By Mike Caravaggio and Norris Hirota, Electric Power Research Institute
Over the past decade, fossil and hydro generation plants have increasingly experienced significant changes in their operating strategies, or “mission profiles,” compared to their original designs. These changes include new operating regimes with increased cycling, extended unit layups, and prolonged periods of low turndown.
The changes, in turn, are creating a multitude of challenges for the plants and their operating staff in areas ranging from component degradation to staffing levels, O&M budgets, and meeting environmental compliance under non-baseload conditions.
|The Brunswick County Power Station. Flexible operation of power generation assets is not new. What is new today is the frequency and level of cycling required. Increased operational flexibility is becoming more commonplace. Photo courtesy: Dominion Virginia Power|
Several years ago, to help the utility industry address these challenges, the Electric Power Research Institute (EPRI) launched a multi-utility, collaborative research project to identify the critical technical issues associated with these trends and to collect best practices and new technologies and processes to mitigate damage and increase flexibility. The project, called “Changing Mission Profiles,” has consisted of a pilot project of power plant case studies to evaluate the experiences of different unit types and an industry working group that convenes to share effective solutions and strategies.
To date, seven case studies have been conducted, and the results have been summarized in an EPRI report entitled Changing Mission Profiles Pilot Project (3002005859). In addition, the working group has held multiple sessions to identify the technical issues and solutions for different combinations of unit types and mission profiles.
Challenges of Increased Operating Flexibility
Flexible operation of power generation assets is not new. What is new today is the frequency and level of cycling required.
Several years ago, a generation planning research study, funded jointly by EPRI and the U.S. Department of Energy, found that the extent and diversity of increased flexible operations will be profound over the next 20+ years. The majority of dispatchable units will experience the necessity for flexible operations, and possibly multiple mission profiles. Flexible operations will also be geographically widespread; most power companies will have to support multiple mission profiles across their fleets in managing current and future assets.
According to this same research, flexible operation is the principal issue arising from increased variable generation, and key hurdles are operations at minimum loads and significant amounts of ramping at high rates (Figure 1). The study also projected frequent, large changes in average hourly generation for combined cycle and conventional fossil assets, as well as significant periods of low-load operations and reserve standby.
As many generation units find that the need for increased operational flexibility is becoming more commonplace, they are facing several challenges:
- Complex Operations. Flexible operations are inherently more complex. As units are operated more flexibly and/or with specific retirement horizons, plants need to operate closer to design limits. As a result, there may be greater levels of dependence on more accurate measurement of key parameters, optimization and process controls, remote monitoring systems, and more burdens on the operator.
- Integrated Impacts. Integrated plant impacts are not well understood or documented. Plant design margins and the consequences of reducing them are integral to understanding the impacts of modifications, installation of new systems, and alternative operational and maintenance strategies. For example, an equipment design life consumption rate under off-design operation might be an important consideration in a decision to balance costs or risks. Not understanding these margins or impacts can lead to unanticipated consequences.
- Burden on Staff. Changes to plant design and/or operational mission tend to place a larger burden on plant staff. These changes can place more responsibility, knowledge requirements, restrictions, and error traps on plant staff. For example, the technical basis (that is, failure modes, degradation rates, preventive maintenance frequencies, equipment monitoring needs, etc.) for maintenance may need to be modified, implemented, and maintained by limited staff resources.
- New Skill Sets Needed. Implementation of advanced technologies and data integration in response to increased flexible operations requires new skill sets. Driven by the availability of increasing amounts of quality data, a changing work force, and the economic need for a leaner O&M staff, data analytics and centralized monitoring and diagnostics will become increasingly important tools for managing overall plant performance.
The Pilot Project portion of Changing Mission Profiles sought to drill down to better understand how these challenges manifest themselves in different unit types and under different mission profiles. The Pilot Projects consisted of in-depth assessments of central station generation units that are experiencing operational modes that differ from their original design basis. These assessments were comprehensive, unit-level “deep dives” into the technical issues associated with specific generation types, the relative importance of these issues, and the extent to which these issues are generic.
The approach consisted of on-site visits by utility and EPRI subject-matter experts (SMEs) to generation units to discuss with the host utility personnel the actual experience of units managing significant flexible operation.
Central generation station units-including hydro, coal, and gas plants-were visited. Unit types and their missions were as follows:
- Francis-type hydropower plant (four units) / load following and part-load operation; increased diversion (spillage) during times of low demand.
- Subcritical natural gas boiler units (one plant, two units) / load following with frequent low turndown and shutdowns.
- Supercritical natural gas boiler units (one plant, two units) / load following with frequent low turndown and shutdowns.
- Gas turbine combined cycle (six units) / increased variability in operations centered on higher capacity factor; significant decrease in starts/year; minimum load during evenings.
- Subcritical coal plant (one unit) / extended layup with months that separate operational periods; load-following mode.
- Subcritical and supercritical coal plant (seven units) / subcritical units experience combination of load follow and extended layup (shutdown) ranging from two days to two months; supercritical units experience load-follow mode.
- Supercritical coal plant (three units) / long-term layup; turndown to as low as possible.
The site visits incorporated on-site workshops with SMEs and plant personnel. These workshops employed a standardized process to identify, discuss, and prioritize key technical issues for that unit / mission configuration. For each issue, the process captured critical assumptions, options and tradeoffs, key interrelationships, vulnerabilities, unanticipated consequences, and gaps and uncertainties in industry knowledge and understanding.
Ranking of Issues
The site visit workshops produced a ranking of technical issues related to plant operations, equipment, and environmental controls. For each unit / mission, experts and stakeholders applied evaluation criteria to the issues via a template. The criteria for ranking included:
- Impact on corporate metrics-How does this issue affect corporate performance and related metrics?
- Acceptable options available-Are there good solution alternatives?
- Technical resources available-Are solutions available through EPRI or other organizations?
- Degree of uncertainty-Do we understand the technical basis for the issue and for the solution path forward?
- Impact on plant staff and O&M-How will the solution to the issue be sustained and affect the plant staff?
For each issue, the criteria were used in a scoring system that established a numerical rating. Examples are shown in Tables 1 and 2 on pages 18 and 20.
This perspective was useful in understanding the complex interrelationships between these issues from the standpoint of both the technical/scientific disciplines and the power company organization, plant processes, and related resources.
|Bayside Power Station was among the nation’s top 20 power-producing combined cycle plants in 2015. Photo courtesy: Tampa Electric|
Key Insights on Technical Issues from Pilot Project Site Visits
The site visit workshops conducted for the Pilot Project identified a number of key insights on technical issues:
- Increased Changing Mission Profiles. Discussions at the site workshops supported the projection that an increasing number of coal and gas units are likely to continue to see changing mission profiles. Moreover, units experiencing new missions may likely experience multiple mission profiles in the future.
- Timeframe. Given that most existing coal units were designed based on the assumption of baseload operations, and considering the relatively slow turnover in the existing coal fleet, it will take a long time (on the order of decades) for new generation technologies inherently designed for multiple mission profiles to significantly penetrate the U.S. generation fleet.
- Ramp Rates. With increased shutdowns and low-load operation, high ramp rates and the corresponding impacts on equipment and systems are becoming an even more critical concern.
- Low-load Operation. Low-load or low-turndown operation was the predominant operating regime experienced by the pilot sites. The second most prevalent mode was shutdown where the duration was uncertain, thus complicating the planning and layup/equipment preservation efforts.
- Avoiding Shutdowns. Enabling lower-load operation of coal plants was identified as a key strategy for avoiding or minimizing shutdowns and the damage associated with on/off cycling. An alternative strategy was switching fuel to a lower-cost coal and/or gas co-firing. Both strategies come with plant-specific technical challenges.
- Layup Practices. Developing effective and “progressive” layup practices (that is, where the selection of layup practices and associated costs/ resources are commensurate with the duration of shutdown and startup demands) are perhaps the most widely felt, high-priority need across all unit types.
- Component Issues. Key issues involving major components that are experiencing low-load operation and frequent shutdown/startups included chemistry concerns and mechanical stresses of turbines, pitting damage and corrosion fatigue of boilers/heat recovery steam generators (HRSGs), and core integrity issues with main generators.
- Environmental Controls. For coal units experiencing low-load operation and frequent shutdown, issues related to the performance and reliability of the environmental controls equipment consistently ranked very high. The nature of these issues ranges from the cost of compliance and balancing the impacts on air/water/solids emissions to maintenance and effective layup practices for environmental controls equipment. For coal units with selective catalytic reduction (SCR) for NOx control, the minimum operating temperature of the SCR is often the first limit to low-load operation.
- Teams. Cross-functional, multi-disciplinary teams provide significant value in identifying and evaluating technical issues of importance from the standpoint of: (1) determination of new issues and areas of concern; (2) identification of solution options; and (3) assessment of impacts on plant staff and related resources.
- Holistic Approach. Breakout sessions at all pilot site workshops yielded new issues that were not initially identified. Both the identification of technical issues and potential solutions have benefitted from a holistic perspective. It ensured that insights from all aspects of the plant design, operations, and maintenance were included in the evaluation, as well as broader company-wide concerns such as emissions compliance strategies.
- Cost Factors. Budget uncertainties limit available options. The viability of solution options was an important consideration during discussion of each issue. In many cases, the cost of the solution option and the uncertainty in getting corporate financial support to deploy that option were key factors. This observation reinforces the need for multiple solution options with clear assessment of the compromises that would be made with a non-optimum “fix.”
- Monitoring and Diagnostics. A key aspect of the sustainability of potential solutions and the impact on plant staffs is the expectation of increased monitoring and diagnostics and the resultant significant increase in data. Data acquisition/monitoring centers could play a key role in addressing this challenge.
Key Insights on Generic Solutions
The site visit workshops also identified a number of insights on generic solutions:
- New Design or Technical Basis. With the advent of changing missions across the central station generation fleet, developing an improved understanding of the off-design operational impacts on plant processes and equipment becomes an important first step. Many plants are conducting tests to measure these impacts and the effectiveness of remedial actions. This includes characterizing specific impacts of flexible operation on major components, such as the boiler, turbine, etc. These efforts enable the development of a new design basis or technical basis for actions to correct or mitigate the negative consequences of the new mission.
- Monitoring Equipment. Possible damage could be avoided if the operations staff had information on the unit’s condition. For example, the addition of wireless thermocouples in and around the furnace could assist with monitoring operating conditions and also facilitate tuning of the burners. Novel high-temperature semiconductor strain gages would facilitate more effective monitoring of conditions that exacerbate creep and thermal fatigue.
- Measurement. Measurement of key process parameters are needed for automated control of critical and complex processes, which lessens the burden on operators and reduces human errors.
- Burden on Staff. Impacts of more frequent startups challenge the plant staff. Industry guidance on proper layup, greater levels of automation, and on-line monitoring systems may help alleviate this burden.
- Training. It is essential to capture the knowledge from current O&M staff before they retire. It may be necessary to retrain staff as they shift from one generation asset to another. A plant simulator capable of simulating several different operational modes would be helpful but may be seen as too expensive.
- Cost Impacts. Discussions during several site visits suggest that quantifying the impacts of new missions such that costs can be determined would help in risk management and resource (O&M and capital) planning.
- Need for Holistic Solutions. The issues associated with changing mission profile require solutions that span technical disciplines. For example, environmental compliance under changing mission profiles requires understanding the technical basis of key processes (e.g., mercury oxidation, sorbent effectiveness) such that effective process controls can be implemented. However, to successfully implement and sustain this solution, new instrumentation and controls, advanced monitoring and diagnostic systems, a new maintenance basis, and operator training will likely be needed.
Future EPRI Activities
The Pilot Project indicated the need for future EPRI activities, including the following:
- Industry Resource. There is a need for an industry resource that defines, acquires, organizes, and disseminates unit-level lessons learned and experiences.
- Review Process. For plants undergoing new missions and planning an analysis of technical issues, it would be beneficial to have a systematic, cross-discipline SME review process that identifies issues, solutions options, and unanticipated consequences.
- Environmental Controls and O&M. A number of needs have arisen involving the connection between environmental controls and O&M. For example, an analysis is needed of air/water/solids emissions such that capture of a pollutant does not inadvertently cause a more expensive or complex problem of disposal.
To proactively address the R&D needs identified above and to bring the value of the Pilot Project to all companies with generation assets facing new missions, a broad new industry collaboration was established. The collaboration, called the Mission Profiles Working Group (MPWG), consists of 13 utilities, representing over 200,000 MW of fossil generation, and organizes groups that seek to produce insights on how individual units can most cost-effectively accommodate new mission profiles.
Over two years, the MPWG has organized multiple working groups around different high-priority design/mission profile combinations. These groups consist of cross-disciplinary subject-matter experts who are identifying key issues and solution options or work-arounds, not just “gold-plated fixes.” The MPWG is also establishing an industry resource in the form of a living database of issues and solutions. Under development, too, are a self-assessment process based on the database that provides a systematic approach to assessing issues in an individual unit, and a progressive layup guidance.
Mike Caravaggio is EPRI Senior Program Manager, Major Component Reliability.
Norris Hirota is a Senior Technical Executive at EPRI.
Using Creep-FatiguePro Software to Monitor Boiler Life
Like other coal-fired plants in Spain, the As Pontes Power Station in Galicia, Spain is required to cycle extensively and operate relatively infrequently due to the power market and the extensive base of renewable energy that dispatches first.
This cyclic (start-stop) operation can result in accumulation of fatigue damage, particularly in thick-walled components such as high-temperature headers that experience the greatest thermal transients. In addition, components exposed to high temperature and pressure experience creep damage over time. Tracking the accumulation of fatigue and creep in these components is a critical element of an overall strategy to manage the life of boiler components.
In the early 1990s, EPRI developed Creep-FatiguePro™ (CFPro) software as a way to trace the accumulation of creep and fatigue damage, while a plant is being operated, based on actual operating conditions-temperature, pressure, and flow rate. Over the years, the software has been upgraded and new features added.
Recently the international utility Enel employed CFPro software to monitor accumulation of creep and fatigue damage at several locations in its As Pontes Power Station. The software helped the utility to track the level of damage occurring, to understand the direct effects on the plant of operational changes, and to plan remedial actions.
CFPro software was installed in the As Pontes Power Station and integrated with the plant instrumentation (PI) system. Eight locations were monitored on the super¬heater (SH) outlet header, and three locations were monitored on the hot reheat steam piping.
Over a several month test period, the CFPro system at the As Pontes plant clearly showed the impact of plant start-up on fatigue accumulation in the SH outlet header and identified an increase in the creep damage rate as a result of a 15°C temperature increase in the SH header. These findings provided important insights into the operation of the plant to minimize fatigue and creep damage accumulation as a result of flexible operation. The online component life assessment, made possible by the CFPro system, is being used to support timely decisions regarding: (1) unit operational strategy management, (2) impact of plant operating modes on component life, (3) component inspection need and schedule, and (4) run/repair/replace decisions for detected damage.