Efficient Fired Heater Operation

A fired heater is not operated uniformly over the entire run as it eould run light in turndown operation and harder in full capacity and toward the end of run for reaction heaters. To estimate the effects of changing tube wall temperature, corrosion rates, and pressure, a metaUurgic examination can be applied to estimate the remaining life of tubes. Knowing the tube life not only prevents premature tube failure, but also identihes the need for metal upgrade if the operating skin temperature increases over time. 

It must be stated that optimal 2% is the balance between safety and efficiency. There are several signs visible when a firebox is short of combustion air a hazy flame, regular thumping sound, and long flame touching the tubes. 

The cost-effective activities include seal welding of the casing, mudding up header boxes, and using high-temperature sealants. Leaks through roof penetration are also a major source of air leak, which should be inspected during turnaround. These activities are especially important for NO control. 

Proper control of combustion air is the key to make complete combustion and stable flame and thus avoid flame impingement. Lower fuel pressure also helps to avoid flame impingement. When the amount of excess air is appropriate, flame is orange and flue gas from stack is light gray. With sufficient air, if flame is long with much smoke, burners may have problems. 

Operators understand the importance of maintaining fired heaters in a safe and rehable condition. The response from operators to this priority could go to another extreme run fired heaters with too much excess air. The result of much excess air is much reduced flame length and thus the risk of flame impingement is minimized. 

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Modem fired heaters operate at thermal efficiencies of between 80 to 90 per cent, depending on the fuel and the excess air requirement. In some applications additional excess air may be used to reduce the flame temperature, to avoid overheating of the tubes. 

Fired heaters operate under very severe conditions, which make reliability and safety the highest priority. High reliability leads to higher performance and longer life. At the same time, efficient operation of fired heaters can improve fuel efficiency. Proper design and operation can achieve both high reliability and energy effieiency. 

Fired Hester a.s a. Reactor. When viewed as a reactor, the fired heater adds a unique set of energy considerations, such as. Can the heater be designed to operate with less air by O2 and CO analy2ers How does air preheating affect fuel use and efficiency How can a lower cost fuel (coal) be used Can the high energy potential of the fuel be used upstream in a gas turbine ... 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

In practice, the efficiency of a fired heater is controlled by monitoring the oxygen concentration in the combustion products in addition to the stack gas temperature. Dampers are used to manipulate the air supply. By tying the measuring instruments into a feedback loop with the mechanical equipment, optimization of operations can take place in real time to account for variations in the fuel flow rate or heating value. 

Next to the efficiency of the trays in a distillation tower, the efficiency of a fired heater is the most critical factor in saving and/or making money for the process plant. The primary objectives in operation of a fired heater are to... 

Reaction condition optimization considers reaction severity in terms of temperature and pressure profiles in accordance with catalyst performance in the entire run length. Optimizing reaction conditions, selecting better catalysts, and maintaining catalyst performance in operation have significant effects on both yields and energy efficiency. Consider reaction temperature as an example. In the catalyst cycle, the catalyst performance deteriorates, which affects the reaction conversion. To compensate, the reaction temperature may be increased. However, more severe reaction conditions require more heat from hot utilities such as fired heaters, while severe conditions also produce more desirable products as well as undesirable by-products. The question is how to determine the optimal reaction temperature, which is a function of reaction conversion, production rate, and energy use. 

Fired heaters provide a major part of heat source for reaction and separation. Reliability is the major concern for furnace operation with heat flux for large heaters and tube wall temperature (TWT) for small heaters as the most important reliability parameters. Increasing either heat flux or TWT could increase furnace efficiency. When operating heat flux is much lower than the maximum limit, this is an indication of the furnace being underutilized and thus presents an opportunity for increased feed rate. Increase feed rate is a win-win operation since more feed also results in reduced energy intensity. 

Therefore, maintaining a fired heater in rehable operation is the highest priority. With this priority in place, process plants can strive to maximize fired heater efficiency and hence reduce its running costs. This is because of a simple fact, fired heaters are the largest energy consumers in process plants and accounts for majority of total energy use. 

Efficient heater operation requires that excess air entering the convection section be minimized, whieh is indicated by a very small negative pressure at the convection section inlet. To achieve this, it should have a well balanced draft pressure profile between the firebox and stack. The hot gas pushes so that the pressure is always greatest at the firewall while the stack draft pulls. When this draft is eorreetly balanced, the pressure at the bridge waU should be around 0.1-0.2 WG (water gauge). Too mueh draft allows cold air leakage into the fired box resulting in wasted fuel. 

Guidelines for Fired Heater Reliability and Efficient Operation... 

Flue dampers that block the flue when the burner is not firing increase the efficiency of gas-fired water heaters. These can operate electrically or thermally. Because gas-fired water heaters lose so much heat up the flue during standby periods, this can provide significant savings. These are readily available on larger water heaters used in commercial settings but haven t been applied in the residential market because of their cost. 

In modem, packaged horizontal FT boilers, the furnace is the most important heat-transfer component, typically providing 50 to 60% of the total heat transfer from only 30% or so of the total available heating surfaces. This level of heat transfer, coupled with the additional heat extraction obtained by the various multiple-pass designs (four passes is a practical maximum) provide efficiencies of 80 to 83% GCV. As a result, there generally is little additional benefit to be obtained from the use of economizers or air heaters, especially when using oil-fired boilers, which can operate at up to a 3% or so higher efficiency level compared to gas-fired units. 

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