Knowing your fuel is key

Among the renewables, bioenergy can be considered one of the technologies closest to being commercially viable, with some biomass projects managing to operate in competitive energy markets without huge subsidies.

It could even be argued that a significant increase in renewable generation in the short term only can only be achieved with biomass, which is an appropriate fuel for cogeneration.

Despite the political will in many countries to expand the renewables, siting has proved to be a constraint on bioenergy development. It sometimes appears that a single 30 m high stack is perceived to have more visual impact than ten high wind turbine towers.

Also, in some countries biopower has gained a reputation for poor reliability. It turns out that most availability problems to date have been caused by auxiliary systems, not the combustion technology. These "low-tech" parts, like fuel and ash handling equipment, need more attention, and many "do-it-yourself" developers of bioenergy projects have learnt this the hard way.

However, perhaps the biggest constraint to bioenergy expansion has been securing reliable access to the right kind of fuel.

Challenge of "new" fuels

In Scandinavia and Northern America, biomass fired power plants have been in operation since the 1970s, most fuelled with virgin wood chips or waste from the wood processing industry.

The recent pressure to increase biomass generation has led to the need to widen the fuel base to less virgin sources and unconventional biomass fuels. Materials such as rice husks, short rotation crops (willow, eucalyptus, switchgrass), olive industry residues, demolition wood, agricultural wastes, even meat and bone meal, have been introduced to the fuel mix. Many of these "new" fuels pose a challenge due to their chemical composition and handling difficulties.

However, experience with recent projects in different parts of Europe has shown that bioenergy need no longer be the sole preserve of countries rich in forest and that recycled biomass materials and agricultural wastes can be used in energy production on a commercial basis. This has required not only technology development on the manufacturer's side but also an informed approach from the investor's side, with the adoption of a what might be called a holistic approach, including life cycle cost optimisation.

The Finnish BioMAC bubbling fluidised bed (BFB) technology is particularly tolerant of problematic fuels. Also, modern combustion technologies such as BFB have the advantage that the operator is not obliged to commit to the original design fuel, but can introduce new fuels without major technology changes.

But the fuel mixes to be encountered at each project, and anticipated combustion issues, must be very thoroughly investigated on a case-by-case basis to keep commercial risks predictable and under control.

Among the techniques used for BioMAC is the computerised "Fuel Tool" which combines information on fuel chemical structure with advanced laboratory analyses and pilot-plant-scale combustion tests. Fuel Tool analysis provides a basis for furnace modelling and boiler dimensioning.

Key issues are slagging and fouling due to high alkali metals (Na, K) in the fuel, hot corrosion due to chlorine, and bed sintering.

The quantities of Na, K and Cl are relatively low in clean wood biomass but higher in short rotation crops and waste.

Alkalis, as well as chlorine, play a major role in the fouling of the heat transfer surfaces. Partially molten ash particles present in the flue gases impact and deposit on heat transfer surfaces causing deteriorated heat transfer and finally unavailability of the boiler. With high alkali fuels the boiler needs to be designed with low flue gas velocities, wide tube spacing and low flue gas temperature before the heat transfer tube banks. The steam temperature is also relatively low.

A special chemical equilibrium model within Fuel Tool is used to estimate the melting behaviour for the particular fuel mix to be encountered by a particular boiler. The flue gas temperature before the superheaters is selected based on the ash melting temperature. Typically, 15 per cent of melt in the ash is used as a threshold for severe fouling.

Taking the example of rice husk, ash melting occurs at about 700-750°C, but the amount of melt in the ash is less than 9 per cent. Correct boiler design allows the ash behaviour to be managed.

Hot corrosion of boiler tubes due to high chlorine fuels is exacerbated by high steam temperatures, while ash melting and fouling of heat transfer surfaces radically increase the corrosion rate. Typically corrosion may become a problem at steam temperatures over 460-480°C and in the furnace walls at pressures higher than 55 bar. When fuel chlorine content exceeds 0.1 per cent the corrosion must be taken into account in boiler design.

Bed sintering is caused by the alkali metals reacting on the surface of the fluidised bed sand particles to form low melting point alkali silicates. These make the surface of the particles gluey, causing them to stick together. This disturbs fluidisation and causes local hot spots, which accelerate the sintering. Unless the sinters are removed, the boiler needs to be shut down.

Sintering may be avoided by lowering the bed temperature, changing the bed material, or using alternative bed material or additives to bind volatilised alkali before reacting with silica. As alternative bed materials, volcanic ash and other pre-sintered and melted materials have been tested.

Recent BFB projects

The table opposite shows recent biomass projects using BioMAC BFB technology.

A comparison between the Cutro and Afferde projects, for example, illustrates the use of Fuel Tool in practice.

Cutro is a greenfield project in Italy, with handover due in autumn 2002, designed to burn various wood species, including wood chips, forest residues, eucalyptus residues and olive pumace. The possibility of alkali-induced sintering and corrosion was a decisive factor in boiler design.

For Cutro, Fuel Tool showed that the southern Italian wood species were prone to fouling and corrosion. Sintering problems were found to be a possibility due to fuel induced alkali compounds, although the indications were not very strong. However, there were clear indications pointing towards fouling and corrosion. This was mainly due to chlorine content and due to a combination of ash sintering/melting at high temperatures. The analysis led to the adoption of furnace dimensions which allow flue gases to cool sufficiently before entering the superheater section.

In the case of Afferde, the same analysis was carried out for demolition wood. This former coal-fired BFB condensing power plant (commissioned in 1986), in Hameln, central Germany, has been converted to burn this material. The conversion contract was placed in summer 2001 and commercial operation on demolition wood started in May 2002.

For Afferde, Fuel Tool suggested a minor tendency for fouling but a higher probability of corrosion due to the low S/Cl ratio in the fuel.

As in the Cutro case, the Afferde furnace design was adjusted to increase flue gas cooling, although here flue gas recirculation was used.

In both cases Fuel Tool results were fed into computational fluid dynamics (CFD) models, with the main emphasis on cooling the flue gases below the critical ash sintering temperatures before entering the superheaters.

In the case of the new boiler, Cutro, this was done by providing sufficient evaporator wall area in the furnace and an additional cooling chamber. In the refurbishment case, Afferde, flue gas recirculation was used.

The modelling results were then applied to boiler dimensioning, which again was different for each project.

In Cutro the main emphasis was laid on finding the right way to cool flue gases below critical ash sintering temperatures. The best solution was found to be a sufficient furnace volume for final combustion after which the cooling chamber reduces the flue gas temperature below critical alkali compound condensing temperatures.

In the Afferde case, the main focus of furnace dimensioning was to locate the secondary air nozzles and flue gas recirculation nozzles so that efficient combustion could be combined with rapid flue gas cooling. Here the secondary air nozzles were located above the bed, while the recirculation gas nozzles were placed above the air nozzles. This proved to allow a high turbulence zone to be created in the freeboard area while flue gas cooling could be achieved in a limited space between the secondary air zone and the superheater section.

Similar analyses are being carried out for other recent projects shown in the table. At Kotka, Finland, where the fuels will be wood biomass, forest residues and peat, and at Herbrechtingen, Germany, which will use wood chips, bark and waste wood, the FB500 BFB will be used.

At Lilla Edet the FB400 BFB will be employed, running on de-inking sludge from a paper factory and from old landfill where the sludge has been deposited for 15 years. The difficulties here will be the high moisture and ash of the sludge. This has been modelled and taken carefully into account in the boiler design.
Tables

BioMAC projects