The bulk of the energy requirements of the world is presently derived from fossil fuel sources. These sources are getting depleted at a faster rate. Since alternative sources have not been developed to full extent, conservation of energy has assumed great importance today, in particular when the energy cost is also increasing day by day.
It may be very impressive to note that a refinery with moderate to low furnaces efficiency is firing the equivalent of 4% of the crude if processes and that improvement of efficiency up to 90% may save 0.5% to 1% of total processed crude. This means, in a 12 MM t/y refinery a saving of more than 100.000 t/y.
Following are the factors considered in design and operation of fired heaters to make it more fuel efficient.
Complete combustion with minimum excess air
For complete combustion with minimum excess air one should excercise control on the following:
- Excess air monitoring
- Air Infiltration
- Combustion air pressure
Besides abnormal increase in stack losses, with the increase in excess air, the ingress of too much excess air lowers the flame temperature and consequently reduces furnaces heating rate. If too little excess air were used, the combustion would be incomplete and lot of fuel will be wasted and will be carried away by flue gases in form of unburnt combustible gases such as carbon monoxide, hydrogen and unburnt hydrocarbons. Thus it is significant to use appropriate control system to monitor proper Air/Fuel ratio to enable the heater to operate with adequate excess air level. Besides this, proper handling of fuel oil can also contribute to the Energy Conservation. While handling liquid fuel oils, some of the points which everybody should always remember are:
- Split oil is not retrievable. Plug all leakages
- Impurities in furnace oil affect combustion. Filter oil in stages
- Oil has to be preheated to obtain the right viscosity to the burn. Provide adequate preheater capacity.
Performance at Turndown Conditions
Proper selection and sizing of burners should be done to ensure that these can be operated within a fairly wide range of maximum firing rates.
Proper heat distribution
Selection of the type of burners, their layout and geometry of the furnace are quite significant factors which, duly supported by other control features would ensure engineered placement of heat which is vitally important for energy utilisation and saving.
Reducing heat losses from furnace openings
The substantial heat losses may occur through furnace openings. Hence, it is imperative that all necessary openings of the furnace be sealed. Doors should be tightly shut and made of light material that is durable and heat proof. To minimise heat losses, the refractory lining on doors should be sufficiently thick and of good insulation quality.
Controlling and maintenance of the correct pressure conditions inside the heater helps in fast and uniform heat distribution, avoid outside air infiltrations and thereby achieving better utilisation of fuel. The aspect of furnace control is very significant for energy conservation.
Minimising wall losses
The appropriate choice of refractory and insulation materials goes a long way in achieving fairly high fuel savings in industrial furnaces.
Waste Heat Recovery from Furnace flue gases
A major part of the heat being wasted can be recovered by utilisation of sensible heat of the furnace flue gases. In addition to heating of the process stream in the convection section, heat recovery can be affected by steam generation and/or combustion air preheating. The most common and effective way to utilise the waste heat in fired heater is preheating of combustion air.
The preheating of combustion air may be:
- Direct type with flue gas – combustion air heat exchanger in finned cast iron tubes.
- Indirect type with flue gas – heating medium convection exchanger and heating medium – combustion air exchanger at ground. In this case a drum and pumps system is installed to assure heating medium circulation.
The introduction of preheated air into existing furnace requires a redesign of the furnace due to change in heat load distribution. The investment required to incorporate air preheat system in the existing furnaces are very attractive having a pay back period between one to two years.
Increase of furnace efficiency by air preheating is one of the powerful contributions of energy conservation. The fundamental concept is to reduce the temperature of flue gases and transferring heat from the flue gas to incoming combustion air. An air preheater system saves furnace fuel by transferring heat from flue gases to combustion air. Furnace flue gas temperature is reduced and operating efficiency is correspondingly increased.
A schematic diagram of a typical air preheater installation is shown in following diagram:
The main advantages of heating of combustion air can be listed as below:
- Saving in fuel consumption
- Increase in flame temperature
- Improvement in combustion
- Reduction in initial heating time
- Reduction in scale losses
Thus we can say, that higher the air preheat, higher would be the saving in fuel consumption. But to what extent, the air can be preheater is influenced by various factors like:
- Volume and temperature of exit flue gases
- Suitability of burners employed, to withstand higher temperature
- Economic sizing of Air Preheater
- Economic evaluation of the choice of stack temperature Considering sulphur dew point or sulphur corrosion phenomenon
- The above is again depending on the sulphur content in the fuel being used.