By: J. Allen, K. Parker & A. Sanyal, International Environmental & Energy Consultants, UK & USA
Many software products are available to identify and optimize operational parameters for the highest plant capacity, utilization and efficiency. Although effective in tuning the unit, they are applied to an operating plant firing a given coal for the given performance, but what if there was a way of predicting how a coal will perform with respect to every single parameter involved in the use of that coal for capacity, efficiency maximization and emission control without firing even a single pound of coal? This would enable the operator to know in advance what to expect and thereby adjust the operating variables to get the best out of that coal.
Evaluating the coal
A software product called Steam Coal Evaluation & Services (SCES) has been developed, based on the fundamental principles of combustion and mineral matter transformation, to benchmark all the operational parameters involved in combustion without the need to fire any coal.
These parameters are: grindability; abrasion of the grinding elements; combustibility and unburnt carbon; slagging, fouling, corrosion and erosion; and emissions of particulates, oxides of sulphur and nitrogen. The only input required is ASTM or standard coal and ash analyzes.
The implementation of SCES in providing meaningful rankings alerts the operators to the performance they can expect and the measures required to operate the plant at its highest load factor and efficiency, while meeting emission compliance with minimum impact on operation and maintenance (O&M) costs.
To use the SCES programme the person logs into a web site (www.steamcoal.co.uk) and inputs their coal and ash data. These data can be for a single type of coal, a mathematical mixture of coals or a combination of coals plus biomass, and is applicable for any PF furnace firing mode or fluid bed combustor. The rankings of each parameter with explanatory notes and recommendations are instantly available to the user.
The programme benchmarks the performance of a coal into the following categories:
- – completely satisfactory, no action necessary
- – satisfactory, but attention may be required
- – reeds action/attention to minimize any difficulty
- – requires immediate attention, with respect to the above parameters
Advantages of SCES
SCES has the advantage over a short test burn, which cannot fully assess the coal’s corrosion, abrasion and erosion characteristics which have significant impact on O&M costs. Some other evaluation products require coal samples and laboratory test work related to specific coals. One programme, for example, in addition to coal analyzes requires specific plant design and operating data, which being regarded as proprietary may not necessarily be readily available.
The application of SCES has been validated on power plants in the USA, UK and India in both wall and corner fired units with capacities ranging from 200 MW to 660 MW, firing bituminous & sub-bituminous coals from the United States and bituminous coals from the UK, Russia and Colombia, as well as Indian sub-bituminous coals, all having a wide range of sulphur, ash and heating values.
The software was used by a utility in the southwest of the United States on two units each of 380 MW, where eastern bituminous coals (Coals C & D) and western sub-bituminous coals (Coals A & B), are fired. Coal E is a mixture of Coals A & B). The prediction of the coals’ operational parameters are shown overleaf.
The predictions from the programme matched the experience of the plant firing for each of these coals as reflected in the following statement from the plant manager: “The findings do correlate with the operator experience. The report was helpful. To someone without any prior knowledge of potential coal impacts, it could be very helpful. I do believe the technical services department here, with the maintenance engineers could monetarize the impact of ranking to derive the difference in O&M costs.”
Interpretation of assessment of different coal types
An intrepretation of the assessment conducted is as follows:
Grindability – the only Hardgrove Index reported is for Coal B and is ranked Category 2, which means that with the appropriate pulverizer properly operated the grindability of the coal should be satisfactory in producing the specified throughput of fineness of a minimum of 70 per cent through 200 mesh.
Abrasion – the evaluation of the quartz and pyrites mineral matter shows Coal A is satisfactory with respect to its impact on the grinding elements. Coal B and the Blend E are also expected to be satisfactory. The abrasion characteristics of Coal D is found to be of Category 3 and Coal C of Category 4, indicating high wear rates on the milling equipment, which are likely to need early replacement.
Erosion – The erosion propensity of the convective pass tubes is mainly determined by the concentration of free quartz and heavy silicate compounds in the ash. A quantitative determination of these minerals shows that Coals A, B and E are expected to be very satisfactory, with a minimum of around 100 000 hours tube life. Coal D is classified to be of Category 3 and Coal C of Category 4, indicating higher potential frequencies of tube failures. If the unit was designed for the bituminous coals, C or D, with a low gas pass velocity, baffles or sacrificial shield tubes, the unit could operate with controllable tube leak incidence.
Combustibility and unburned carbon – the indices, which define these characteristics, involve a function of the fuel ratio (fixed carbon/volatile matter) in conjunction with other factors such as HHV, ash and fixed carbon. If any coal performs below the expectation as indicated by the indices then remedial work is required.
In general the higher the fuel ratio, the tendency will be for the coal to demonstrate a high or severe ranking level in the SCES classification system. This does not preclude the use of a coal for power generation, but highlights particular aspects of the combustion system, which will need special attention to achieve satisfactory coal utilization. All the coals analysed exhibit a relatively low fuel ratio, which is conducive to low nitrogen oxides (NOx) combustion characteristics and good char burn out factors, which assist in achieving lower carbon carryover.
Furnace slagging – a complex phenomenon is broadly a function of the ash chemistry, gas temperature/composition and the concomitant viscosity profile under the thermal regime of and particles’ residence time in the furnace. The propensity of these fundamental physico-chemical properties is strongly influenced by the furnace design-residence time, burner zone rating, cooling factor (heat released and available/extended projected radiant surface) and effective soot-blowing.
If the unit were designed for Coal C or D, with appropriate furnace area rating, heat release rate, residence time and a furnace exit gas temperature (FEGT) compatible with the ash fusion properties, it is likely to cause little concern. If, however, Coals A, B or E are fired, the unit is likely to experience significant furnace wall deposition and consequently may find it difficult to achieve the design load.
Greater care would be required to achieve specified load, as for example, increased fuel fineness, more uniform distribution of coal/air within and between burners, increased soot-blowing, higher excess air, or additives to control FEGT. Non-compliance to these measures might well lead to too high a gas temperature, and hence higher gas flow at the inlet of the electrostatic pecipitator causing the opacity level to be exceeded.
Fouling – based on the quantitative evaluation of the alkaline constituents, the propensity of fouling deposition on the convective passes is very satisfactory for four of the five coals. However Coal A, with a high sodium oxide level, is evaluated to be of Category 4 and expected to cause severe fouling of the convective heat transfer surfaces giving rise to high FEGT gas temperatures affecting ESP performance, high draft loss requiring an increased frequency of soot-blowing and possibly derating to meet opacity compliance unless the unit were designed for this coal.
Corrosion – each coal is characterized to be of Category 1 and hence should pose no concern regards high or low temperature corrosion.
Particulate control – the electrical resistivity values of the ash, for Coals A, B and E, are evaluated to be of Category 1, and therefore should not present precipitation difficulties. The resistivity values of the ash for Coals C & D are found to be of Category 3 and 4, respectively, i.e. it will be difficult to precipitate and hence would necessitate generously sized ESP (SCA – >300 ft2/1000 acfm) and/or conditioning of ash.
Sulphur dioxide control – the sulphur dioxide emissions range from 0.73 to 1.43 lb/mmbtu. Hence to meet 0.3 lb/mmbtu would require some means of control, e.g. wet or dry scrubbers, to achieve removal efficiencies of 60 and 80 per cent, respectively.
Primary deNOx potential – each of the five coals is evaluated to exhibit a low fuel NOx index, means that whatever boiler they are fired in – wall, corner or cyclone type – the uncontrolled NOx emission will be at the lower end of the range calculated for that particular firing mode, which will reduce the duty required for any secondary NOx system.
Each coal analyzes shows satisfactory combustibility and char burn out factors, which make them compatible with any NOx reduction measures employed, i.e. low NOx burners and/or over-fire air. Any problems in the application of an in-furnace NOx control system will not be due to the intrinsic properties of the coal, but to operational factors, such as, coal preparation, fuel/air distribution to the burners, burner design and internal boiler aerodynamics.
Secondary deNOx control – with the advent of recent Clean Air Interstate Rule (CAIR) and EPA’s existing SIP Call Rule, all these coals will need secondary deNOx measures, preferably by selective catalytic reduction (SCR).
Application of an SCR system for the three Coals, A, B & E, will need to adopt greater measures, compared to Coals C & D, in order to achieve the required removal efficiency in order to minimize catalyst poisoning/blockage and/or ammonia slip. These measures include use of catalysts of higher volume and pitch, more uniform ammonia distribution, more rigorous catalyst management and extensive soot-blowing.
Making Economic Sense
Using the software, and confirmed by information from worldwide installations, plant operators will be alerted to the consequences of the use of any coal in advance, and thereby should be able to operate the unit at the desired load factor by taking the measures described above to minimize loss of revenue and high O&M costs.
For a 500 MW unit, where a one per cent improvement in boiler efficiency equates to annual coal cost savings of approximately $500 000, while a five per cent improvement in unit capacity utilization can equate to additional revenues of up to $12 million per annum, plus potential reductions in O&M costs.