How to Optimize the Energy Efficiency of Crude Distillation Unit (CDU) Furnaces
Oil and gas refineries are highly energy intensive and require large amounts of fuel to operate. Optimizing the energy efficiency of their furnaces is a key factor in achieving cost savings and reducing emissions. This article will look at the energy components and guidelines of a Crude Distillation Unit (CDU) furnace and explore ways to optimize its energy efficiency.
Understanding the Components of a CDU Furnace
A CDU furnace is designed to convert chemical energy into thermal energy, with the average thermal efficiency being estimated at 75-90%. To calculate the efficiency of the furnace, the equation is Heat Absorbed by the Process Stream (Useful Heat Output) divided by Heat Input.
Heat Input is calculated as the summation of fuel rate multiplied by the Low Heating Value (LHV) of the fuel, plus any external heat inputs to the system. Heat losses in the furnace can be divided into two categories; major and minor losses. Minor losses are typically plus or minus 2% and include radiation losses, convection section losses, incomplete combustion, and the latent heat of any atomizing steam. Major losses occur in the enthalpy/heat content of the stack gases, which can be minimized by reducing the quantity and temperature of the flue gas and recovering heat.
Optimizing the Energy Efficiency of CDU Furnaces
There are a number of steps that can be taken to optimize the energy efficiency of CDU furnaces. The first is to reduce heat losses by ensuring that the furnace is well insulated and that any combustion air preheating is done using an external source. Additionally, the quantity and temperature of the flue gas should be reduced through careful control of excess air and the use of heat recovery applications.
Heater Efficiency = Heat absorbed by process stream (useful heat output)/Heat Input
Another way to optimize the energy efficiency of CDU furnaces is to use fuel with a higher LHV. This will increase the efficiency of the furnace and reduce fuel costs. Additionally, it is important to ensure that the fuel is being burned efficiently and that any atomizing steam is properly managed.
Heat Input = Summation of {(Fuel rate* LHV)i }+ External heat input to the system
Finally, it is important to regularly monitor and maintain the furnace to ensure it is running optimally. This includes checking the furnace for any signs of wear and tear, such as cracks or corrosion, and ensuring that all components are in good working order.
By following these tips, oil and gas refineries can optimize the energy efficiency of their CDU furnaces, leading to cost savings and reduced emissions.
Combustion
Combustion is a rapid, runaway reaction between fuel and oxygen or air at the ignition temperature. The released heat is the desired product of the chemical reaction, rather than the end products. The amount of heat released can be controlled by the amount of fuel used. If more fuel than the stoichiometric amount is supplied, the efficiency of the reaction decreases because unreacted materials leave the heater at a higher temperature, taking away some of the heat produced.
For an efficient combustion process, fuel should be burned completely to CO2 and H2O with no excess air. However, since the mixing of reactants is never perfect, excess air is necessary to guarantee complete combustion in the fired heater.
Effects of Excess Air on Combustion
When excess air is used in combustion, energy is lost for two reasons. First, the extra air needs additional fuel to heat the air from ambient to stack temperature. This extra air cools down the heater, as it takes away some of the heat produced. Secondly, the extra air cools down the flame, reducing the amount of heat transferred in the radiation section of the heater.
Potential Fuel Savings from Reducing Excess Air
Modern heaters have software that can calculate the potential fuel savings from reducing the excess air. Heat available is defined as the LHV minus the enthalpy change of the products of combustion between the indicated temperature and 60°F, expressed as Btu/Lb of fuel. Heat available curves can be used to determine the amount of heat absorbed in a process heater if the stack temperature and excess air are known.
Plant operators can use heat available curves to calculate the efficiency of a fired heater, using the unit’s operating conditions of excess air and stack temperature. The heat available at the stack temperature is the heat absorbed in the unit/heater per pound of fuel fired. By dividing this by the fuel LHV, the heater efficiency can be calculated, neglecting minor losses.
In the past, high excess air levels of up to 30-50% of theoretical air was accepted. However, with rising fuel prices, there is an incentive to control the excess air and it is now common to see process heaters operating in the 10% excess air range.
Quick Graphs to Calculate Fuel Savings
Many quick graphs are available in books and other sources that can be used to get the percent of fuel saving upon given the original oxygen content and stack temperature. This calculates the potential fuel savings in fired heaters and boilers by reducing the oxygen in the flue gas from the current operating condition to 2-5%, which is considered a practical goal of most of the furnaces, according to the type of the fuel.
Improving Efficiency and Reducing Excess Air in Fired Heaters
Fired heaters are an essential part of many industrial processes, and ensuring that they are running efficiently is a key part of ensuring successful operations. Reducing excess air in these heaters is one of the most effective ways to improve efficiency and save money. The use of an excess air curve can help operators quickly determine the consequences of not making furnace adjustments to keep excess air low.
What is Excess Air?
Excess air is the amount of air that is added to the combustion process in addition to that which is required for complete combustion. Too much excess air results in a decrease in efficiency, as energy is wasted by cooling the flue gases and increasing the mass of flue gas that needs to be handled.
How to Reduce Excess Air
The most common cause of high oxygen levels in the stack is air leakage through holes and openings in the fired heater. To reduce air leakage, adjustments should be made to the fired heater draft. Additionally, a CO analyzer can be used instead of an oxygen analyzer to more accurately calculate the amount of excess oxygen in the flue gas.
The Cost of Air Leaks
Ignoring the cost of air leaks in fired heaters can be costly, with estimates of up to $60,000 per year depending on the stack temperature and draft as well as the size of the leak. Common sources of air leakage include flanges, cracks, casing corrosion, and open peep doors.
Constraints on Reducing Excess Air
While reducing excess air is beneficial for fired heaters, there are some constraints that can hinder its optimum adjustment, such as smoke, combustible breakthrough and flame shape. Each of these constraints may have a different threshold value of excess air, and each heater or boiler must be evaluated individually to determine its minimum excess air level.
Smoking
Smoking is caused by incomplete combustion of carbon particles or soot. Poor atomization, poor mixing of the air with fuel, and inadequate residence time in the radiant section can all lead to soot formation. Uneven distribution of combustion air can lead to smoking in one burner and non-smoking in another, and fuel mal distribution can be caused by plugged fuel tips, partially closed shut-off valves, or unsymmetrical burner piping.
Combustible Breakthrough
Combustible breakthrough is usually a result of unburned fuel due to air or fuel mal distribution.
Flame Shape
Maintaining proper flame shape is important to prevent tube over heating and impingement. When the excess air is reduced, the flames tend to become longer and to lose definition, which can lead to impingement if the tubes are close to the burners.
Conclusion
Reducing excess air in fired heaters is essential to ensure efficient operation and save money. Operators should be aware of the constraints that can prevent optimum adjustment, such as smoke, combustible breakthrough, and flame shape, and should evaluate each heater or boiler individually to determine its minimum excess air level.