Biomass market seeks higher reliability

Achieving high reliability is becoming a key issue for biomass plants. That this can be done is amply illustrated by the successful commercial operation record of the 25 MWe Cuijk power plant in the Netherlands - now listed by the German Ministry for Environmental Affairs as an AIJ ("Activities Implemented Jointly") project under the Kyoto Protocol.

However, the biomass market poses special challenges when it comes to reliability. As well as the usual commercial imperatives for feasibility and profitability there is a need to gain wider acceptance of biomass as an energy source.

Many of the industries in which biomass plants might be appropriately located, eg, timber, paper and cellulose, food processing - where it is possible to close the cycle almost perfectly, since the biofuel itself frequently occurs as residual waste during production - also attach particular importance to high availability. In these applications the biomass power plant is usually using the waste to supply steam and electricity to the process, but when the power plant stands still, so does production.

A second important group of potential customers for biomass electricity generation are the IPPs (Independent Power Producers). But they also totally depend on high power plant availability due to their strong focus on high returns and, very often, lack of equity capital. As a rule, they do not have the financial "buffer" in the balance sheet to offset unscheduled downtimes.

Early involvement of experienced operators in the power plant design process and modularisation are two things that can help address these reliability issues in the biomass sector.

Experienced operators of wood-fired power plants, for example, can give power plant designers insights into such areas as:

• boiler design in accordance with fuel specifications found in practice, which must permit a high degree of flexibility in procuring fuel;

• fuel handling, which must take into account dust problems and the percentage of material unsuitable for processing (stones, metal etc);

• dry ash removal; and

• optimisation of the operating scheme to allow for seasonal changes in fuel mix.

Fuel handling

Supplying the life blood of a power plant, fuel handling is clearly of central importance for trouble-free operation and special emphasis must be placed on its optimisation in the case of biomass. The range of possible biomass fuels is wide, and apart from wood, can include residues from the food production industries, such as bagasse, leaves, etc. The appropriate solutions depend on such factors as the wetness of the fuel and the required level of automation to be aimed at.

In the 11 MWe Altenstadt biomass power plant, in Germany, for example, grapple cranes were selected for the fuel store to achieve low noise emissions, a high degree of automation and high availability due to their sturdiness and redundancy.

A different solution was adopted at Cuijk. At this plant, the wood chips are stored in completely sealed silos, each with a storage capacity of 5000 m3. Removal from the silo is via a 12 m long conical worm screw. The screw rotates at the floor level, pushing the fuel towards the centre of the silo from where it is thrown onto belt conveyors through a hole in the floor. This extremely robust solution has proved itself in numerous other plants throughout the world.

Furnace and boiler

A variety of different combustion technologies are possible, depending on fuel composition, size distribution, emission thresholds, economic plant size etc.

For the Gütersloh cogen power plant, for example, a travelling grate with spreader-stoker firing system (from Detroit Stoker) was selected as the best solution. The Gütersloh plant serves Pfleiderer AG's particle board plant and this technology was judged to have several advantages over other combustion systems of a similar output for the combustion of used woods with a high proportion of grinding dust from the chipboard process. The plant has a firing capacity of 60 MWt and max electrical output of 13.3 MWe, with main steam conditions of 18.8 kg/s, 70 bar, 455 degrees C and boiler efficiency 89.9 per cent.

The fuel is fed into the furnace by the spreader-stoker. Air transports it from a blast table into the boiler, which leads to very even distribution of the fuel. Thus dust-like particles burn in suspension above the grate whilst larger pieces with high kinetic energy are catapulted right to the end of the grate and spend the longest time on the travelling grate in the combustion chamber. Lighter pieces do not fly quite as far and therefore spend a shorter period in the furnace corresponding to their burn-up time.

This leads to advantages in terms of the energy density and, in turn, the size of the boiler. The optimisation of the air supply makes it possible to achieve very low emissions. Also, the relatively small mass of fuel on the grate leads to greater flexibility and operating safety.

In the Cuijk power plant on the other hand, a Kvaerner Pulping bubbling fluidised bed furnace was chosen as the ideal system given the initial conditions prevailing there. Cuijk power plant is one of the largest power plants in Europe running entirely on biomass fuel.

The operator had a major input into this choice of furnace technology. Fuel quality, the need for low emissions and high efficiency requirements were among the particular considerations that pointed to the fluidised bed as the right choice of firing system. In plants of this size the bubbling fluidised bed represents an increasingly attractive alternative to grate firing from an investment point of view.

Standardised design

Input from power plant operators is also reflected in Siemens' standardised biomass technology concept, called SiecoLine BioPower.

Having set a goal of being able to offer high-availability plants to the market, SiecoLine BioPower was developed as a product type biomass power plant. Following contributions from experienced operators and suppliers to the development of the power plant it is based on a design-to-operate principle.

The power plant is based on a modular system. It incorporates not only modules used in traditional fossil-fired power plants, such as turbine-generators, the condenser plant, feedwater system, etc, but also biomass-specific modules such as the steam generator and fuel handling systems. Investigations also looked at process engineering, economics and availability criteria to guide selection of the optimum elements from a modular system. The scope of standardisation covers software, engineering and plant equipment. Experience gathered in the construction of numerous industrial power plants, including several biomass-fired plants in particular, was channelled into the system.

Three turnkey SiecoLine BioPower plants are currently under construction, two in Germany and one in Italy.

The first German project is at Helbra, 50 km from Halle. This 6 MWe cogen plant, which will fire waste wood, is being built for Umweltkontor Renewable Energy AG, Erkelenz. The Helbra facility is scheduled to go on line end of 2002. Its fuel sources will include waste wood from light industry, municipal waste wood, gardening waste and forestry waste, firing 50 000 t annually. Main steam conditions are 50 bar/420°C. The power will be fed into the local MEAG (Mitteldeutsche Energieversorgung AG) grid. In addition, the plant will supply hot water for the town's district heating network and process steam for industrial purposes.

Helbra is one of several biomass projects underway in Germany, where new legislation encouraging bioenergy was introduced in May 2001.

The second German project is at Malchin. This cogen plant is being built for the regional power and district heating service provider, envia, Energie Sachsen Brandenburg AG located in Chemnitz. Local wood residues and citrus fruit peel from a pectin production facility will be used as fuel. The power plant will supply up to 36 t/h process steam and feed 11 MW electricity into the grid. The plant is especially environmentally friendly as it combines high fuel efficiency (through CHP) with clean biomass fuels and low emission values (new German TA Luft).

In Italy, a 10 MWe SiecoLine BioPower plant is under contract for a local IPP. In addition to forestry waste, the plant will use olive processing residues, agricultural residues, straw and oil seed shells, firing as much as 90 000 t of biomass annually. The plant will be remunerated for the power it generates under the terms of Italian legislation encouraging renewables, feeding into the ENEL grid. Emissions will be well below those of the German TA Luft, which would apply to a similar plant in Germany.