Guru
Introduction
The estimation of calorific value is an important operation in the area of energy exploration which reveals the bound of potential energy in various fuels. One of the most essential parameters in understanding the efficiency and energy content in fuels is the calorific value, or heat of combustion. The main subject of this blog is the process by which we calculate the calorific values in all cases the energy generated through the combustion of fuels is quantified.
Before knowing about different methods to measure Calorific Value,it is necessary to know about what is a calorific value?
Defining Calorific Value
The calorific value measures the heat energy you get when you completely combust one kilogram of a specific fuel. You usually express it in joules per kilogram (J/kg) or British thermal units per pound (BTU/lb). You need to establish the calorific value to assess whether a fuel is suitable for different applications, from industrial processes to electricity generation.
| Constituent | Higher calorific value |
|---|---|
| C | 8080 kcal/kg |
| H | 34500 kcal/kg |
| S | 2240 kcal/kg |
If oxygen is also present, it combines with hydrogen to form `H_2O`.Thus, the hydrogen in combined form is not available for combustion and is called fixed hydrogen.Amount of hydrogen available for combustion = Total mass of hydrogen–hydrogen combined with oxygen.
`[H_2+½\left(O_2\right)=H_2O]`
1 8 9
that is 8 parts of oxygen combines with 1 part of hydrogen to form water or for every 8 parts of oxygen, 1 part of hydrogen gets fixed.If the fuel contains x mass of oxygen then
Fixed hydrogen = 18×X=Mass of oxygen in fuel8
Amount of hydrogen available for combustion = (H-`frac{o}{8}`)
Dulong's formula for calculating calorific value is given as
Gross calorific value (HCV) = `frac{1}{100}`[8080C + 34500(H-`frac{o}{8}`) + 2240S] kcal/kg
Here C, H, O and S are percentages of carbon, hydrogen, oxygen and sulphur in fuel.
Net calorific value (LCV) = (HCV-`frac{9H}{100}times587`) kcal/kg
(HCV – 0.09 H × 587) kcal/kg
(Latent heat of steam = 587 kcal/kg).
Units of calorific value and heat
Unit of calorific value
The units of calorific value for solid, liquid and gaseous fuels are given below.
| System | Solid / Liquid fuels | Gaseous fuels |
|---|---|---|
| CGS | calories/g | cm3 |
| MKS | kcal/kg | m3 |
| BTU | BTU/lb | Btu/ft3 |
These units can be interconverted as follows:
1 cal/g =1 kcal/kg = 1.8 BTU/lb
1 kcal = 0.1077 BTU/
1 BTU/= 9.3 kcal/
Units of heat
1.Calorie:It is defined as the amount of heat required to raise the temperature of 1 g of water by 1 °C ( from 15 °C to 16 °C)
1 calorie = 4.185 Joules = ergs.
2.Kilocalorie:It is defined as the amount of heat required to raise the temperature of 1 kg of water by 1 °C (from 15 °C to 16 °C). 1 kcal = 1000 cal.
3.British Thermal Unit (BTU):It is defined as the amount of heat required to raise the temperature of 1 pound (lb) of water by 1 °F (from 60 °F to 61 °F)
1 BTU = 252 cal = 0.252 kcal = 1054.6 Joule = ergs.
4.Centigrade Heat Unit (CHU):It is defined as the amount of heat required to raise the temperature of one pound of water by 1 °C (from 15 °C to 16 °C).
1 kcal = 3.968 BTU = 2.2 CHU
Gross and Net Calorific Value
- Gross Calorific Value (GCV):It is also called higher calorific value (HCV) and is defined as the total amount of heat produced when a unit quantity (mass/volume) of fuel is burnt completely, and the products of combustion are cooled to room temperature.
Usually all fuels contain hydrogen. During combustion, the hydrogen present in the fuel is converted into steam. When the combustion products are cooled to room temperature, the steam gets condensed into water and heat that equals the latent heat of condensation of steam is evolved. This heat gets included in the measured heat, and so its value is high; hence, it is called higher calorific value.
- Low Calorific Value (LCV):It is also termed as net calorific value (NCV) and is defined as the heat produced when a unit quantity (mass/volume) of a fuel is burnt completely and the hot combustion products are allowed to escape.
In actual practice, when a fuel is burnt water vapor escapes along with the hot combustion gases; hence, heat available is lesser than the gross calorific value. Therefore, this is called low calorific value or net calorific value.
Thus LCV = HCV – Latent heat of water vapour formed.
As 1 part by weight of hydrogen gives 9 parts by weight of water,
H2+ ½ O2 → H2o
LCV = HCV – Weight of hydrogen in unit mass/volume of fuel × 9 × latent heat of steam.
Basically there are two ways to measure the Calorific Value.
- Bomb calorimeter
- Boy's Gas calorimeter
Bomb calorimeter
Principle: You burn a known amount of fuel in excess oxygen, and the heat released is absorbed by a known amount of water. You measure this heat by noting the change in water temperature. You then calculate the calorific value of the fuel using this principle.
Heat liberated by fuel = Heat absorbed by water and the calorimeter.
Construction A simple sketch of the bomb calorimeter is shown in the Figure 1.
Figure 1
It consists of the following parts:
- Stainless Steel Bomb: The stainless steel bomb is a cylindrical container with an airtight, screw-secured lid, two electrode holes, and an oxygen inlet. One electrode holds a ring supporting a crucible with the fuel and a magnesium wire. A platinum lining resists corrosion from acids formed during combustion. The bomb can withstand 25–50 atm pressure.
- Copper Calorimeter: You place the bomb in a copper calorimeter containing a known amount of water. The calorimeter includes an electrical stirrer and a Beckmann thermometer that measures temperature differences accurately up to 1/100th of a degree.
- Air Jacket and Water Jacket: An air jacket and a water jacket surround the copper calorimeter to prevent heat loss due to radiation.
Place 0.5–1 g of fuel in a crucible with a magnesium wire touching it. Add 10 mL distilled water in the bomb, seal it, fill with oxygen at 25 atm, and place in a water-filled copper calorimeter. Note the initial water temperature, ignite the fuel using a 6V battery, and record the maximum temperature reached. Measure the cooling time back to room temperature, then calculate the fuel’s gross calorific value using these readings.
Calculations
Let
Weight of fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of calorimeter, stirrer, thermometer, bomb etc = Wg
Initial temperature of water in the calorimeter = `t_1` ºC
Final temperature of water in the calorimeter = `t_2` ºC
Higher calorific value of fuel= H calorie / g
Heat liberated by burning of fuel = x × H
Heat gained by water = W × ∆T × specific heat of water = W (`t_2` - `t_1`) × 1 cal
Heat gained by calorimeter = w (`t_2` - `t_1`)
Total heat gained = W (`t_2` - `t_1`) + w (`t_2`- `t_1`)
= (W + w) (`t_2` - `t_1`)
But
Heat liberated by the fuel = Heat absorbed by water and calorimeter.
x × H = (W + w) (`t_2` - `t_1`)
H= `frac{(W+w)(t_2-t_1)}x` cal/g (or kcal/kg)
Net (lower) calorific value
LCV = HCV – 0.09 H × 587 cal/g or kcal/kg
(Latent heat of condensation of steam = 587 kcal/kg).
Corrections
The following corrections are applied to get more accurate results.
1.Fuse Wire Correction: The gross calorific value calculated above includes the heat released by the ignition of the Mg fuse wire, so you need to subtract this heat from the total value.
2.Acid Correction:During combustion, sulphur and nitrogen in the fuel oxidise to form `H_2SO_4` and `HNO_3`.
S + `O_2` `rightarrow` `SO_2`
2`SO_2` + `O_2` + 2`H_2O` `rightarrow` 2`H_2SO_4` ∆H = – 144000 cal
2`N_2` + 5`O_2` + 2`H_2O` `rightarrow` 4`HNO_3` ∆H = – 57160 cal
Hence, the formation of acids releases heat, so you should subtract this value from the obtained GCV.
3.Cooling Correction:Heating and cooling are simultaneous processes. As the temperature rises above the room temperature, the loss of heat occurs due to radiation and the highest temperature recorded will be slightly less than that obtained if there was no heat loss. A temperature correction (cooling correction) is therefore necessary to get the correct rise in temperature.
If the water in the calorimeter takes ‘x’ minutes to cool from its maximum temperature to room temperature at a cooling rate of dt per minute, you calculate the cooling correction as x × dt and add it to the observed rise in temperature.
HCV of fuel (H) = (W + w) × (t₂ − t₁ + cooling correction) − (Acid + fuse wire correction) × Mass of the fuel (x) = Calorific Value of Gaseous Fuels
Junker’s Gas Calorimeter:Junker’s Gas Calorimeter measures the calorific value of gaseous and volatile liquid fuels.
Principle:You burn a known gas volume at known pressure in a sealed chamber. Water flowing steadily through a jacket absorbs the heat. By measuring water temperatures, quantity, and condensed water weight, you determine the calorific value.
Construction
It consists of the following parts:
- Bunsen Burner: You use it to burn gaseous fuel. It clamps at the bottom, allowing you to pull it out or push it into the chamber during combustion.
- Gasometer: It measures the volume of the gas burning per unit time. A manometer with a thermometer attaches to it to record the gas pressure and temperature before burning.
- Pressure Governor: It regulates the supply of a gaseous fuel at constant pressure.
- Gas Calorimeter: This setup has a vertical cylindrical combustion chamber where you burn the gaseous fuel. An annular water space surrounds the chamber, allowing water to circulate and absorb heat. A chromium-plated outer jacket prevents heat loss by radiation and convection. The air within the outer jacket acts as an effective heat insulator. Openings at appropriate points allow you to place thermometers to measure the inlet and outlet water temperatures.
Working:You burn a known volume of gas at a constant rate in a combustion chamber with excess air. Water circulating in the annular space around the chamber absorbs all the heat produced.
Observations
- The volume of gaseous fuel burnt at a given temperature and pressure
in a certain time = V`m^3` - Weight of water circulated through the coils in time t = W g
- Temperature of inlet water = `t_1` ºC
- Temperature of outlet water = `t_2` ºC
- Weight of steam condensed in time t in a graduated cylinder = m kg.
Let GCV of the fuel = H
Heat produced by the combustion of fuel = V × H
Heat absorbed by circulating water = W (`t_2` - `t_1`)
Assuming no loss of heat,
V × H = W (`t_2` - `t_1`)
HCV or GCV
H = `frac{W(t_2-t_1)}V` kcal/`m^3`
Weight of steam condensed in a certain time t by the combustion of
V`m^3` of the fuel = m kg
V`m^3` of the fuel = m kg
Mass of `H_2O` condensed per `m^3` of the fuel = m/V kg
Latent heat of steam per `m^3` of the fuel
= `frac{mtimes587}V` Kcal,
= `frac{mtimes587}V` Kcal,
therefore, NCV or LCV = [H - `frac{mtimes587}V`] Kcal/`m^3`
Boy’s Gas Calorimeter
Like Junker’s calorimeter, engineers also use the Boy’s gas calorimeter to find the calorific value of gaseous and volatile liquid fuels. It consists of the following parts.
- Gas Burner: You use a gas burner to combust a known volume of gas at a known pressure. A gasometer measures the volume of gas burned, while a pressure governor monitors the gas pressure.
- Combustion Chamber: The combustion chamber, or chimney, contains copper tubes coiled inside and outside it. Water circulates through these coils. It enters from the top of the outer coil, passes through the outer coils, moves to the bottom of the chimney, then flows upward through the inner coil, and exits from the top.
- Thermometers: Two thermometers t1 and t2 measure the temperatures of the incoming and outgoing water.
You place a graduated beaker at the bottom to collect the condensed steam produced during combustion.
Working
Firstly, the working principle is like Junker’s calorimeter. Indeed, you circulate water and burn fuel to warm it for 15 minutes. Furthermore, once warm, adjust gas flow, consequently burning gas inside. Therefore, water absorbs heat, additionally circulating in tubes. Moreover, heat transfer occurs, thus allowing measurement. Meanwhile, note water temperature rise, since it shows heat absorbed, hence aiding calculations. Similarly, measure gas volume burned, besides water volume circulated, ultimately using these for calorific value. Nevertheless, observe steam condensed; however, ensure accuracy. Likewise, timing matters, accordingly record time t. Otherwise, errors occur. In fact, precise steps matter. Next, compare data, finally, refer to Junker’s calorimeter for calculations.
Applications and Important of Calorific Value
1. Fuel Quality Assessment
The calorific value further determination is especially important in the evaluation of the quality of different fuels. It gives the opportunity to compare energy content and efficiency enabling the choice of the best fuel for particular purposes.
2. Energy Production
Industries largely depend on correct calorific value assessments to precisely optimize combustion processes that are vital in energy generation. This guarantees maximum energy harvest and efficiency in power generation.
3. Environmental Impact
To evaluate the environmental impact of different fuels one has to familiarize with their calorific value. Higher calorific value fuels normally provide a higher energy output with lower emissions per unit of energy produced and this way promote cleaner and greener energy production.
4. Research and Development
Estimation of calorific value is one of the basic stages of a new fuel and energy products researches and developments. It offers an understanding of possible deployments, and how operate about emerging technologies.
Challenges and Considerations
Firstly, calorific value determination is valuable; indeed, it aids energy analysis. Furthermore, it supports fuel assessment, consequently providing data. Therefore, accurate measurements matter, additionally aiding design. Moreover, it boosts efficiency, thus helping industries. Meanwhile, consider impurities, since they affect accuracy, hence requiring checks. Similarly, combustion completeness matters, besides influencing results, ultimately impacting outcomes. Nevertheless, fuel variability exists; however, methods adjust. Likewise, calibration helps, accordingly ensuring precision. Otherwise, errors occur. In fact, consistent testing is vital, next, analysis proceeds, finally, measurements guide optimization.
Conclusion
The assessment of calorific value is a voyage into the place of energy potential, giving essential information about the productivity and the appropriateness of various fuels for a horde of undertakings. Firstly, scientists and engineers use bomb calorimetry and adiabatic flame calorimetry; indeed, these tools are vital. Furthermore, they enable precision, consequently improving analysis. Therefore, data guides research, additionally aiding design. Moreover, methods optimize systems, thus enhancing efficiency. Meanwhile, industries apply findings, since energy demands grow, hence requiring solutions. Similarly, technology advances, besides ensuring sustainability, ultimately producing the key for a future of efficient, green energy use.