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Combustion University

Learn more about the exceptional combustion services and products that Metro Services can provide for you. Metro Services, Inc. is dedicated to exceeding our customer’s expectations by providing world-class service, without exception, across all product and service lines. We pledge to treat each customer, employee, vendor, and subcontractor with integrity, honesty and fairness, thereby building long-term business and personal relationships. Each member of the Metro Services team works for and is committed to our customers and we realize that we would not be here without our customer’s commitment to us.

Boiler Education: Ask the Energy Expert

Dear Energy Expert,
My plant is looking for recommendations for achieving boiler combustion control more efficiently. Do you have any suggestions?

Check Burner Air to Fuel Ratios

Periodic checking and resetting of air-fuel ratios is one of the simplest ways to get maximum efficiency out of fuel-fired process heating equipment such as furnaces, ovens, heaters, and boilers. Most high temperature direct-fired furnaces, radiant tubes, and boilers operate with about 10 to 20 percent excess combustion air at high fire to prevent the formation of dangerous carbon monoxide and soot deposits on heat transfer surfaces and inside radiant tubes. For the fuels most commonly used by U.S. industry, including natural gas, propane, and fuel oils, approximately one cubic foot of air is required to release about 100 British thermal units in complete combustion. Exact amount of air required for complete combustion of commonly used fuels can be obtained from the information given in one of the references. Process heating efficiency is reduced considerably if the air supply is significantly higher or lower than the theoretically required air.

Combustion Theory

In light of the continued rise in fuel and labor costs, a good understanding of basic combustion theory is more important today than ever before. In addition, increasingly stringent environmental regulations and concerns make the selection, design, and maintenance of combustion control systems of paramount importance. There must always be more air supplied to the combustion process than the theoretical or stoichiometric air requirement. This is because no burner made is this “perfect”. This “extra” air is referred to as “excess air”. If 20% more than the theoretical air requirement is supplied, we say that the burner is operating at 20% excess air. Another way of stating the same thing is to say that the burner is operating with 120% “total air.”

Eclipse Combustion Engineering Guide

Click to open the engineering guide to Eclipse products. The Eclipse combustion engineering guide (EFE 825, 1/00 Electronic Edition Published by Eclipse, Inc. All Rights Reserved) contains chapters on Orifices & Flows, Fan Laws & Blower Application Engineering, Gas, Oil, Steam & Water, Electrical Data, Process Heating, Combustion Data, Mechanical Data, and more. Please reference the Eclipse combustion engineering guide below.

Furnace Pressure Controllers

Furnace draft, or negative pressure, is created in fuel-fired furnaces when high temperature gases are discharged at a level higher than the furnace openings. This is commonly known as the chimney effect. The negative pressure in a furnace that operates at a fixed temperature changes with the heat input rate or mass flow of flue gases moving through the stack. This negative pressure causes ambient air to leak into the furnace. Furnace pressure controllers regulate and stabilize the pressure in the working chamber of process heating equipment. Pressure controllers use a pressure gauge in the furnace chamber or duct and regulate the airflow to maintain a slightly positive pressure (a few inches of water gauge) in the furnace chamber. Airflow can be regulated by varying the speed of draft fans or by changing damper settings for the incoming combustion air or the exiting flue gas.

Install Waste Heat Recovery Systems for Fuel-Fired Furnaces

For most fuel-fired heating equipment, a large amount of the heat supplied is wasted as exhaust or flue gases. In furnaces, air and fuel are mixed and burned to generate heat, some of which is transferred to the heating device and its load. When the heat transfer reaches its practical limit, the spent combustion gases are removed from the furnace via a flue or stack. At this point, these gases still hold considerable thermal energy. In many systems, this is the greatest single heat loss. The energy efficiency can often be increased by using waste heat gas recovery systems to capture and use some of the energy in the flue gas.

Load Preheating: Load Preheating Using Flue Gases from a Fuel-Fired Heating System

The thermal efficiency of a heating system can be improved significantly by using heat contained in furnace flue gases to preheat the furnace load (material coming into the furnace). If exhaust gases leaving a fuel-fired furnace can be brought into contact with a relatively cool incoming load, heat will be transferred directly to the load. Since there is no intermediate step, like air or gas preheating, in the heat recovery process, this can be the best approach to capturing waste heat. Load preheating is best suited for continuous processes, but it can sometimes also be used successfully with intermittently operated or batch furnaces.

Oxygen Enriched Combustion

When a fuel is burned, oxygen in the combustion air chemically combines with the hydrogen and carbon in the fuel to form water and carbon dioxide, releasing heat in the process. Air is made up of 21% oxygen, 78% nitrogen, and 1% other gases. During air–fuel combustion, the chemically inert nitrogen in the air dilutes the reactive oxygen and carries away some of the energy in the hot combustion exhaust gas. An increase in oxygen in the combustion air can reduce the energy loss in the exhaust gases and increase heating system efficiency.

Reduce Air Infiltration in Furnaces

Fuel-fired furnaces discharge combustion products through a stack or a chimney. Hot furnace gases are less dense and more buoyant than ambient air, so they rise, creating a differential pressure between the top and the bottom of the furnace. This differential, known as thermal head, is the source of a natural draft or negative pressure in furnaces and boilers. A well-designed furnace (or boiler) is built to avoid air leakage into the furnace or leakage of flue gases from the furnace to the ambient. However, with time, most furnaces develop cracks or openings around doors, joints, and hearth seals. These openings (leaks) usually appear small compared with the overall dimensions of the furnace, so they are often ignored. The negative pressure created by the natural draft (or use of an induced-draft fan) in a furnace draws cold air through the openings (leaks) and into the furnace. The cold air becomes heated to the furnace exhaust gas temperature and then exits through the flue system, wasting valuable fuel. It might also cause excessive oxidation of metals or other materials in the furnaces.

Save Energy Now in Your Process Heating Equipment

Process heating accounts for about 36% of the total energy used in industrial manufacturing applications. And in some industries, this percentage is much higher. In the glass industry, for example, process heating accounts for about 80% of energy consumption, according to the U.S. Department of Energy’s (DOE) Manufacturing Energy Consumption Survey. The thermal efficiency of a heating system can be improved significantly by using heat contained in furnace flue gases to preheat the furnace load (material coming into the furnace). If exhaust gases leaving a fuel-fired furnace can be brought into contact with a relatively cool incoming load, heat will be transferred directly to the load. Since there is no intermediate step, like air or gas preheating, in the heat recovery process, this can be the best approach to capturing waste heat. Load preheating is best suited for continuous processes, but it can sometimes also be used successfully with intermittently operated or batch furnaces.

Waste Heat Recovery 101

Thermal efficiency of process heating equipment, such as furnaces, ovens, melters, heaters, and kilns is the
ratio of heat delivered to a material and heat supplied to the heating equipment. For most heating equipment, a
large amount of the heat supplied is wasted in the form of exhaust or flue gases. These losses depend on various factors associated with the design and operation of the heating equipment. This technical brief is a guide
to help plant operators reduce waste heat losses associated with the heating equipment.
This technical brief supports or complements the software tool Process Heating Assessment and Survey Tool
(PHAST) developed jointly by the Industrial Heating Equipment Association (IHEA) and the U.S. Department
of Energy’s (DOE) Industrial Technologies Program.