By Sanna Alitalo, Tauno Kuitunen
Fortum Engineering Ltd., Finland

A new software tool developed by Fortum Engineering Ltd. is helping the company optimize the design and operation of biomass plants by predicting the behaviour of certain fuels and modelling their effect on boilers.

Biomass power plants are becoming more popular as a means to generate clean energy. But unfortunately, using this type of fuel can damage power generating equipment and cause financial losses for the plant operator. However, new software technology from Fortum Engineering Ltd. could provide the answer to many of the problems.


Peat and pine bark: heat surface fouling and slagging are probable due to the high CaO content of the ash
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There are many physical and chemical differences between biomass fuels and conventional fuels such as coal. The chemical quality of the biomass fuel affects the selection of feasible combustion process, flue gas cleaning equipment, steam parameters in the boiler and, therefore, the steam turbine process.

The special properties of biomass fuels are low heat value, high moisture content and a large amount of volatile matter. Furthermore, the ash of biomass fuels contains significant amounts of alkali. These properties can cause heat surface fouling, slagging and corrosion, and fluid bed sintering and agglomeration when used in fluid bed combustion (FBC) boilers. This has to be taken in to consideration when designing and running a biomass power plant.

The physical properties of biomass fuels also have to be taken into consideration when designing the fuel handling systems, but unlike the chemical properties, they have no effect on process values.

Therefore, the chemical composition of the feasible fuel mixture to be used in the power plant and its ash should be identified and quantified as this chemical quality affects the economics of biomass power plants. A software tool from Fortum Engineering Ltd., can be used to make a preliminary estimation of fuel quality, help in the design of the power plant, and to reduce these problems and optimize plant procedures.

Common problems

Fuels with high alkaline levels, such as biomass fuels, provide low melting point alkaline compounds that react with sulphur and chlorine to form deposits or agglomerate silica particles for ash or fluidized bed media. Even when burned alone these fuels require low temperature furnace conditions and frequent fly ash or deposit removal.

These deposits can retard the heat transfer and lead to a decline in boiler efficiency and capacity. They also grow to the extent that flow through the boiler is restricted, often bridging across tubes and tube bundles, and causing mechanical damage. These deposits have also been associated with corrosion so it is essential to remove them, which results in large maintenance costs.

The boiler may have to be shut down in order to clean deposits that are difficult to remove. Alkali and alkaline earth metals in the fuel are important elements in the formation of fireside deposits. For biomass fuels, potassium is the major alkali element of concern. Silica in combination with alkali and alkaline earth metals, especially with the readily volatilized forms of potassium present in biomass, can lead to the formation of low melting point compounds which readily slag and foul at normal biomass boiler furnace temperatures.


70 per cent forest residue chips and 30 per cent pine bark: the high water soluble potassium in the mixture will cause several problems
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Deposit formation also depends on the boiler design and operation. Superheater fouling depends to a large extent on the furnace exit gas temperature

Sintering and agglomeration

Sintering and agglomeration are common problems in FBC boilers, where reactions in the bed can lead to the formation of large aggregated composites of bed media and ash. These composites can eventually cause the defluidization of the bed and lead to plant shutdown.

Agglomerates are composed of sand and ash particles bound by fused, glassy materials arising from reactions between the fuel elements or other compounds in the furnace. Fuel quality programmes to control the quantities of sodium and potassium entering the fuel procurement activities become too restrictive. But there are other methods available to reduce agglomerates which include: controlling the bed chemistry, using additives such as kaolin or limestone, and increasing the rate of bed blowdown and fresh sand make-up.

Heat surface corrosion

The material loss in superheaters is one of the most expensive phenomena as it results in extensive maintenance costs for the commercial fluidized bed boilers. Superheater corrosion is also one of the most common reasons for boiler shutdowns in the case of combustion of fuels containing corrosive substances such as biomass. Corrosion often results in a shutdown for maintenance and superheater reparation. This normally causes a major increase in operating costs for the power plants.

Chlorine-induced corrosion is the most common corrosion mechanism in biomass fuel combustion. This type of corrosion takes place in the combustion of fuels containing chlorine. Although biomass contains relatively small amounts of chlorine, it does contain significant levels of alkaline, which is an accelerent for this type of corrosion.


Olive pits: a difficult fuel to use due to a low S/Cl molar ratio, high K2O content in the ash, high potassium levels and high concentrations of alkali
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The corrosion is initiated at high temperatures when the fuel contains sufficient amounts of chlorine, and the superheater tube temperature is sufficient for chlorides to form molten eutectics. If there is sulphur present the chlorine corrosion can cause very high metal loss rates.

The chlorine corrosion rate depends on the chlorine concentration in the flue gas, flue gas temperature, sulphur content of the fuel, tube metal temperature and tube steel composition. By controlling these matters, with the aid of the Fortum Engineering software tool, chlorine corrosion can be avoided.

Emissions

Acid gas emissions (SO2 and Nox) are typically controlled in a biomass-fired FBC by maintaining proper combustion and temperature conditions in the combustor. Due to the low sulphur content of most biomass fuels and the high alkaline levels of the fuel ash, it is not always necessary to add a calcium-based sorbent to the bed for SO2 control. In some facilities, limestone is also added to help mitigate slagging and fouling induced by the formation of molten salt from sodium and potassium.

Nox emissions are also affected by combustor temperature, the nitrogen content of the fuel, and excess air. The nitrogen content of most biomass fuels is low compared to typical coals. Since, at FBC combustor temperatures, most of the Nox originates from fuel bound nitrogen and not from the combustion air, Nox emissions inherently meet most requirements without additional controls. In addition, staged combustion reduces Nox emissions even further. Particulate emissions of biomass fuel combustion are relatively high due to the high ash content of the fuels. Both fabric filters and electrostatic precipitators can be used to control particulate emissions.

Estimating problems

The tool that Fortum Engineering has developed models the problems caused by the combustion of biomass fuel mixtures. When the properties – calculated and fundamental to the arise of problems – of the biomass fuel mixture and its ash are known, they are compared to the limiting values that have been determined by researchers at Fortum. If the calculated value is higher than the limiting value, it is probable that problems will occur in the boiler.

The most significant problems in fluidized bed combustion are heat surface fouling, slagging and corrosion, and fluid bed sintering and agglomeration. The limiting values are based both on experiments and empirical knowledge.

When defining the limiting values, their applicability to the power plant under the study has to be taken into consideration because the type and size of biomass power plant affects the limiting values.

Figures 1, 2 and 3 demonstrate some of the common effects of using biomass fuels within a basic power plant design. In each case FBC technology is used.

Figure 1: This example shows shows a fuel mixture of 70 per cent peat and 30 per cent pine bark. This mixture causes heat surface fouling and slagging. The reason for the problem is the high CaO content of the ash.

Figure 2: In the second case the fuel is a mixture of forest residue chips and pine bark. This mixture causes heat surface fouling, slagging and corrosion, and fluid bed sintering and agglomeration. This is caused by a high amount of water soluble potassium in the fuel mixture.

Figure 3: In Figure 3, olive marc/pits are used as a fuel. This causes heat surface corrosion because the S/Cl molar ratio of the fuel is too low. The combustion of the fuel causes heat surface fouling, slagging and corrosion, and fluid bed sintering and agglomeration. The reasons for the problems are the high K2O content of the ash, the high amount of water soluble potassium in the fuel and the high concentration of alkali.

Using the data

When the problems and their causes have been estimated, the effects of different biomass fuels on the basic design of power plants can be determined by comparing different scenarios.

The software tool can support the product development process of a medium size biomass power plant where the combustion takes place in a bubbling fluidized bed boiler. The tool is used in product development to find the optimal fuel mixture, in risk management and in concept development.

The tool developed by Fortum Engineering helps to find an optimal biomass fuel mixture that causes none or only few problems. These problems can be controlled without significant costs being incurred by the plant operator. The competitiveness of the biomass power plant can therefore increase even though the fuel mixture includes fuel components that are difficult to burn.