FUNDAMENTALS OF COMBUSTION AND HEAT LOSSES

COMBUSTION PROCESSES have been, are and will be for the near future, the prime generator of energy in our civilization, which is burning fossil fuels at an ever-increasing rate. The processes must be managed well for the sake of the environment and the sustainability of civilization.

The principles of combustion are common to heaters, boilers and other forms of industrial combustion, e.g. in furnaces and kilns. In this sense, the term "boiler" is interchangeable with "heater" throughout this text (unless stated otherwise).

Conventional fuels consist mainly of two elements carbon and hydrogen. During combustion, they combine with oxygen to produce heat. The fuel value lies in the carbon and hydrogen content. Non-fossil fuels, such as biomass and alcohol, also contain oxygen in their molecular structures.

Ideally, combustion breaks down the molecular structure of the fuel; the carbon oxidizes to carbondioxide (CO2) and the hydrogen to water vapour(H2O). But an incomplete process creates undesirable and dangerous products. To ensure complete combustion, even modern equipment with many features must operate with excess air. That is, more air (carrying about 21 percent oxygen by volume) is passed through the burner than is chemically required for complete combustion. This excess air speeds up the mixing of fuel and air.

On one hand, this process ensures that nearly all the fuel receives the oxygen it needs for combustion before it is chilled below combustion temperatures by contact with heat exchange surfaces. It also prevents fuel that is not burned completely from exploding within the boiler.

On the other hand, excess air wastes energy by carrying heat up the stack. A fine line exists between combustion efficiency and safety in ensuring that as little excess air as possible is supplied to the burner.

Boiler owners and operators will want to know if their operations are efficient. As the objective is to increase the energy efficiency of boilers, reviewing the causes of heat loss in boiler operations maybe useful.

HEAT LOSSES in a boiler are well described by the American Society of Mechanical Engineers (ASME) in its rigorous PTC4.1 power test code (1973). The test code applies to any type of fuel used. However, natural gas or fuel oil fire most boilers and heaters in Canada. In such systems, many of the losses listed in the code do not apply. And other systems are small enough for their losses to be rolled into an "unaccounted for" category, for which a value can be assumed. A simplified method for quantifying boiler efficiency uses this equation:

Efficiency (E) % = (Output ÷ Input) X 100,where: Output = Input – Losses

Alternatively,

Efficiency (E) % = 100 – losses, where losses can be calculated according to the ASME power test code.

Since this code uses Imperial units, it is necessary to convert temperatures to degrees Fahrenheit (ºF) and heating units to British thermal units per pound (Btu/lb.), which can be done with the following conversion formulas:

ºF = (1.8 X ºC) + 32
Btu/lb. = 0.4299 X kJ/kg

The following four significant types of energy losses apply to natural gas and heating oil systems.

Dry flue gas loss (LDG)

Heat is lost in the "dry" products of combustion, which carry only sensible heat since no change of state was involved. These products are carbondioxide (CO2), carbon monoxide (CO), oxygen(O2), nitrogen (N2) and sulphur dioxide (SO2).Concentrations of SO2 and CO are normally in the parts-per-million (ppm) range so, from the viewpoint of heat loss, they can be ignored. Calculate the dry flue gas loss (LDG) using the following formula:

LDG = [24 x DG x (FGT – CAT)] ÷ HHV, where

DG (lb./lb. fuel) = (11CO2 + 8O2 + 7N2) x (C + 0.375S) ÷ 3CO2
FGT = flue gas temperature, ºF
CAT = combustion air temperature, ºF
HHV = higher heating value of fuel, Btu/lb.
CO2 and O2 = percent by volume in the flue gas
N2 = 100 – CO2 – O2
C and S = weight fraction in fuel analysis

Minimizing excess air reduces dry flue gas losses.

Loss due to moisture from the combustion of hydrogen (LH)

The hydrogen component of fuel leaves the boiler as water vapour, taking with it the enthalpy – or heat content – corresponding to its conditions of temperature and pressure. The vapour is a steam at very low pressure, but with a high stack temperature . Most of its enthalpy is in the heat of vaporization. The significant loss is about 11 percent for natural gas and 7 percent for fuel oil. This loss (LH) can be calculated as follows:

LH (%) = [900 x H2 x (hg – hf)] ÷ HHV, where

H2 = hydrogen weight fraction in fuel analysis
hg = 1055 + (0.467 x FGT), Btu/lb.
hf = CAT – 32, Btu/lb.

Where hg is the enthalpy of water vapour at 1 psig (pounds per square inch gauge) and the flue gas temperature (FGT), and hf is the enthalpy of water at the combustion air temperature (CAT).

Only a condensing heat exchanger will reduce this loss appreciably.

Table 1. Direct Method for Calculating Boiler Efficiency

  1. Measure steam flow via kg (or lb.) over a set period, e.g. one hour. Use steam integrator readings, if available, and correct for orifice calibration pressure. Alternatively, use the feedwater integrator, if available, which will in most cases not require a correction for pressure.
     
  2. Measure the flow of fuel over the same period. Use the gas or oil integrator, or determine the mass of solid fuel used.
     
  3. Convert steam flow, feedwater flow and fuel flow to identical energy units, e.g. Btu/lb. or kJ/kg.
     
  4. Calculate the efficiency using the following equation: Efficiency = 100 x (steam energy – feedwater energy) ÷ fuel energy

Loss due to radiation and convection (LR)

This loss occurs from the external surfaces of an operating boiler. For any boiler at operating temperature, the loss is constant. Expressed as a percentage of the boiler's heat output, the loss increases as boiler output is reduced. Hence, operating the boiler at full load lowers the percentage of loss. Since the boiler's surface area relates to its bulk, the relative loss is lower for a larger boiler and higher for a smaller boiler. Instead of making complex calculations, determine the radiation and convection loss using a standard chart available from the American Boiler Manufacturers Association (ABMA).

Losses that are unaccounted for (LUA)

For reasons mentioned earlier, use an assumed loss value of 0.1 percent for natural-gas-fired boiler systems and 0.2 percent for oil-fired systems.

Then, calculate efficiency as follows:

Efficiency (E) % = 100–LDG–LH–LR–LUA, where

LDG = Dry flue gas loss
LH = Moisture from hydrogen loss
LR = Radiation and convection loss
LUA = Unaccounted for losses

Begin a boiler plant program of energy management by assessing current boiler efficiencies. Then monitor boiler performance regularly to gauge the effect of established energy-saving measures and to set improvement targets.

The simplest way to calculate fuel-to-steam efficiency is the direct method of calculation (see Table 1), using steam generation and fuel consumption data from operating logs. However, this method may not be as accurate as the indirect method due to errors in metering fuel flow and steam flow.

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