Richard Kelly, D.I.T.1 Facilities Management and the Environment BSc in Electrical Services and Energy Management

FUEL COMBUSTION AND BOILER OPERATION

What is a Boiler? A boiler is an enclosed vessel in which water is heated and circulated, either as hot water, steam, or superheated steam for the purpose of heating, powering, and/or producing electricity. The furnace of the boiler is where the fuel and air are introduced to combust; Fuel/air mixtures are normally introduced into the furnace by using burners, where the flames are formed. The resulting hot gases travel through a series of heat exchangers, where heat is transferred to the water flowing though them. The combustion gases are finally released to the atmosphere via the stack of exhaust section of the boiler.

Richard Kelly, D.I.T.4

8 Good boiler design will: Seek to maximise the transfer of the energy released to heating of water, while at the same time Attempt to minimise the inevitable pollution of the environment

Richard Kelly, D.I.T.9 The most significant loss of energy from the overall process is the stack heat-loss due to the exit of warm combustion gases through the boiler flue. These losses can be estimated using combustion theory.

Richard Kelly, D.I.T.10 Air is mixed with the fuel in the boiler to provide oxygen for the combustion process. Excess air above the theoretical amount is needed in boiler operation to compensate for incomplete mixing of fuel and combustion air.

Richard Kelly, D.I.T.11 Complete combustion occurs when all of the fuel is converted to Carbon Dioxide and water. This is also called stoichiometric combustion.

Richard Kelly, D.I.T.12 The quantity of excess air provided is important. Too much excess air leads to unnecessary loss of heat from the boiler. A deficiency of combustion air, leads to fuel- rich combustion. This is dangerous because highly toxic carbon monoxide (CO) may be present in the flue gases in appreciable amounts.

The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than the heat provided by more efficient combustion.

Typical excess air quantities to achieve highest efficiency for burning fuels are: Natural Gas 5%-10% Fuel Oil 5%-20%

Richard Kelly, D.I.T.16 Large, unnecessary amounts of excess air can occur because of: ♦ Burner/control system imperfections ♦ Variations in boiler room temperature, pressure, and relative humidity ♦ Need for burner maintenance ♦ Changes in fuel composition

Richard Kelly, D.I.T.17 Air is comprised of approximately 21% oxygen and 79% nitrogen When air is delivered for combustion, the nitrogen absorbs heat and is carried up the stack, resulting in energy losses. If there is excess air, the result is unused oxygen as well as even more nitrogen to absorb heat that is carried up the stack.

Richard Kelly, D.I.T.18 Boiler efficiency can be improved by incorporating an excess air trim loop into the boiler controls. A stack gas oxygen analyzer can be installed to continuously monitor excess air and adjust or “trim” the boiler fuel-to-air ratio for optimum efficiency. A carbon monoxide trim loop, used in conjunction with the oxygen analyzer, assures that incomplete combustion cannot occur due to a deficient air supply.

Richard Kelly, D.I.T.19 For combustion calculations we deal with quantities of materials in terms of kilomoles of a substance 1 kilomole of a substance contains x atoms or molecules of that substance x is known as Avogadro's Number

Formal definition of the kilomole The amount of a substance which contains as many atoms or molecules as there are atoms in 12 kg of Carbon 12 (i.e. 12 kg of Carbon 12 contains 1 kilomole of carbon 12).

Ratio Nitrogen/Oxygen 23.3%21%Oxygen 76.7%79%Nitrogen By MassBy Volume Approximate composition of Air

Calorific Value of Some Fuels

Chemical reactions in symbolic form. C → C0 2 + Heat Solid carbon reacts with oxygen gas to form carbon dioxide gas with the release of heat. A single molecule of carbon (one atom) combines with a single molecule of oxygen (containing two atoms). 1 kmol of C + 1 kmol of 0 2 = 1 kmol of C0 2 Mass is conserved: 12kg Carbon + 32kg Oxygen = 44kg CO 2

2H 2 +O 2 → 2H 2 O+ Heat 2 Hydrogen molecules combine with one one Oxygen molecule to produce water and heat

Air Needed for Complete Combustion “Write the combustion equation for Octane (C 8 H 18 ) burning with the stoichiometric quantity of air.” - An idealised reaction is considered, i.e. additional products such as CO, NO and NO 2 are ignored.

The reaction will involve the Octane reacting with Oxygen and Nitrogen to produce CO 2, water and Nitrogen. The kilomolar quantities of Nitrogen and Oxygen are set equal to their approx volumetric proportions in air. 8 kmol of O 2 will combine with the 8 kmol of Carbon to produce 8 kmol of CO 2 The 18 Hydrogen atoms combine with 9 Oxygen atoms (4.5 kmol of O 2 ) to produce 9 kmol of water Total O2 required = 8+4.5=12.5 There will be 3.76 times more N2 than there is O2, so the total N2 is 12.5*3.76=47 The stoichiometric equation is then: C 8 H N 2 → 8C H N 2 C 8 H 18 + x xN 2 → yC0 2 + zH xN 2

Calculating the Volumetric Composition of Flue Gases A fuel oil, C 12 H 26, is burned with 25% excess air. Determine the volume percentage of the products of combustion.

First, determine the stoichiometric combustion equation: C 12 H 26 +x xN 2 →12C H N 2

Stoichiometric combustion required 18.5 Oxygen molecules. Next, increase the quantity of Oxygen and Nitrogen by a factor of 1.25 (25% excess air)‏ The final combustion equation becomes: C 12 H 26 +x xN 2 →12C H O N 2

Adding the products on the right we get: kmol We can now express each of the products as a percentage of : C0 2 : 12/ =10.29% H 2 0: 13/ = 11.15% O 2 : 4.63/ = 3.97% N 2 : 86.95/ = 74.58% Total 99.9%