While an average industrial power boiler has a lifespan of up to 50 years or more, replacing one can be a costly investment. The natural consequence of an aging boiler is reduced reliability, increased maintenance costs and lower performance. Depending on the age of a particular boiler and its projected remaining useful life, manufacturers usually have strong economic incentives to retrofit system components to keep the boiler operating at optimum capacity and efficiency.
The decision of when and how to upgrade an existing boiler can be driven by a number of factors. For example, how old is the boiler and when was it last upgraded? Is the system meeting desired reliability and efficiency goals? Are major operating components becoming costly to maintain or replace? Is there a need to replace the burners to accommodate a different type of fuel? Are there any impending regulatory changes on the horizon that may require an update in operating or control specifications?
A good starting point is examination of the boiler’s operation and maintenance costs. Are downtime or labor costs excessively high? Are replacement parts becoming overly expensive or difficult to find? Many times these costs are hidden within your overall maintenance budget. Obviously, it doesn’t make sense to keep investing in an outdated unit when a reduction in the ongoing operation and maintenance costs will justify a new or substantially upgraded unit.
Another variable that can factor into the upgrade equation is fuel costs. For example, if your existing unit is designed to fire low-grade fuel oil, you may want to evaluate a higher-performing, more cost-efficient fuel alternative. In this case, the next step would be to examine the conversion costs, along with operation, maintenance and efficiency projections to see if it makes sense to consider replacing the existing burner to accommodate a different fuel source.
Upgrading the Controls
Due to the complexity and importance of efficient boiler operation, one type of update that usually pays high dividends is a control system upgrade. Today’s advanced boiler automation and combustion control systems are capable of reducing costs while providing resources for greater flexibility in plant management and control. Boiler steam loads are always fluctuating and today’s sophisticated control systems can automatically detect changes and respond to conditions faster and more accurately than manually operated devices.
Boiler efficiency, in the simplest terms, represents the difference between the energy input and energy output. To achieve optimum efficiency, operators typically try to run boilers at approximately 80 percent load. In applications with multiple boilers and variable loads, achieving the most efficient combination of boilers may mean occasionally shutting down some to allow others to operate at a more efficient firing rate.
One effective strategy in periods of light production demands is to have your less efficient boilers operate in standby mode and engage the more efficient boilers to meet the load requirements. This can be accomplished by programming the boiler control system to automatically manage the desired boiler switchover sequence. The controls must be properly adjusted and coordinated for continuous delivery of steam or hot water to these dynamic processes. This includes online operation as well as control and monitoring of burner start-up and shutdown sequences.
Boiler control methods can improve operational consistency and reliability and protect against damage to combustion process equipment and surrounding areas due to explosion or other undesirable events. Following are some advance control techniques.
Minimize excess air – Efficient operation of any combustion equipment is highly dependent on a proper air-to-fuel ratio. The amount of unburned fuel and excess air in the exhaust is an indication of a burner’s combustion efficiency and requires energy to heat and move excess air. In actual operation, boilers and other fuel burning systems do not do a perfect job of mixing the fuel and air, even under the best possible conditions.
Regular monitoring of stack gas oxygen content will indicate how much excess air (O2) is available in the stack gas after the fuel/air combustion. High levels of O2 in the stack gas can be corrected by incorporating an excess air trim loop into the boiler controls. A stack gas oxygen analyzer can be installed to continuously monitor excess air and adjust the boiler fuel-to-air ratio for optimum efficiency.
The reduction of excess air in the boiler combustion process provides a better heat-transfer rate, advanced warning of potential flue gas problems and significantly lower fuel costs. By reducing the amount of air going through the combustion chamber, the boiler is able to absorb more of the heat in the process. Since the percentage of oxygen in exhaust stack is closely related to the amount of excess air, by adding oxygen trim controls, operators have tighter control over flue gas emissions, more precise control of excess air to oxygen set point, and faster return to set point following disturbances.
Plants that use a jackshaft (single point positioning), parallel positioning or other mechanically linked control system can gain significant advantages by converting to a cross-limiting, fully metered combustion control strategy. This control method helps improve safety by minimizing the chance of a dangerous ratio of air and fuel within a combustion process. This is implemented by always raising the air flow before allowing the fuel to increase, or by lowering the fuel flow before allowing the air flow to drop. Cross-limiting combustion control is highly effective and can easily provide: better optimization of fuel consumption; safer operating conditions by reducing the risk of explosion; faster combustion characterization setup; improved diagnostics and troubleshooting; and better process visibility. Combination firing of multiple fuels simultaneously can also be easily accomplished within this type of system.
A carbon monoxide trim loop, used in conjunction with the oxygen analyzer, can also be used to correct the basic fuel/air ratio for combustion quality and incomplete combustion. Carbon monoxide trim minimizes excess air while maintaining low stack CO emissions. It also provides additional process visibility and allows operators to respond quickly to needed changes in the fuel rate set point.
Upgrading a boiler with automated controls can help eliminate imbalances and upsets in system processes that impact plant operations and allow operators to be more responsive to changes. For example, by implementing an integrated control system connected to a high-speed communication network, manufacturers are able to extract data from control devices and quickly deliver it to high level business systems. This gives managers up-to-date information on fuel usage, peak demands and operational efficiency, allowing them to make quicker, more accurate decisions.
In addition, many plants have multiple boilers that are not the same size and don’t operate under the same load demands. By automating these multi-boiler systems and providing process information to managers, plants can better coordinate production and increase the operational efficiency and availability of the units. More specifically, the automated sequencing and load modulation capabilities of the control system allow plants to reduce fuel consumption while maintaining the same level of production.
One of the biggest benefits of a control system upgrade is the potential to significantly reduce operation and maintenance costs. With manual controls, boilers must be staffed by operators around the clock to monitor the system and respond to problems. Through the use of automated controls, pager systems can notify designated operators of triggered alarms. This allows plants to redeploy workers to perform other tasks and use their resources more efficiently.
Automation also gives manufacturers greater flexibility in how they meet their steam requirements. Many plants need the majority of their energy for first shift, which results in a huge increase in steam requirements to run their processes. Today’s advanced automation systems include microprocessor controls that anticipate heating load demand by calculating rate of change of system temperature or pressure, plus they can provide adjustable reset points for setback. As a result, boilers no longer need to run continuously to maintain constant steam header pressure during non-peak times. This gives users greater scheduling flexibility and allows them anticipatory response to steam demand changes.
Another key advantage of boiler automation is demand-load management. One of the primary goals in operating a boiler plant is to ensure that the working steam pressure is sustainable for any load demand placed on the plant. The purpose of demand-load scheduling is to distribute the steam demand in an optimized manner and to adjust the boiler plant output to meet working requirements. This helps ensure that boilers are fired only when needed and allows the system to use the best distribution of load between the boilers, resulting in the lowest overall cost.
Important Upgrade Considerations
When upgrading boiler controls, it’s important not to overlook the value of effective diagnostics and alarm capabilities. The sophisticated graphical and visualization capabilities of today’s human machine interfaces typically found with burner management/flame supervision systems help increase process uptime by reducing the amount of time necessary to identify process faults and system failures. With fewer veteran boiler operators in the workforce, the detailed diagnostics and alarm messages help less experienced operators to quickly identify oversights, while providing more access to information for more effective troubleshooting.
The addition of variable frequency drives is another technique that can help improve the performance and efficiency of the boiler process. Rather than relying on dampers and louvers to control the airflow through the fan, using variable frequency drives allows “trimming” of fan speed to exactly match field conditions. This helps reduce the air and electrical energy needed to operate the boiler and eliminates the need for fan over sizing, damper “short-stroking” or linkage requirements.
Companies can maximize their return on investment in their upgrade project by using automation technology that is scalable and compatible with other parts of their plant. For example, by implementing standard, off-the-shelf hardware that is similar to other technology already employed elsewhere in the plant, companies can minimize their maintenance, training and inventory costs and leverage the investment they may have already incurred in these areas.
In any boiler upgrade project, it can be valuable to look into the future and carefully consider all potential needs and operational requirements, including changes in regulatory standards, communication needs, and fuel usage issues. For example, with more complex solutions, it’s important to take into account the potential additional expenses associated with maintaining more sophisticated devices or instrumentation, such as ongoing testing and calibration. Therefore, companies should secure in-house or outside expertise to handle this task and factor this expense into the project’s long-term cost.
Finally, it’s important to work with an experienced boiler system consultant to review the efficiency and condition of your existing unit. This includes performing a comprehensive evaluation of your boiler room requirements, including boiler size, load characteristics, backup requirements, fuel type, control requirements and emission requirements.
In the end, the time spent evaluating the potential savings in fuel, maintenance and boiler efficiency will be well worth the effort. Incorporating high efficiency burner technology and high-performance controls will pay off every time your boiler is fired, for the entire life of the equipment.