Furnace

Improve Fuel Efficiency with Economizers

Boilers, fired furnaces, and kilns are extremely inefficient and are often the major consuming of energy in many different processes. The ability to recover wasted heat is critical to achieving optimal energy efficiency, capacity and profitability.

The majority of heat and energy losses in a furnace can be split into the following categories:

  • Waste gas
  • Shell losses
  • Heat in the product - i.e. steam

Waste gas energy losses can then be further divided:

  • Quantity of waste gas - excess air
  • Temperature - enthalpy
  • Composition - CO content, humidity, incomplete combustion

The amount of heat lost through the waste gas is a significant portion of every unit operation involving combustion, however it can be harnessed. This will improve the furnace’s thermal efficiency and reduce total plant fuel consumption. There are two main ways this can be done

  1. Reduce the quantity of waste gas through excess air control
  2. Reduce the temperature of the waste gas by installing an economizer

Simply put an economizer is a heat exchanger which preheats a boiler’s water feed by using the hot waste gas.

For furnaces other than boilers, for examples calciners or kilns, the same concept can be used, however another liquid stream may be required to reduce total plant energy requirements.

The economic justification for upgrading a boiler by installing an economizer can be made by simply looking at the potential in temperature reduction on heat loss. The potential enthalpy that can be recovered by reducing the stack temperature by 10 degrees can often be significant, and the temperature drop is usually much greater than this.  From this the design of an economizer in terms of heat exchange area, and liquid flows can be made. From this it is virtually the same as designing a heat exchanger, although the conditions particularly for high sulfur fuels can be extreme.

General rule of thumb’s suggest that more than 5% fuel consumption reductions can be achieved by installing an economizer, with additional 5% improvement possible with a condensing economizer - which condense the stack gases to additionally recover the latent heat of vaporization.

The re-use of heat is the primary method for improving fuel efficiency, and an economizer is the re-using heat in the most basic sense. Looking further afield synergy between unit operations can be taken into consideration when looking for even greater energy gains.

More Reading:

Consider installing a condensing economizer - US Department of Energy

Improve Combustion Efficiency with Excess Air Control

Boilers, fired furnaces, and calciners are among some of the most common pieces of equipment in any processing plant, and often consume the majority of the site’s fuel. The increase in their fuel efficiency will often result in significant economic benefits and the simplest way to do this is through excess air control.

Combustion is used in an extremely wide range of practices - boiling water into steam as a heat transfer media, calcining product such as limestone or alumina, transforming chemical energy into mechanical energy or electricity. Combustion requires a certain amount of oxygen for the reaction to fully occur, which is provided through air. Unfortunately air only contains approximately 21% oxygen, with the majority of the remainder being nitrogen. Due to the exothermic nature of the combustion reaction the nitrogen is also heated, which is then vented to the atmosphere through a stack. This is a direct loss of energy as heat to the atmosphere.

Most furnaces are run with too much excess air to ensure complete combustion, as well as to reduce the risk of an explosion if there is a potential for secondary ignition - i.e. the presence of an electrostatic precipitator to clean the waste gas on fired calciners. Insufficient air will also result in incomplete combustion, leading to high levels of Carbon monoxide, soot, ash, and unburnt fuel in the waste gas emissions.

Depending on the accuracy and confidence in the air flow meters, an air to fuel ratio control can be used to stabilize the excess air regardless of the furnace load. From this an online O2 or CO monitor can be used as a trim to ensure the optimal amount of air is being used at all times. The optimal excess O2 can be determined by measuring the CO concentration in the flue gas and reducing the oxygen content until the CO concentration begins to increase.

This control system requires strict control over both the fuel flow rate and the air flow rate through blower output/speed.

With the ever decreasing cost of instrumentation control systems such as this can be installed on smaller and smaller pieces of equipment and still prove to be economically viable.

The potential energy savings can be easily estimated:

  • Calculate the stoichiometric oxygen requirement for a set fuel flow
  • Determine an appropriate level of excess oxygen - usually 2-3%
  • Convert the oxygen requirements into total mass flow of air
  • Calculate the total enthalpy of the stack gases
  • Compare the difference between current stack energy losses and with excess oxygen control
  • Determine the cost of energy through fuel cost and calorific value

Introducing an excess air control system will also result in the following additional benefits:

  • Increased combustion efficiency
  • Reduced fuel costs
  • Reduced NOx emissions

Once excess air control is in place further fuel efficiency upgrades can be made, such as economizers to reduce stack temperature, or extra insulation to reduce shell losses.