3.5 Power generation, CO2 capture and coal type
The coal type used for power generation and its characteristics can vary significantly between locations and impact the design of the power generation facility. Typically in selecting the coal to be used as a fuel, economic analyses are performed based on a specific location, the available coals and their cost. To account for the potential variation in coal types between regions, correlations were developed between a factor to relate the capital costs, plant heat rate, and O&M costs to the coal rank and facility designs. These factors were used to adjust the reference plant operating parameters and costs, so that they would be representative of a typical coal used in the regions.
The following paragraphs provide a description of the methodology used to derive the correlations between performance and cost implications on various power generation technologies and on a variety of fired coals. The power generation technologies include supercritical (SC) and ultrasupercritical (USC) PC power generation with/without CO2 capture,with/without CO2 capture and SC and USC oxyfuel combustion power generation.
3.5.1 General methodology
To obtain the correlations, plant performance and cost data for different coals were extracted from various publications and thedatabase. The data was then screened and analysed. The data was selected for producing the correlations only when it was from a single study and covered a relatively wide range of coals, typically one or two bituminous coals, one sub-bituminous coal and/or a lignite coal. Selecting studies that consider multiple coal types at the same location guarantees the data is on the same basis. The factor correlations were developed by considering the ratio of a parameter, that is capital cost, at different coal heating values, to a reference coal heating value of 30.8MJ/kg, the heating value of Pittsburgh No. 8 coal. Extrapolation or interpolation was applied in some cases when necessary. The best fit curves and correlation equations were found through selected data series using regression analysis techniques in Excel (Trendline function in Chart).
The correlation equations present the percentage change in plant performance or capital cost as a function of coal heating value from the reference coal heating value. The higher heating value (HHV) was used throughout this work. It is acknowledged that coal constituents have impacts on plant performance and cost to different degrees of extent. However, only HHV is used to represent coal categories in this study.
If multiple data sets from different sources were available, the correlation equation/curves were based on all data, which basically averages the selected parameters and hence reflects generic correlations. If the correlation equations were developed based on a single data set due to limited information, the correlations reflect the conditions defined in referred studies. In such case only the studies with relatively generic design conditions were chosen.
The following assumptions were also made to derive the correlations between performance and cost implications on various power generation technologies and on a variety of fired coals.
- Correlation equations are developed by data fitting technique if multiple data sets are available, which basically averages the subject parameters and hence reflects only generic trend of coal heating value. If the correlation equations are developed by single data set due to limited info availability, the equations will only reflect the data based on specific conditions of the referred study.
- All the technical and cost data is based on studies in US region. It should be acknowledged that cost varies with the plant site, significantly sometimes. The studies are either public or from WorleyParsons database.
- The data collected is based on the consistent system configurations for US region, and does not intend to reflect specific region or country’s pollutant emission requirements and other constraints such as water.
- PC plant data is based on SC and USC plant. The IGCC plant data is based on Technology only considering it is relatively abundant data for different coal including low rank coal.
- All the SC and USC PC plants analysed with or without CO2 capture are equipped with flue gas desulphurisation (FGD) for sulphur reduction and selective catalytic reduction (SCR) for NOx reduction from the stack.
- The SC or USC PC plant performance curve does not necessarily reflect the impact of sulphur content in coal on SC steam condition. The PC heat rate curves are based on consistent steam condition for the range of coal.
- All cases with CO2 capture efficiencies are assumed to be approximately 90 per cent.
- The SC/USC PC plant performance (heat rate) of each data set is based on new plants specifically designed for different coals.
- The SC PC plant capital cost is based on sites in US. It should be acknowledged that cost varies with the plant site, dramatically sometimes.
- PC FGD option data was based on FGD using limestone or lime in US corresponding to coals.
- While specific component selection has an impact on IGCC system performance (for example an IGCC choosing Selexol as acid gas removal (AGR) system has slightly different performance from one choosing system), the performance curve intends to reflect such differences.
- IGCC with or without CO2 capture heat rate comparisons are based on Shell technology (dry feed). It should be noted that for other technologies, the comparison result may change to different degrees of extent.
Oxyfuel PC plant
- Based on SC/USC power plants (24.1MPa/599°C/621°C, 3500 psig/1110°F/1150°F)
- The plants are equipped with low NOx burner (LNB)/overfire oxygen (OFO)/flue gas recirculation (FGR) and FGD
- Oxygen (95 per cent pure) used for combustion is produced from cryogenic unit (ASU) system.
- CO2 specification is for storage in saline formation (raw combustion product produced using 95 per cent oxygen and dehydrated to 0.015 per cent (by volume) H2O).
- Assume the efficiency difference for an oxyfuel boiler firing different coal is similar to the difference for an air fired boiler firing different coal while the oxyfuel boiler efficiency has slightly higher efficiency.
- CO2 compression uses integrally geared, multistage centrifugal with indirect water intercoolers.
FGD option factor
- FGD performance and cost comparisons are based on a SC PC plant firing #6 bituminous coal with a wet limestone forced oxidation positive pressure absorber.
Cooling options factor
- The factor is based on mechanical draft wet cooling options and 100 per cent dry cooling.
- Comparison is assumed on average annual ambient temperature of about 16°C (annual average). The performance impact is of average corresponding to the ambient. It should be noted that ambient temperature has significant impact on the performance when the plant switches between wet and dry cooling options at high temperature, for instance 0°C.
3.5.3 Correlations developed
To facilitate presentation, the correlations were given in the form of a ratio of studied plant performance/cost to a referred plant performance/cost as a function of heating value difference between studied coal and the referred coal heating value. The performance and cost of plants firing bituminous coal, i.e. Pittsburgh No. 8 coal, was used as the referred data point. Correlation diagrams between net plant heat rate (HHV) and coal heating value, and plant capital costs and coal heating value for a SC PC plant without CO2 capture are illustrated in Figure 3-2 and Figure 3-3.
Figure 3-2 Heat rate factors as a function of coal heating value
Figure 3-3 Capital cost factors as a function of coal heating value
Specifics regarding the data sources and specifics of the analyses for the various technologies are provided in the following sub-sections.
PC power plant
Correlations between performance/capital cost and coal heating values for SC-PC power plant without CO2 capture were derived using data from several public studies and a confidential WorleyParsons study (Holt, N & Booras, G 2007; WorleyParsons 2002; EPA 2006; WorleyParsons 2009a; DOE/NETL 2007; WorleyParsons 2009b). Performance and capital cost correlations for SC-PC power plant with CO2 capture were developed using data from various reports (WorleyParsons 2009a; DOE/NETL 2007; WP 2009b). Information were applied in correlations between coals and O&M cost for SC PC power plants with and without CO2 capture (DOE/NETL 2007; WorleyParsons 2009b).
For the case of heat rate correlation of USC-PC without CO2 capture, data from a WorleyParsons report (WorleyParsons 2009a) was used. Due to the sparse information the correlations for USC power plant capital cost, O&M cost, the data for SC PC plants were adopted. USC-PC plants have different capital and O&M costs from SC-PC plants. However using the ratio of the cost mitigates the impact to some extent.
IGCC power plant
Correlations for IGCC power plant were derived based on Shell coaltechnology because of its suitability to relative wide range of coals and good information availability.
Data from several sources were used to develop the correlations for IGCC plant with and without CO2 capture (WorleyParsons 2009a; DOE/NETL 2007; WorleyParsons 2009b; Maurstad et al 2006).
Oxycombustion power plant
Oxycombustion technology used for power generation is still in development. There is currently little published information on the impact of different coals on the plant performance and costs. However, many studies for power generation utilising oxycombustion boiler for single specific coal have been done, and some studies from technology vendors indicate that a oxycombustion PC boiler has similar or slightly higher efficiency as a conventional PC boiler, and the oxycombustion PC power plant essentially has the same system and equipment as the conventional PC power plant except the boiler island (US DOE 2008a; DOE/NETL 2006; DOE/NETL 2001). With these considerations in mind, the performance and cost for an oxycombustion power plant were estimated based on the performances and costs of an SC oxycombustion power plant based on a specific coal (US DOE 2008a) and a conventional SC PC power plant firing different coals (DOE/NETL 2007; WorleyParsons 2009b). The correlations for SC and USC PC oxycombustion power plants then were derived from these estimated performance and costs.
Flue gas desulphurisation and wet/dry cooling design options
The impact of a SC PC power plant with or without a FGD on cost and performance was estimated. The derived factor (percentage) was based on information from the ‘Cost and Performance Baseline for Fossil Energy Plants’ (DOE/NETL 2007). The study utilising wet limestone forced oxidation type FGD.
The impact for switching wet cooling to drying cooling on the performance and cost of PC and IGCC power plants were estimated based on information from various reports (DOE/NETL 2007; WorleyParsons 2007; Power, W.E 2005; Power, W.E 2009). It should be noted that the design ambient temperature may have significant impact on the performance and cost of a power plant. In this study the data was based on design ambient temperature of 60-65°F, which is considered to be an annual average temperature.
3.5.4 Limitations of approach
The above approach is not intended to provide for detailed accounting of the coal type and process design requirements but rather a rapid method of assessing the impact of these parameters in an economic model. As illustrated these correlations are strictly empirical and should be treated as such. Limitations of this approach include:
- not considering variation in process designs which account for many interacting case specific factors;
- trends utilise coal heat heating as the primary driver for cost and heat rate changes, coal characteristics leading to coal heat rate are not necessarily considered; and
- impact of high coal ash content and ash composition/properties not included.