BOILER EFFICIENCY

Boiler Efficiency is a term that establishes a relationship between energy supplied to the boiler and energy output received from the boiler. It is usually expressed in percentage. As a general rule, “Boiler efficiency (%) = heat exported by the fluid (water, steam) / heat provided by the fuel x 100.

Boiler efficiency is a combination of:

1. Combustion Efficiency

2. Thermal Efficiency

Apart from these efficiencies, there are some other losses that also impact boiler efficiency and hence need to be accounted for to arrive at boiler efficiency value.

1. Combustion Efficiency

Combustion efficiency indicated the ability of the combustion system–burner–grate to burn all combustible matter in the fuel.

The most desirable parameters for optimized combustion efficiency are minimized or no un-burnt carbon and ash with no traces of CO at the minimum possible excess air level.

Given the nature of the fuel, combustion efficiency is highest for gaseous followed by liquid and least for solid.

2. Thermal Efficiency

Thermal efficiency is a measure of heat transfer effectiveness i.e the extent of heat generated from the combustion of fuel that is transferred to the water. Thermal efficiency is impacted by scale formation/soot deposition on heat transfer surfaces.

The combination of thermal and combustion efficiency gives an overall efficiency of the boiler. Overall efficiency is calculated by two methods:

Direct efficiency- It is the simple ratio of output by input (steam generated/fuel consumption) Indirect efficiency- Indirect efficiency is a method based on the calculation of efficiency by accounting for losses

Indirect efficiency= 100%- Losses 100 - (L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8).

 

2.1 Direct Efficiency:

This method needs only the heat output and heat input, hence also called the input-output method. In this method, efficiency is calculated by dividing energy output (steam) by the boiler by energy input as fuel, using the equation:

 

Where,

Q = Steam Generation in kg/hr

hs = Enthalpy of steam at operating pressure in Kcal/kg 

hw = Enthalpy of water in Kcal/kg

NCV = net Calorific value of fuel in Kcal/kg 

q = actual fuel consumption in kg/h

a) Heat Input

Both heat input and heat output must be measured. The measurement of heat input requires knowledge of the calorific value of the fuel and its flow rate in terms of mass or volume, according to the nature of the fuel.

For gaseous fuel: A gas meter of the approved type can be used and the measured volume should be corrected for temperature and pressure. A sample of gas can be collected for calorific value determination, but it is usually acceptable to use the calorific value declared by the gas suppliers.

For liquid fuel: Heavy fuel oil is very viscous, and this property varies sharply with temperature. The meter, which is usually installed on the combustion appliance, should be regarded as a rough indicator only and, for test purposes, a meter calibrated for the particular oil is to be used and over a realistic range of temperature should be installed. Even better is the use of an accurately calibrated day tank.

For solid fuel: The accurate measurement of the flow of coal or other solid fuel is very difficult. The measurement must be based on mass, which means that bulky apparatus must be set up on

 

the boiler-house floor. Samples must be taken and bagged throughout the test, the bags sealed, and sent to a laboratory for analysis and calorific value determination. There are instruments like load cells, for vapor measurement of rotary feeders.

b) Heat Output

There are several methods, which can be used for measuring heat output. With steam boilers, and installed steam meter can be used to measure flow rate, but this must be corrected for temperature and pressure The density compensation vortex flowmeter gives an accurate reading. Use of orifice plate meter should be avoided. But these values are indicative given the nature of fuel and variation in physical properties.

There are other simple methods for small boilers like measurement of feedwater flow rate or measuring the change in feed tank level. These methods however give a rough estimation of heat output for efficiency measurement and the results are not reliable to be considered for actions pertaining to boiler efficiency.

The direct method using water measurement does not capture losses on account of blowdown.

Advantages and disadvantages of Direct Method

Advantages:

• Plant people can quickly evaluate the efficiency of boilers

• Requires few parameters for computation

• Needs fewer instruments for measurement and monitoring.

Disadvantages:

• Does not give clues to the operator as to why the efficiency of the system is lower. Reasons for the efficiency gap are not known.

2.2 Indirect Efficiency

The indirect method individually calculates the losses and the sum of these losses are then subtracted from 100 to give the value of efficiency %. Indirect method efficiency calculation standards that are available are usually IS8753, BS845, etc.

This method has one big advantage - of each loss being measured separately which provides quantitative data that will give details of gaps. These gaps than can be added by specific actions and changes. Hence this method tells us where we are, and how to get to a better level of efficiency.

 

Indirect efficiency includes the following losses:

Losses due to water and hydrogen in fuel:

This is the difference between GCV and NCV of a fuel that needs to be considered if efficiency is calculated on GCV. Not much can be done to reduce this loss, as it is a function of fuel constituents alone.

 Stack loss:

Stack losses represent the heat in the flue gas that is lost to the atmosphere upon entering the stack. Stack loss constitutes the addition of two types of losses: Dry Flue Gas Losses – the (sensible) heat energy in the flue gas due to the flue gas temperature; and the Flue Gas Loss Due to Moisture – the (latent) energy in the steam in the flue gas stream due to the water produced by the combustion reaction being vaporized from the high flue gas temperature. Improper combustion is responsible for this loss. In most burners, the manufacturer specifies a minimum level of excess air required to ensure that complete combustion of the fuel takes place. However, typically, excess air levels are higher than this specification, so fuel is being spent to heat air from ambient to flue gas temperature. Further, since the amount of air required depends on amount of fuel (which depends on load on the boiler), it varies continuously, making it that much more difficult to ensure that the excess air levels are kept within specified levels.

Stack loss can increase if the damper is not correctly positioned, or if the burner nozzles need cleaning, or in the case of oil, even if oil temperature is not controlled.

 Radiation loss:

Radiation loss is a function of temperature gradient between the boiler water and the ambient temperature, quality of insulation and surface area of the boiler. This is typically specified by the boiler manufacturer at full load conditions (say 1% for a packaged boiler). As this is a constant loss, at half load it will be double as a percentage. Accordingly, if steam flow is known, we can work out the instantaneous radiation loss.

In alternative fuels, other losses need to be considered such as ash bed losses in coal fired boilers or combustibles losses in husk fired boilers.

So, 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) = 100 − (𝑆𝑢𝑚 𝑜𝑓 𝑎𝑙𝑙 𝑙𝑜𝑠𝑠𝑒𝑠)

 

L1-Loss due to dry flue gas (sensible heat) L2- Loss due to hydrogen in fuel (H2)

L3- Loss due to moisture in fuel (H2O) L4- Loss due to moisture in air (H2O) L5-Loss due to carbon monoxide (CO)

L6-Loss due to surface radiation, convection and other unaccounted.

The following losses are applicable to solid fuel fired boiler in addition to above L7-Unburnt losses in fly ash (Carbon)

L8-Unburnt losses in bottom ash (Carbon)

𝐵𝑜𝑖𝑙𝑒𝑟 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 𝑏𝑦 𝑖𝑛𝑑𝑖𝑟𝑒𝑐𝑡 𝑚𝑒𝑡ℎ𝑜𝑑 = 100 − (𝐿1 + 𝐿2 + 𝐿3 + ⋯ + 𝐿8)

1. Heat Loss due to dry flue gas (L1):

Flue gas heat loss is the largest single energy loss in the combustion process. It is an inevitable loss as the individual constituent of flue gas enters the system as cold and leaves at high temperatures. Reducing the amount of excess air supplied to the burner can minimize flue gas heat loss. CO2 and O2 levels are directly related to the amount of excess air supplied.

L1 = % Heat loss due to dry flue gas

m = Mass of dry flue gas in kg/kg of fuel

    = Combustion products from fuel

    = CO2 + SO2 + Moisture in flue gases + O2 in flue gas + Mass of air supplied for combustion of fuel

Cp = Specific heat of fuel gas in kJ/kg K (BTU/lb F) 

Tf = Flue gas temperature in °C (°K)

Ta = Ambient temperature in °C (°K)

 

2. Heat loss due to hydrogen in fuel (L2)

The combustion of hydrogen causes a heat loss. The evaporation of water absorbs the heat in the form of Latent Heat. This loss can be calculated by following formula:

 

Where,

 

h2 = kg of hydrogen present in fuel (per kg of fuel) (lb)

Cp = Specific heat of superheated steam in kJ / kg °K (BTU/lb F) 

Tf = Flue gas temperature in °C (°F)

Ta = Ambient temperature in °C (°F)

2676 = Latent heat corresponding to a pressure of water vapor in kJ/kg ( BTU/lb)

3. Heat loss due to moisture present in fuel (L3):

Fuel also contains some amount of moisture it. On heating, the fuel the moisture in it escapes as superheated vapor. The loss due to moisture is made up: sensible heat to bring the moisture to the boiling point, the latent heat of evaporation of the moisture, and the superheat required to bring this steam to the temperature of the exhaust gas. This loss can be calculated by the following method:

 

Where:

m= kg moisture in fuel on 1 kg basis (lb)

Cp = Specific heat of superheated steam in kJ/kg °C(BTU/lb F) Tf = Flue gas temperature in °C (°F)

Ta = Ambient temperature in °C(°F)

2676 = Latent heat corresponding to a pressure of water vapor in kJ/kg (BTU/lb)

4. Heat loss due to moisture present in the air (L4):

The air entering in the system contains moisture in the form of vapor. This vapor is superheated and travels up the stack which accounts for the boiler loss.

 

Where:

AAS = Actual mass of air supplied per kg of fuel

 

Humidity factor = kg of water/kg of dry air

Cp = Specific heat of superheated steam in kJ/kg K (BTU/lb F) 

Tf = Flue gas temperature in (°C) (°F)

Ta = Ambient temperature in (°C) (Dry bulb temperature) (°F)

5. Heat loss due to incomplete combustion (L5):

Incomplete combustion leads to the formation of products like CO, H2, and hydrocarbons that are present in the flue gas. These products can be mixed with oxygen for complete combustion. The percentage concentration of CO can only be determined in the boiler plant test.

Where:

L5 = % Heat loss due to partial conversion of C to CO 

CO = Volume of CO in flue gas leaving economizer (%) 

CO2 = Actual Volume of CO2 in flue gas (%)

C =      Carbon content kg/kg of fuel

6. Heat loss due to radiation and convection (L6):

This loss in the boiler consists of the loss of heat by radiation and convection from the boiler water temperature to the ambient temperature.

   

Where,

L6 = Radiation loss in W/m

Vm = Wind velocity in m/s (ft/s) 

Ts = Surface temperature (°C) (°F)

 Ta = Ambient temperature (°C)

 

7. Heat loss due to unburnt in fly ash (%) (L7):

This loss is applicable to solid fuels only. Ash delivered in the atmosphere contains heat. This loss of heat can be calculated by analyzing the carbon content in the ash sample. The quantity of ash produced per unit of fuel must also be known. The loss contributes to around 1% in solid fuels.

8. Heat loss due to unburnt in bottom ash (%)(L8):

The gap between Direct and indirect efficiency:

a) Indirect method is a spot check

Indirect efficiency gives us the efficiency of the boiler at a particular time. It does not give us the overall picture of the boiler over a period of time. The boiler is tuned to operate under certain specific conditions, but these conditions are never constant. For e.g. the boiler is set to operate at a certain ambient temperature. This temperature is never constant and changes during the course of time. Hence the efficiency of the boiler also changes with the change in conditions.

It is thus important to measure indirect efficiency over a period of time.

b) Change in load fluctuations

The boiler efficiency is impacted by changes in steam load. Indirect efficiency is generally calculated at stable (constant) load conditions. Direct efficiency captures the changes in efficiency on account of load fluctuations. This adds up to the gap of indirect and direct efficiency.

c) Startup/ Shutdown Losses:

There are inherent start-up and shutdown losses in every boiler. Combustion systems are incorporated with pre-purge and post-purge cycles which are safety measures. During start-up, the burner does not start firing immediately. Instead, it purges air for a period of 30 seconds before the actual atomization. The purpose of pre-purge is to blow away the residual exhaust flue gases that exist in the furnace and the boiler tubes since the boiler is shut down. Similarly, a post purge cycle is carried

out after the shutdown. These purging cycles blow away hot flue gases and also take away a part of the heat in the combustion room. Indirect efficiency does not account for start-up and shutdown losses but direct efficiency does.

d) Blowdown Losses

Blowdown has to be carried out from the drum to maintain the TDS levels in the boiler within limits. The blowdown water is at saturation temperature and draining of this high-quality water is a straight loss. Indirect efficiency does not account for blowdown loss.

e) Ambient temperature Variation

Indirect efficiency is a spot check so does not capture this parameter.

f) Radiation Loss

Radiation loss in indirect efficiency is an assumed value either by standards or information from the manufacturer. Radiation loss depends upon surface area, insulation efficiency, and ambient temperatures. This further adds up to the gap between direct and indirect efficiency.