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 Obviously, it doesn’t make sense to keep investing in an outdated unit when a reduction in the ongoing operation and maintenance costs will justify a new or substantially upgraded unit.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.