By David A. Wilson and Mark J. Upton
Many modern utilities may have a diverse portfolio of generating assets which often includes aging coal fired units which are nearing or at the end of their economic operating life. Many factors can lead to the retirement of coal fired generating plant including lack of competitiveness with other forms of generation such as gas plant, environmental compliance issues and plant condition.
Where the future operating life of existing coal fired assets is under question due to competitiveness or environmental compliance, the recent EPA ruling indicating a shift of position towards the reclassification of biomass as a carbon neutral fuel may present opportunities for extending the useful life of these assets by partially or wholly converting them from coal to biomass firing.
The market in the US for biomass as an alternative fuel to coal has previously been limited due to its classification as a conventional fuel however if biomass is considered to be carbon neutral status coal to biomass conversions can offer a number of benefits ion the right circumstances including a low-cost solution to plant life extension, significant reductions in emissions and a cost-effective solution to increase a utilities renewable energy portfolio.
Due to the different regulatory landscape outside the US, particularly in Europe, there has been significant experienced gained in the conversion of coal fired generating units to fire biomass fuels. This has driven the development of biomass fuel and firing technology to its present state of maturity.
This article is intended to provide a summary of the key factors to be considered when planning a coal to biomass conversion including biomass fuel specifications, implications to plant performance and operation, scope of conversion work and timescales based on recent project experience outside the USA.
Biomass fuels constitute a wide range of materials typically encompassing agricultural, forestry or wood wastes/residues. Some biomass fuels potentially relevant to the North American market are contrasted with a typical bituminous coal in Table 1:
Table 1 illustrates the fact that the typical energy density of agricultural biomass residues, such as Switchgrass and Corn Stover, are less than 20 percent of that of bituminous coal. This has a direct impact on the volume of fuel required, impacting transportation as well as the size of the power plant fuel storage and fuel feeding systems. Additionally, as a general rule, the on-site bulk storage of biomass materials should be minimized, and a ‘just in time’ approach adopted in order to avoid fuel degradation and biological activity. If dry biomass materials get wet, say above 15 – 20 percent, there is a tendency for them to become subject to microbial respiration.
Agricultural biomass residues do not lend themselves to long distance travel and this combined with seasonal availability, usually means their use is limited to smaller scale, localised power plants.
Over the last 10 years wood pellets have become the fuel of choice when considering co-firing in coal fired power plants or 100% biomass firing in converted coal fired power plants. During this period there have been initiatives in a number of European Union countries focusing on the replacement of coal in power plants with wood pellets. This combined with the introduction of associated International Standards for fuel specification has stimulated the market. The Global wood pellet market has shown tremendous growth of around 15 percent year on year over this period. Global pellet production is currently in excess of 20 Mt per year. The European Union region produces around 10 Mt per year of pellets, all of which are essentially consumed domestically. From a pellet production point of view on a country basis the US is the largest source by far producing around 7 Mt, although Canada’s production has seen a rapid increase recently to about 2.5 Mt. The majority of the US and Canadian production is exported. South East Asia has also commenced wood pellet manufacture on a significant scale. The largest single consumer at around 6.5 Mt per year is the UK, with a number of other European countries and South Korea also being large importers.
It is evident from Table 1 that the properties of wood materials are very different from coal. The ash content of wood materials depends largely upon the bark content of the fuel, (bark often containing 5 to 8% ash); the widely available white wood pellets generally have less than 1% ash. Volatile matter contents are very high in the 72 – 75% range. The fuel components often associated with emissions such as nitrogen, sulphur and chlorine are low compared to coal which is obviously advantageous.
Torrified pellets are becoming commercially available and may be of interest to some operators. These use a thermal pre-treatment process to upgrade the biomass to a higher quality. This is achieved by heating it to 250 – 350°C, partially devolatilising it and driving off the moisture. The net result being a product that has a higher heating value, improved water resistance and better grindability in conventional coal pulverisers.
Implications to plant performance and operation
The amount of biomass firing adopted on a boiler can be from a small figure up to 100 percent. This has implications for the extent of plant modification, relative complexity and cost. Three typical schemes are illustrated in Figure 1 which provides an indication of the equipment impacted along with the associated degree of biomass firing applicable. The three schemes shown (1a, 1b and 1c) give a general indication, but there is a spectrum of permutations and options for arranging the equipment which will be dictated by individual site circumstances.
Around 10 years ago significant operational experience, especially in Europe, was gained with direct co-firing systems based on a pre-blending approach, i.e. where the biomass and coal are pre-mixed and fired through the installed coal handling, milling and firing system. The positive operational experiences accumulated from these projects provided the confidence for the subsequent upsurge of 100 percent biomass firing conversion projects, the majority of which now utilise wood pellet fuel.
The energy density of wood pellets is significantly higher than many other biomass forms although still only 50% of that of bituminous coal. This clearly has implications for the storage and metering facilities at the power plant. Wood pellets in their ‘as-delivered’ form are not suitable for feeding directly to the furnace of a PF coal fired boiler. Size reduction either in the existing boiler coal mills or via hammer mills is necessary to produce fuel particles which will result in acceptable furnace residence times.
It should be noted that wood pellets, (and other pelletised biomass fuels) have a low bulk density and modest fines content, and it is considered unlikely that the inventory contained in a bunker or hopper above the mill will provide an adequate seal. A rotary valve is therefore usually installed in the coal feed pipe between feeder and mill.
Compared to bituminous coals, extra care in terms of good housekeeping and the avoidance of dust accumulations is necessary with biomass materials. This applies to the fuel feeding and storage areas and is also important inside the fuel preparation/pulverising equipment with explosion suppression equipment being provided at strategic locations.
Biomass materials are more sensitive to heat than are coals. When most biomass materials, including wood pellets, are heated to temperatures in excess of 350 – 400°F, there is an increasing tendency to release significant quantities of combustible volatile material. This is undesirable for safety reasons and has particular implications for pulveriser performance. Under bituminous coal operation, pulveriser inlet air temperatures are usually in the range 450 – 600°F.
For biomass operation pulveriser inlet air temperatures need to be reduced to around 320°F or less. This can be accomplished by adding cold tempering air – although this will have a small negative impact on overall efficiency – as the regenerative airheater becomes less effective. A superior technical solution is to incorporate a ‘primary air cooler’ heat exchanger. This maintains regenerative airheater effectiveness, recovering unwanted heat in the primary air stream and transferring it to the turbine feedwater heating system – leading to an efficiency improvement – refer to Figure 2.
The fact that the energy density of wood pellets is only around 50 percent that of bituminous coals means that pulveriser throughput will be negatively impacted when exclusively grinding wood pellets. Experience indicates that pulverisers can provide around 70 percent of the heat input from coal when firing wood pellets. The deficiency in boiler heat input can be rectified via a number of possible routes including; (i) utilising a standby mill, (ii) introducing an additional biomass stream (similar to the scheme shown in figure 1b), (iii) replacing the existing pulverisers with a larger size selected specifically for the biomass.
Biomass fuels with their high volatile matter content do not need to be ground to a very fine particle size distribution to combust readily in a pf fired boiler. However, particles generally need to have a maximum dimension of less than 1/8” (3.2 mm) in order to burnout effectively given the relatively short residence times available within the furnace. Coal pulverisers are not able to influence biomass particle size distribution to any great extent. While they do impart a small comminution effect the biomass particle size distribution exiting the pulveriser is largely dependent upon the size of the particles constituting the original wood pellet.
To avoid drop-out of the larger biomass particle sizes of the fuel leaving the pulverisers it is necessary to use somewhat higher velocities (than would be used for coal) in the downstream pf pipework. In some instances the existing coal burners can be used without modification for the biomass, while in other instances purpose designed biomass burners may be advantageous.
The low levels of ash in wood pellets compared to a typical bituminous coal could lead one to believe that biomass ash was benign as regards boiler slagging and fouling. However, it is important to note that biomass ashes differ from coal ashes. Coal ashes are essentially an alumina-silicate system, whereas biomass ashes comprise quartz and the simple inorganic salts of potassium, calcium, magnesium and sodium with relatively high levels of potash. These alkali and alkaline earth metals have the effect of reducing the ash fusion temperatures typically 100 – 200°C below those of bituminous coals. This introduces the possibility of increased ash deposition on both boiler furnace and convective heat exchange surfaces. However, boiler plants vary significantly in their sensitivity to ash deposition, if and where the deposits accumulate, their impact on performance and the ability of the on-line cleaning systems to control deposition to acceptable levels. Experience to date indicates that ash deposition issues with wood pellets normally fall within manageable limits.
The different ash characteristics (compared to bituminous coals) and the low sulphur content associated with biomass fuels means that the performance of flue gas dust removal equipment, principally electrostatic precipitators, should be carefully considered. Minor modifications and changes to operating practices may be appropriate.
Scope of conversion work
Thermal digital modelling of the boiler under consideration is normally the starting point in any project. This involves understanding and benchmarking the existing coal fired boiler operation by calibrating the digital model against actual plant performance. Projections can then be made as to how the boiler will behave when firing biomass. Making these projections requires specific knowledge of how the biomass fuels behave – this is something a suitably experienced contractor can undertake based on proven references.
Thermal modelling and subsequent plant operation at numerous sites has shown that boiler pressure part modifications are usually unnecessary. Conversion work therefore focuses essentially on fuel feeding, preparation, and combustion equipment alterations as mentioned previously
Details of two recent biomass conversion projects executed by Doosan are provided in Figures 3 and 4. Both of these projects represent very comprehensive coal to 100 percent biomass conversions. Lower cost and less extensive conversions are possible but are largely dependent upon the objectives/goals of each individual project.
Biomass, particularly biomass pellets, are now a commercially traded fuel worldwide with a significant proportion of these pellets being produced in North America and exported for firing in European and worldwide power plants.
In view of the renewable energy classification of biomass and the various renewable energy subsidy regimes prevalent in Europe and other markets it is economical to import pellets from sources such as the U.S. despite the considerable logistics and transportation costs. It is likely that domestic consumers of US produced biomass pellets would benefit from reduced fuel pricing due to lower transportation costs.
Based on recent projects, typical power plant coal to biomass firing conversion costs and timescales, including boiler modifications, fuel handling and storage are summarized in Table 2.
The potential for classification of biomass as a carbon neutral fuel provides utilities with the opportunity to economically lowering their CO2 footprint and improving emissions compliance by re-purposing aging coal fired assets.
The conversion of coal fired plant to co-fire or to fire 100 percent biomass can be considered as a mature and proven technology which can readily be implemented while retaining fuel flexibility to meet future changes in environmental regulations and market conditions.
In Europe and in many other parts of the world biomass is already playing a significant part in the drive to utilise an ever-greater proportion of renewable energy in electricity production. It is hoped that the current direction of the EPA towards a carbon neutral classification for biomass, as evidenced by its recent ruling, is a major step towards stimulating a similar trend in the USA.
Dave Wilson is operation manager for Doosan Power Services. Mark Upton is specialist engineer in boiler design+development at Doosan Babcock