Boilers are the source of energy in form of steam, electricity or both for industries. They supply steam for various heating requirements of the plant. Moreover, provide electricity to drive motors and lightening requirements, which we generate using steam turbines. Therefore, boiler efficiency directly related to cost of steam and electricity. In industries we generally find three types of boiler installations as below,
- Only for the steam supply, this we can see in small size industries and steam generation pressure is in the range of 2 – 16 bar.
- For power generation, as in power plants, these are high pressure boilers operating around 90 bar. This steam is used to run a turbine to generate electricity.
- Other type of installation is for both steam supply and power generation to the plants, also known as co-gen boilers. Here we can see, operating pressure can be in the range of 40 – 90 bar. We extract steam from various stages of the steam turbine based on the plant requirements.
We can classify boiler in two classes Fire-tube (also known as smoke tube) and Water-tube. In water-tube boiler water circulates inside the tubes. A high-pressure boiler is of this type as this will be an economical design. As high pressure require high shell thickness. Since, shell thickness depends on diameter, therefore keeping water in tube side will reduce the boiler cost.
For fire-tube boilers, water is outside the tubes or in other words hot gases are in tube side and water is in shell side. Small capacity and low pressure boiler are of this type.
In a boiler we use fuel to generate steam. Commonly used fuels are coal, furnace oil and natural gas. Other than these you can see distillery slop, bagasse, rice husk, coconut shell and municipal sewage is also used as a fuel.
For any industry efficient & safe boiler operation is very important. This is required for profitability, sustainability and environment compliance. In this article we will discuss how to calculate boiler efficiency. Moreover, we will find out various available opportunities for boiler efficiency improvement also.
Table of Contents
Schematic Diagram of a Boiler
To understand boiler process, you can refer below figure. In our discussion we are considering coal fired boiler. As coal fired boilers are most common in industries than furnace oil & natural gas fired boilers. This is because coal is cheapest fuel and easily available. The efficiency calculation and improvement basic remain same for all type of boilers.
You can see in above schematic diagram; we are using coal as a fuel and air for combustion. Feed water is going inside boiler and converting into steam taking heat from coal combustion. Therefore, energy converted into steam is our area of interest. While other areas are area of concern such as flue gas, boiler blowdown, boiler ash, fly ash and surface heat loss. So, in boiler operation it is our objective to maintain and improve boiler efficiency to increase business profitability and sustainability.
Steam/Fuel Ratio & Rated Efficiency of Boilers
Below are the data from various literature and based plant experience which you can refer for benchmarking:
Boiler Efficiency
In coal fired boilers steam generation (kg steam/kg coal) depends on coal GCV (gross calorific value). For higher GCV coal we get more steam generation per kg of coal and vice versa for low GCV coal. In a boiler we can estimate efficiency by two method, first is direct and second is indirect method.
Direct Method
Using direct method we can calculate the boiler efficiency using below formula. In this method we do not consider various losses of energy in boiler process.
Where, m = steam generation (kg/h), Hg = Enthalpy of steam (kcal/kg), Hl = Enthalpy of feed water (kcal/kg), W = Coal feed rate (kg/h), GCV = Gross calorific value (kcal/kg)
Indirect Method
Indirect method is based on all the losses in boiler. In this we estimate all the losses and subtract them from 100 to get indirect boiler efficiency. We can write down the formula for indirect method of efficiency as below:
Total loss includes following contributors:
L1 = heat loss with flue gas through stack
L2 = heat loss with moisture in fuel
L3 = heat loss with moisture in combustion air
L4 = heat loss because of hydrogen in fuel
L5 = heat loss due to incomplete combustion and CO formation
L6 = Unburnt carbon in fly ash
L7 = Unburnt carbon in boiler bed ash
L8 = Heat loss to the surrounding (due to radiation & convection)
Therefore, Total Loss = L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8, subtract this total loss from 100 to get indirect efficiency of the boiler.
Coal fired boiler efficiency ranges between 75 – 80%. Ideally estimated boiler efficiency from both the methods should be close to each other. Any gap is this tells us about unidentified inefficiencies or special causes such as air leakage, heat loss from damaged insulation, error in coal GCV data etc.
We can see the direct method of boiler efficiency is simpler. While, indirect method comprises many loss estimations. So, now we will discuss how to estimate the various losses as below.
Calculation of Various Losses in Boiler
Heat loss with flue gas: or stack loss is the largest contributor to the total loss. Typically, it can be in the range of 8 – 12%. It depends on the excess air flow and stack temperature. We generally feed 15% excess air to ensure the complete coal combustion and should be optimized monitoring the CO % and O2 % in flue gas. Higher excess air will decrease the boiler efficiency. Moreover, stack temperature is maintaining higher than 150 0C, to avoid the possibility of stack raining. This will cause acid formation because of SO2 in flue gases and subsequently damage the stack.
Where, Ma = Air flow rate to boiler (kg/h), Cpa = Heat capacity of air (kcal/kg-0C), W = Coal feed rate (kg/h), GCV = gross calorific value for coal (kcal/kg of coal), Ta = Ambient temperature (0C), Ts = Stack temperature (0C)
Heat loss of water formation due to H2 in fuel: To estimate this we need the H2 wt.% in coal. As you know H2 combustion equation is H2 + 1/2O2 = H2O. Hence, you can estimate heat loss due to water formation using below equation.
Where, H2 = weight % of H2 in coal and rest terms are mentioned as above
Heat loss with the moisture in fuel: You can understand higher moisture, higher the loss of energy. Open coal yards, which are the coal feed storage for boilers face this problem more in rainy season.
Where, M = weight % of moisture in coal
Heat loss because of moisture present in air: however, this loss is not significant and remain around <0.5%. We can estimate it as below:
Where, Ma = Mass flow rate of combustion air (kg/h), H = air humidity (wt.% of water in air)
Unburnt carbon in bottom ash: is the cause of significant loss and occurs poor fluidization and fluctuation in air to fuel ratio. Moreover, coal particle size distribution impacts this loss prominently. For AFBC boilers unburnt carbon in ash can be in the range of 5 – 6%. We can estimate the loss because of unburnt carbon in bed ash as follows:
Where, BA = kg of bottom ash/kg of coal feed, CB = weight % of carbon in bed ash
Unburnt carbon in fly ash: is a loss due to carryover of fine unburned carbon particles. This comes out with fly ash which trap in electrostatic precipitators. For AFBC boilers it should be in the range of 2 – 3%. We can estimate the loss because of unburnt carbon in fly ash as follows:
Where, FA = kg of fly ash/kg of coal feed, CF = weight % of carbon in fly ash
Ratio of bed ash to fly ash is around 90% : 10% of the total ash content in coal.
Surface Heat Loss: Finally, surface heat loss from insulation to surroundings due to radiation and convection heat transfer mechanism. For smaller capacity boilers this heat loss is in the range of 1.5 to 3%, while for large capacity boiler such as in thermal power plants, it can be in the range of 0.2 to 1.0%.
Opportunities for Boiler Efficiency Improvement
So far, we have discussed various sources of heat loss in a boiler. Having this insight now we can work out over various opportunities to improve our boiler efficiency.
- Proper treatment of water is very important before feeding into the boiler. Otherwise it will scale the boiler tubes and reduce heat transfer coefficient. Moreover, frequent cleaning increases maintenance cost and in due course of time it will damage the tubes also. Therefore, feed water TDS and pH controls are very important for efficient boiler operation.
- Coal should be stored under shed to eliminate dust contamination and avoid excess moisture content during rainy season.
- Preheating of combustion air using hot flue gas will increase boiler efficiency. As a thumb rule 20 0C decrease in stack temperature will reduce approximately 1% of coal consumption.
- Air to coal ratio is very important for the efficient and complete combustion of coal. Various factors affecting the coal combustion or boiler efficiency are as below:
- Fluctuation in air to fuel ratio – higher the ratio more heat loss through stack. And, in case of lower ratio incomplete combustion will be there. Which will increase CO generation. Hence, excess air control is important, a 2% oxygen decrease in flue gas can reduce fuel consumption by 3%. Theoretically, for 15% excess air, oxygen concentration in flue gas should be around 3%.
- Coal particle size – fine particles will increase unburnt carbon loss in fly ash, while higher size particles will increase unburnt carbon loss in bed ash. Therefore, proper sizing of coal particles is very important for the boiler efficiency.
- Boiler feed water preheating – available waste heat inside the plant should be used to preheat the boiler feed water. You should know 6 0C increase in boiler feed water temperature can reduce 1% of coal consumption approximately.
- Use of soot blowers – to clean the heat transfer surface. In boiler furnace 3mm soot deposition on tube can increase the fuel consumption by 2% approximately.
- Proper insulation – is very important to reduce surface heat losses. Any, damaged insulation or bare heat surface in boiler system must be insulated to avoid unnecessary heat loss.
- Boiler blowdown automation and flash steam recovery from boiler blowdown will increase the boiler efficiency.
- Precise control of boiler parameters – is the key to achieve consistently high boiler efficiency. Various parameters like furnace bed pressure drop, furnace pressure, stack O2%, feed water temperature, air to coal feed ratio, boiler pressure should be controlled using industrial control system.
Conclusion
In this blog I am sure you got insight of boiler operation and could be able to understand various factors responsible for boiler efficiency. Therefore, we should evaluate the performance of operating boilers in our plants and should plug the gaps to achieve benchmark boiler efficiency. This way you can increase the profitability of your business.
You should also look into the opportunity of boiler operation digital transformation. This digital transformation of boiler process can open next level of performance and operational excellence opportunities. Which will enable you to achieve sustained and improved boiler efficiency with asset health check, using real time data analysis. Moreover, use of real time data analytics will help you to understand boiler failure causes in advance. And, will prescribe the remedial action plan for abnormal process conditions. This action plan will eliminate the unplanned shutdown requirements for boiler maintenance. This will also help to manage the unpredictable feed and demand for boiler operation.
Finally, we can say, improve boiler efficiency, save energy, enhance business profitability and save the environment.
Thanks.