Fig. 1. Fuel-air ratios[2]

Man has always had a primeval fascination with fire and the heat treater is no exception, particularly since he has learned to harness this energy source as a cost-effective solution to heating and processing materials. We are on a continual quest to understand what combustion is and how we can safely and efficiently use this valuable resource. Let’s learn more.

The Industrial Heating Equipment Association (IHEA) defines combustion as “the rapid oxidation of a fuel resulting in the release of usable heat and the production of a visible flame.” By its very nature, combustion is a complex interaction of exothermic (i.e. heat producing) chemical reactions between a fuel and an oxidant producing both heat and light. The transport of energy, mass and momentum are the physical processes involved in combustion, while the chemical processes are conduction of thermal energy, diffusion of chemical species and the flow of gases, all of which follow from the release of chemical energy in the exothermic reaction. Combustion accounts for approximately 85% of the world’s energy.

For “perfect combustion,” natural gas and air are combined in a ratio of 10 parts air to one part gas (10:1, pronounced “10 to 1”) to produce approximately 1,000 BTU (252 kCal) of energy (Eq. 1). Perfect combustion is defined as the exact stoichiometric ratio (i.e. the balance of products and reactants) in which no excess fuel or air is produced (Fig. 1). In other words, perfect combustion is achieved when 100% of the air (or oxygen) required for combustion is provided. For each cubic foot (0.028 m3) of air input, 100 BTU (25 kCal) of heat is liberated. This is generally true for all hydrocarbon fuels (hydrocarbon gases, oil, coal, biomass, etc.).

(1) CH4 + Air (2O2 + 8N2) → CO2 + 2H2O + 8 N2

Deviations from perfect combustion also impact the analysis of the exhaust (flue) gas. Certain heat-treat applications require either excess fuel to, for example, provide a protective non-oxidizing atmosphere or excess air to, for example, provide an oxygen-rich atmosphere or improve low-temperature uniformity. Adding even small amounts of excess air lowers the flame temperature and the speed of heat transfer (i.e. the radiating “power” of the flame).

Fig. 2. Fuel-air ratios and resultant flame characteristics[3]

TOP: On-ratio (stoichiometric) combustion (all fuel consumed, minimum exhaust volume). The flame is primarily blue near the burner tile with a yellow cone shape. The highest flame temperature (hottest flame) is produced; CENTER: Lean combustion (flue products oxidizing – free oxygen). All fuel is combusted but the flame temperature drops due to heating the excess air. Note the pale blue color of the flame and the sharper more conical shape; BOTTOM: Rich combustion (incomplete, air starved or fuel rich, CO and H2 formed, creating a reducing atmosphere). Flame is predominately yellow, and the shape is less defined. The flame temperature also drops.

Combustion Basics

The onset of combustion begins with ignition from a source (e.g., pilot, spark igniter or Glo-plug) that provides heat to at least 1200°F (650°C). Once ignition commences, requirements for good combustion (Fig 2) are: (a) the proper proportioning of fuel and air to achieve stable combustion, (b) thorough mixing of the fuel and air within the flammability limits of the fuel, (c) initial and sustained ignition of the mixture, and (d) removal of the products of combustion from the process.

The scientific subject areas most relevant to combustion are the fields of thermodynamics (e.g., stoichiometry, gas and gas-mixture properties, heat of formation and reaction, equilibrium and adiabatic flame temperature), transport phenomena (heat transfer by conduction, convection and radiation, mass transfer) and chemical kinetics/fluid dynamics (e.g., laminar and turbulent flow, inertial and viscosity effects, combustion aerodynamics).

Fig. 3. Heat losses in a continuous furnace[2]

In the practical world, a Sankey diagram (Fig. 3) can be used to represent heat loss in industrial furnaces and ovens. Calculations of each individual heat-loss component allows us to determine the exact amount of available heat – that is, the energy left over from the total heat input provided after all losses are accounted for. This is the energy “available” to perform useful work – in other words, how much energy remains to heat the work and balance (make up for) thermal losses.

The available heat can be determined (Fig. 4 - online) from measurements of the flue-gas exhaust temperature and the percentage of excess oxygen in the exhaust. This latter fact is used to determine the amount of excess given the fact that 1% O2 » 5% excess air.

Air for combustion can be supplied to the burner in several ways. Primary air is a term used to describe air supplied and mixed with fuel prior to ignition. This is usually controlled through orifices and valves where all combustion air is mixed with the fuel and is ready to ignite as soon as it reaches the burner nozzle. Sealed-nozzle mixed burner systems depend entirely on primary air. By contrast, secondary air is supplied to the flame after it is ignited and is brought in at the burner. An example of both types is in atmospheric burners that use about 70% primary air and 30% secondary air.

The percentage of fuel savings[5] for any operating condition (preheated air, excess air, oxygen-enriched air) or combination of conditions can be determined (Eq. 2).

Fig. 4. Available heat input[2]

Classification of Heating Systems

Combustion systems can be divided into two general categories: direct-fired and indirect-fired systems. In direct-fired applications, the products of combustion are exposed to the work, whereas indirect systems fire into radiant tubes or find the work protected from the flame by such items as retorts and muffles. In most direct-fired combustion systems (e.g., box furnaces), secondary air is pulled into the furnace through leaky doors, other openings and negative furnace pressure.

Recuperative systems also have provided many benefits. There are two types of air preheaters: recuperators (Fig. 5) and regenerators. Recuperators are gas-to-gas heat exchangers placed on the furnace stack. Internal tubes or plates transfer heat from the outgoing exhaust gas to the incoming combustion air while keeping the two streams from mixing. Recuperators are available in a wide variety of styles, flow capacities and temperature ranges. Regenerators include two or more separate heat-storage sections. Flue gases and combustion air take turns flowing through each regenerator, alternately heating the storage medium and then withdrawing heat from it.

Typical Payback Period = (Cost of combustion air preheating system, obtained from the supplier or contractor) ÷ (Reduction in fuel usage, Million Btu/hr ´ Number of operating hours per year ´ Cost of fuel per Million Btu)

Fig. 2. Recuperative systems[2]

Good Combustion Practice

Understanding the basics of combustion will provide tangible benefits to the heat treater. Benefits include:
  • Faster heat-up times and load recovery (due to higher flame temperatures and greater heat transfer)
  • Greater efficiency (more available heat)
  • Reduced pollution (minimum exhaust volumes, reduced fuel use)
  • Cost savings (more cost competitive versus alternative energy sources).