Combustion

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Combustion is the quick oxidation of fuel, releasing heat. Complete combustion of a fuel is then desired to allow the maximum delivery of its chemical energy content. Ideally, stoichiometric combustion occurs when fuel and oxygen in exact proportions react completely and yield maximum heat energy. However these ideal proportions of fuel and oxygen are just theoretical and cannot be applied for two reasons. First since it is a chemical reaction, to displace equilibrium towards products and to consume all fuel, an excess of oxygen is required and second, oxygen from air comes with nitrogen that reduces energy efficiency by absorbing heat from combustion and eventually reacts to form oxides. So too much excess oxygen cools the flame and increases NOx pollutants, while too little oxygen leads to incomplete combustion producing CO, unburned carbon or sooting and leads to a delayed combustion condition, which can result in furnace explosion.

Good combustion is obtained by mixing fuel and as little excess air as possible, to promote complete fuel burning and generate energy to match process needs, coping with safety and economics. This is accomplished by controlling the burner temperature to be high enough to initiate and maintain fuel ignition, turbulence at burner tip to promote intimate mixing of fuel and oxygen, and time to allow complete combustion. These parameters are provided by burner design and careful operation and maintenance of furnaces and burners.

The main energy efficiency opportunity for operating furnaces is controlling excess air. Combustion controls should be used to decrease the amount of excess air to a point where the amount of excess O2 is set to the optimal minimum, almost at the limit at which actual waste of energy occurs. Searching for this maximum energy efficiency point is a critical operation; small reductions in excess air, still guaranteeing complete fuel burning can represent significant energy savings. But if the air inlet is slightly less than the minimum oxygen requirement, there is not enough oxygen to complete the reaction, CO and smoking can appear. While less energy is released per mass of fuel and more fuel is demanded to cope with the absolute heat requirement of the furnace, heat receiving tubes can become fouled, and the equipment performance is reduced. Since combustion conditions vary through the operational period, finding a setting point a little higher than the minimum avoids these occurrences. Excess air can be measured by the amount of O 2 in the stack gas. It can be done periodically by sampling flue gas with an Orsat analyzer or continuously, with a recording gas analyzer. Of course continuous readings give more chances for operational improvement and better efficiency results. Eventually the presence of continuous CO and CO2 analyzers make this procedure more accurate. Excess air control is managed by draft control.

Draft provides air for the combustion process while exhausting flue gases to atmosphere. When induced by a stack alone it is called natural draft. Flue gas behaves similarly to air because both have nitrogen as the major component, and since its temperature is higher than ambient external temperature it is much lighter than the outside air. This pushes the flue gas up making the outside air flow in through the burners into the furnace. This kind of draft can only be controlled by hand-operated dampers in the stack and by flaps around the burners. Control is not precise and air flow depends mainly on heater load. But draft can also be controlled more accurately when air or flue gas flows are produced by fans. It is possible to blow air to the furnace through a blower that is called a forced draft, draw flue gas from the stack by a fan, inducing air to enter in the furnace in an induced draft or to use both to balance the air fed to the furnace, known as a balanced draft. When air is forced in, furnace pressure becomes higher than the external atmospheric pressure, when induced, slightly below and when balanced, it may vary from positive to negative relative to atmospheric. Whatever form of fan arrangement is used, a mechanical driven draft enables more accurate control.

Openings in the furnace can also lead to significant heat losses. If furnace pressure is below atmospheric, undesired cold air may infiltrate into the furnace and depending where it happens, can either increase excess air in a non controllable mood or simply cool heating surfaces and hot flue gas streams before they leave their energy in the proper spots. The other way around is if the furnace pressure is above atmospheric, hot streams leak out of the furnace, increasing heat losses to the ambient. The opportunity is to prevent and track openings in the furnace while ensuring that furnace is operated at a pressure slightly positive when the draft system allows.

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