts them to a suitable size before they are returned to the conveying path.

The CFB-gasifier has a steel construction with a brick and concrete refractory. The gasification chamber is a vertical tube without internal components or heat exchangers. All the fluidizing and combustion air is fed through an open grid of air nozzles located at the bottom. The air is taken from the recuperator at 270°C. Fine sand is used as the bed material.

The hot gas enters the boiler via a specially designed burner nozzle that guarantees rapid ignition, stable flame, deep penetration into the coal flame and good mixing. The combustion behaviour was modelled by Enel with a CFD programme. For these calculations, Enel defined the optimum point of injection of the product gas into the boiler.

The product gas is used as a reburning fuel in the coal boiler to reduce the NOx emissions through the conversion of NOx to nitrogen.

The biomass partially combusts, raising the temperature to 850°C, and partially gasifies because of the lack of oxygen in the upper part of the furnace.

Reaction temperature and bed behaviour are the main parameters to be controlled by varying the air flow. Biomass particles remain in the fluidized bed until (due to gasification and attrition) they are small enough to pass the hot cyclone. All fine particles, mainly wood char dust and ash, leave the gasifier with the gas through the hot gas duct to the boiler. Larger particles recirculate and enter the gasification reactor near the nozzle grid, where an oxygen surplus is available for combustion.

A water-cooled screw conveyor at the bottom of the gasifier allows the discharge of bed sand and incombustible (mineral and metal) parts. Ash is not expected in this bottom sand since it will leave the gasifier with the gas.

Control and instrumentation for the whole project were developed by ESB in cooperation with Verbund and Austrian Energy. The sophisticated system allows manual or automatic operation and manages the complicated switch from combustion to gasification mode.

The main challenge during this switching period is the enormous change in specific air demand. Biomass and air flow have to be changed to keep the bed under stable conditions. Since the control logic is reverse in the two operation modes it has to be changed from air flow to fuel flow control.

Operating experience

Hot commissioning of the demonstration took place in November and first gasification was reached on December 10, 1997. A complex measurement and testing programme was carried out in January this year. Tests, measurements and monitoring phases are planned for the next three years.

So far, more than 1000 tons of biomass have been gasified since startup. The main fuel during this period was spruce bark with a water content of about 55 per cent. Chopped wood and saw dust from larch trees has also been used.

Operating experience has been promising. Ignition and gasification behaviour in the gasifier has been good. The critical changeover from gasification to combustion and back is smooth with a slight but acceptable temperature increase.

The power range of the gasifier was varied between 5 MWth and 13 MWth depending on the humidity of the fuel. The quality of the product gas is similar to the precalculated values (see Table 3). The heating value of the gas is optimized by adjusting the air inlets and distribution. The burnout of carbon is excellent with almost no carbon found in the discharged sand.

Reburning has had a dramatic effect on boiler performance. The use of sufficient biomass to provide three per cent of total thermal input, results in a decrease of 10 to 15 per cent in ammonia consumption. Work will continue in learning more about the effects of reburning.

Ongoing measurements will increase experience with a goal to increasing the biofuel share to 15 to 20 per cent of total fuel input.

Further reading

“Thermal use of biomass in coal fired power plants in Austria – the EU-demonstration project BioCoComb for biomass gasification Co-combustion grate project,” Dr Andreas Mory, Josef Tauschitz, Verbund-Elektrizitätserzeugungs-GmbH Klagenfurt, Austria. Presented at Power-Gen 1998, Milan, Italy

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Figure 1. Schematic of the Zeltweg biomass project

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Figure 2. Main components of different biomass concepts

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Figure 3.The BioCoComb gasifier