Source: CMS Energy
Most technical problems experienced when retrofitting biomass co-firing capabilities to existing pulverized coal fired plants have been associated with the reception, storage and handling of the biomass and, where relevant, the preparation of the biomass-coal blends. The nature of the problems depends largely on the type of biomass, but it is possible to draw some general conclusions.
One of the key properties of biomass is its total moisture content. This can vary widely from negligible up to around 60 per cent for some green biomass materials. Dried biomass is hydrophilic and has a tendency to absorb moisture from the atmosphere, even in covered storage.
The long-term storage of wet biomass can be problematic because with moisture contents above 20 per cent, relatively rapid microbial respiration activity can cause heating of the storage pile, loss of dry matter and significant deterioration in the physical quality of the fuel. It is also possible that high dust and spore concentrations in the stored fuel can result in health and safety issues during subsequent fuel handling operations. This can be particularly troublesome during and after fuel drying operations, when dust and spores can be released into the working environment.
There are four possible courses of action that can be taken to minimize biological action during the long-term storage of biomass.
First, storing biomass in billets or in larger pieces to reduce the cut surface area available for biological activity. Second, using fungicides and other chemical agents to suppress biological activity. Third, pre-drying the fuel to a moisture content at which biological activity is reduced. And fourth, cooling the stored fuel to temperatures at which biological activity is reduced. All of these options will add to the delivered costs of the fuel.
Granular and pelletized materials, such as solid residues from the palm oil and olive oil industries, handle reasonably well at normal delivered moisture contents, and their flow behaviour can be accurately modelled. The handling behaviour of woody biomass, in the form of chips, chunks and sawdust, is generally harder to characterize, due mainly to the wide range of particle sizes and moisture contents.
Pelletized biomass is generally free flowing, but some of these materials handle poorly when wet. Pellets can absorb moisture from the surrounding air, and can grow mould and swell. Dust generation is probably the most important problem area when storing and handling pelletized materials in bulk.
Grassy biomass and straws are usually handled, transported and stored in bales. Specialized equipment for handling and storing baled materials in bulk is available, but tends to be relatively expensive. Baled materials are generally not suitable for co-firing by pre-mixing and co-milling. These materials can be pelletized successfully, but this process can be relatively expensive, and when the pellets break up during handling, the liberated fine material can cause major handling problems.
Overall, the single most important issue when handling and storing biomass has been the generation and accumulation of dust. Dust extraction system have been successfully used, but the fine materials collected in them can be very cohesive, and there can be problems with the disposal of dusts collected, particularly when using wet removal systems. Special arrangements, including explosion vents and fire suppression systems may be required in biomass storage and handling areas.
If the accumulated dust in the fuel store or handling system gets wet, it can swell and there can be mould growth. The experience with water misting systems for dust suppression has been mixed. These systems tend to increase the relative humidity in the store, and this can further encourage mould growth.
“Most technical problems experienced when retrofitting biomass co-firing capabilities to existing pulverized coal fired plants have been associated with biomass reception, storage and handling”
Co-milling of biomass with coal
The pre-mixing and co-milling of chipped, pelletized and granular biomass materials with coal in large coal mills has been practised in a number of coal fired power stations on a commercial basis for a number of years. The maximum achievable co-milling ratio, and hence the level of co-firing, is limited and depends on the design and operation of the coal mill and the nature of the biomass material. In most cases, co-firing of the biomass could technically achieve around ten per cent on a mass basis, but in practise, the normal co-firing ratio is lower than this.
In general terms, conventional coal mills break up the coal by a brittle fracture mechanism, and most biomass materials tend to have relatively poor properties in this regard. There is a tendency, therefore, for the larger biomass particles to be retained within the mill to some extent, and this can act to limit the co-firing ratio that is achievable in this way.
In vertical spindle coal mills, there may be a tendency for the mill differential pressure and the mill power consumption to increase with increasing biomass co-firing ratio, and this may represent a limiting factor. There may also be an increase in the particle size of the mill product when co-milling biomass because of the relatively low particle density of most biomass materials. When co-milling very wet biomass there is a significant impact on the mill heat balance, and this can also be a limiting factor.
There may be a mill safety issue in most conventional coal mills, where hot air is applied to dry the coal in the mill. Biomass tends to release combustible volatile matter into the mill body at temperatures significantly lower than those that apply when milling bituminous coals. It may be necessary, therefore, to modify the mill operating procedures to minimize the risks of overheating the coal-wood mixture, and thereby causing temperature and pressure excursions in the mill.
Despite these potential difficulties and limitations, the co-milling and co-firing of a number of chipped, granular and pelletized biomass materials through most of the more common designs of conventional coal mill has been carried out successfully on a fully commercial basis in a number of coal fired power plants.
“In countries where coal will continue to be a significant portion of the fuel mix for power generation, co-firing has a key role to play”
Impact of co-firing on boiler performance and integrity
The general experience has been that provided the mill product is acceptable, and that there are no very large biomass particles passing to the burners, then the combustion behaviour of the blended fuel has been acceptable. Biomass materials are much more reactive in combustion systems than coal, and do not require particle size reduction to the same levels as for pulverized coal.
Randers CHP plant in Denmark was retrofitted in 2002/03 with an AET biomass injection co-firing system Source: AET
At the relatively low co-firing ratios applied in stations to date, the impacts on the combustion efficiency, on flame stability and on the burner turndown capabilities have been modest.
The co-firing of biomass materials, and particularly of wet biomass, can have an impact on the maximum achievable boiler load, depending on the mill constraints and on the boiler efficiency. At low biomass co-firing ratios and with dry (less than ten per cent moisture content) biomass materials, the impacts have been modest.
Biomass materials generally have lower ash contents than most power station coals, but the nature of biomass ashes is very different from that of most coal ashes. In general, biomass ashes have relatively low fusion temperatures and have relatively high levels of the alkali metals, particularly potassium. They have a greater propensity towards the formation of both slagging and fouling deposits than the majority of coal ashes.
At the low co-firing ratios that have been applied in most European plants to date, very few significant operational problems due to increased ash deposition on boiler surfaces have been reported. At higher co-firing ratios, and particularly with the higher ash biomass ratios, the risks of excessive ash deposition are significant.
There is also a technical issue with the potential for increased fouling of selective catalytic reduction (SCR) catalysts. However, the experience in Europe to date indicates that at the biomass co-firing levels that currently apply in large power plants, there have not as yet been any significant operational problems. The results of a number of side stream tests at low co-firing ratios on operating plants have also been relatively encouraging in this regard.
In the event of significant catalyst deactivation, it is possible to water wash the catalyst blocks to remove alkali metal and other salts and recover the catalyst activity. Avedore power station in Denmark, which has co-firing of wood pellets with heavy fuel oil and natural gas, has had such a system in commercial operation for a number of years.
The co-firing of biomass materials in coal fired power plants is a relatively recent development and, to date, the biomass fuels co-fired have generally had low chlorine levels and are co-fired at relatively low co-firing ratios. There have been no reports, to date, of accelerated metal loss from boiler tubes due to gas-side corrosion in large pulverized coal fired boilers.
Overall, it is considered that the risks of excessive corrosion associated with biomass co-firing are modest, except at elevated co-firing ratios, where co-firing activities can have the effect of increasing hydrochloric acid (HCl) concentration in the flue gases significantly.
Impacts of co-firing on boiler environmental performance
In all cases, the commercial co-firing of biomass in power plants is commonly preceded by the performance of trials during which the key environmental performance parameters of the boiler plant when co-firing are measured. If there are significant additional environmental impacts, the authorization from the environmental regulator required for commercial co-firing may be withheld.
The general experience from the environmental monitoring testwork carried out to date on boiler plants co-firing biomass at ratios up to around ten per cent on a heat input basis, can be summarized as follows.
- There has been very little negative impact on the combustion conditions or the combustion efficiency. There has been little negative impact on flame stability, burner turndown capabilities, the unburned carbon levels in the ash discards or on the emission levels of carbon monoxide (CO)and the other organic pollutants.
- The measured nitrogen oxides (NOx) emission levels when co-firing biomass have been similar to or a few percent lower than those measured when firing coal.
- The total dust and trace element emission levels have been similar to those when firing coal.
- The sulphur dioxide (SO2) emission levels have been similar to or lower than those when firing coal, in line with the co-firing ratio and the sulphur content of the fuels.
- There have been no significant problems with the quality of the ash discards, or on ash sales or disposal.
There has been little or no evidence of any significant environmental impacts associated with the co-firing of biomass at ratios of less than ten per cent on a heat input basis. The only significant environmental concerns have been associated with the generation of fugitive dust emissions from the biomass storage and handling facilities and, in some cases, the smell from the biomass reception and handling activities when processing materials with a distinctive odour.
IMPORTANT PART OF EUROPE’S FUEL MIX
There has been very rapid development of biomass co-firing potential. In the UK, all of the large coal fired power plants have been co-firing biomass, principally by pre-mixing the biomass with the coal and co-milling. It is expected that biomass co-firing activities in Europe will continue to expand over the next few years.
The scope for expansion will depend on government policies, and the effectiveness of the policy instruments put in place to encourage carbon dioxide (CO2) emission reductions from the electricity supply industry. The availability of suitable biomass materials for co-firing however, may be a constraint on co-firing activities.
In countries where coal will continue to be a significant portion of the fuel mix for power generation, co-firing has a key role to play in helping to meet the challenges represented by climate change, and the imperative to develop the means of progressively decarbonizing fossil fuel utilization.