Combustion is a specific group of chemical reactions where a fuel and oxygen burn together at sufficiently high temperature to evolve heat and combustion products. The fuel can be a gas (eg Hz or natural gas), a liq- uid (eg alcohol or oil), or a solid (eg Na, pure carbon, or coal). Combustion can vary in rate from a very slow decay to an instanta- neous explosion. The objective of the combustion engineer and plant operator is to obtain a steady heat release at the required rate.
Most industrial fuels are hydrocarbons, so called because their elements carbon and hydrogen, are oxidised to rriease heat durmg combustion. The chemistry of this oxidation process is a very complex chain reac- tion. However, for our purposes we can reasonably simplify the chem- istry to four basic reactions.
The Complete Oxidation of Carbon
C + O2 -> CO2 + 394 kJ/mole (94kcal/mole) The Complete Oxidation of Hydrogen
+ 572 kJ/mole (137kcaJ/moJe) - water condensed 2H2+ O2 -> 2H2O
+ 484 kJ/mole (116kcal/mole) - water as steam
The difference in the physical states of the water produced as a result of the oxidation of hydrogen is the reason for the complexity of the gross (GCV) and net calorific values (NCV) for hydrocarbon fuels. The gross
heat release is that which is released when the hydrogen is oxidised and the water condensed, whilst the net calorific value is the
heat which is released while the water remains as steam. The former is also referred to as the high-heating-value (HHV) and the latter low- heating-value (LHV).
The Incomplete Oxidation of Carbon
In the event of imperfect combustion, not all of the carbon in the fuel will be oxidised to carbon dioxide but some will be partially oxidised to carbon monoxide. The main effect of carbon monoxide production is to reduce the heat release from the fuel.
2C + O2 → 2CO + 221 kJ/mole (53 kcal/mole)
It can be seen that only just over halt" of the heat is released in the pro- duction of carbon monoxide, compared with the complete combustion of carbon. Thus any burners producing carbon monoxide as a result of bad fuel/air mixing will cause a significant drup in combustion effi- ciency. It is therefore absolutely essential to prevent the production of
significant levels of carbon monoxide in any combustion system. The Oxidation of Carbon Monoxide
Carbon monoxide is the unwanted repository of considerable combus- tion energy in inefficient combustors, and also an important air pollu- tant, a poisonous gas in high concentrations. In many instances where hydrocarbons are burnt, the oxidation reactions proceed rapidly to the point where CO is formed and then slow greatly until CO burnout is achieved. Carbon monoxide may be further oxidised to carbon dioxide according to the following reversible chemical reaction:
2CO + O2 ↔ 2CO2 + 173 kJ/mole (41 kcal/mole)
The combustion of dry carbon monoxide is extremely slow, however, if H-containing radicals are present in the flame, the combustion rate of (wet} carbon monoxide increases significantly.
9.2 Fuels
Hydrocarbon fuels may be solids, liquids or gases. Gases may be nat- ural or manufactured, generally from oil or coal. Both natural and man- ufactured fuels vary widely in chemical composition and physical characteristics. Each of these fuels is considered below.
Gases
Natural gas has been known for many years and utilised for much of this century. The characteristics of some typical gases are given in Section B6.3. It can be seen that while the basic constituent of all is
methane, the presence of other gases affects both the calorific value and the density. Methane has narrow flammability limits and the presence of higher hydrocarbons widens these limits and assists with flame sta- bility. Owing to the low carbon content of natural gas, conventional burners have low emissivity flames. This has a detrimental effect on the radiant heat transfer from the flame and can seriously affect the effi- ciency of the plant. The high hydrogen content means that natural gas requires more combustion air per kJ of heat released than most other fuels, and produces more exhaust gases, though these have a smaller proportion of CO2.
Oil Fuels
Oil fuels are produced by the refining of crude oil or can be manufac- tured from coal. Waste lubricating oil is currently being used as a
supplementary fuel in a number of plants, but supplies are limited. Oil fuels are classified as distillate fuels, such as kerosene and diesel oil or residual fuels. The latter come in a range of viscosities and are classified differently in,different countries. Typical characteristics of oil fuels are given in Section B6.2. Residual fuels have to be heated to render them pumpable and to reduce the viscosity to enable atomisation. The heav- ier the fuel, the more it has to be heated. Owing to the tendency of these fuels to solidify when cold, great care has to be taken with the design of oil fuel handling systems to minimise 'dead legs'. Since the lighter 'white' oil products have a higher value than black fuel oils, refineries increasingly manufacture more light products, leading to heavier and heavier back fuels containing increasing quantities of asphaltenes. These augmented refining processes involve 'cracking' the oil and pro- duces black oils which have different characteristics from the former residual oils. These cracked fuels vary in character, depending on the source of crude and the refining process and are not necessarily compatible with each other. Under some circumstances, fuel oils from different sources can form 'gels' in tanks and fuel handling systems with disastrous results. Proposed fuels should therefore always be test- ed for compatibility with the existing fuel before purchase.
Atomisation of fuel oil is important because the initial drop size deter- mines the size of the cenosphere which is formed and hence the length of time taken for the particle to burn. The oxygen diffusion is dependent on the surface area but the oxygen demand is dependent on the mass of the particle. Since the surface area is dependent on diameter2 and the
mass on diameter3 it follows that the larger the drop the longer it takes to burn.
Droplet sizes are normally measured in microns, a micron being 10-6 of a meter. This means that a 100 micron drop is 0.1 mm and 1000 micron drop is 1 mm diameter. Most atomizers produce a range of drop sizes with the smallest being in the order of a few microns in diameter, and the largest anything from 100 micron to 1000 micron or even more, A 100 micron particle takes about half a second to burn in a typical indus- trial flame, therefore a 500 micron particle takes about five times as long and a 1000 micron particle 10 times as long. Since the residence time of a droplet in a flame is typically 1 second or less, it follows that drops larger than about 200 micron will not be fully burnt out at the end of the flame and will either drop into the product as un-burnt fuel or end up in the dust collector. Anyone who makes a light coloured product but sees discoloured dust - dark grey or black - is suffering from just this sort of problem.
For optimum combustion performance, an oil sprayer with a range of drop sizes is ideal, fine drops to facilitate ignition and flame establish- ment and then some larger drops to maintain a controlled burning rate.
However, for the reasons outlined above, there should be a limit on the largest drops in the spray. Depending on the particular application, this upper limit should be in the order of 100-250 micron to minimise the risk of unburned fuel at the tail of the flame.
Equally as important as the drop size is the angle of the spray.
Essentially, most sprays are conical and two types are common; hollow cone and solid cone, Hollow cone atomisers are generally preferred,
since this enables the air to mix most effectively with the fuel- The small number of drops in the core of a hollow cone spray allows the estab- lishment of an internal recirculation zone which assists in maintaining a stable flame front.
Most burners are required to operate over a range of heat liberation and therefore fuel flow-rates. It is especially important that the atomiser per- formance is satisfactory over the entire operating range, since cement plants do not operate consistently at full load all the time. The drop size of many types of atomiser increases rapidly as the fuel flow-rate is turned down and this can present special problems for plant operation.
The turndown performance varies with different types of atomiser and is an important consideration when choosing an atomiser for a particu- lar application,
Coals
Great care has to be taken handling and burning coal owing to the risk of spontaneous ignition, fire and explosion. As a result, the design and
operation or coal firing systems requires greater specialist knowledge than gas and fuel oil systems. The characteristics of coals vary even more widely than other fuels, from anthracite, which has a high calorif- ic value and very low volatile and moisture content, to the lignites with
moisture and volatile contents of up to 6U%. Typical properties of some commonly traded coals are given in Section B6.1. The characteristics of the
coal and its ash have a dramatic effect on the performance of the plant in which it is burnt and on the plant maintenance requirements. Relevant properties include:
Volatile content - The higher the volatile content the more rapidly the coal ignites and bums. High volatile coals (above 35%) tend to present a significantly higher explosion risk than those below 25%. Coals with volatile contents above 45% require special precautions.
Swelling properties - Once the volatiles have been driven off a coke par- ticle is left behind. If this is larger than the original particle then it is more open and the particle will burn more rapidly than if it shrinks.
Moisture content - Coals have two types of moisture, surface moisture and inherent water. Generally the higher the inherent water the greater the reactivity of the coal and the higher the consequential fire and explosion risk. For pulverised coal tiring the surface moisture has to be removed when grinding. Removal of the inherent water should be minimized otherwise moisture from the atmosphere recombines with the coal and causes spontaneous heating which can result in a fire or explosion.
Ash content - Chemical composition of the ash has a significant effect on some processes and appropriate coals have to be selected accordingly.
Hardness and Abrasion Indices - The hardness of the coal affects the capac- ity of coal mills, the harder the coal the less can be ground and/or the coarser the resulting pulverised coal. The abrasion index is mainly
dependent on the ash characteristics. Very abrasive coals with high sil- ica ashes cause high wear rates in coal mill grinding elements,