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To more efficiently measure the heat released by combustion reactions, physicists use a bomb calorimeter, which is a sealed vessel that contains a smaller container called a 'bomb.' A chemical reaction heats a quantity of water in an insulated container. However, in this case, the reaction occurs inside a sealed compartment, or bomb. The bomb contains the chemical to be analyzed and enough oxygen to ensure complete combustion. The bomb is submerged in a container of water, and ignition wires initiate the combustion.
Since the reaction occurs in a rigid, sealed container, no pressure-volume work is done by the reaction; all the energy will be released as heat and none as work. In other words, a bomb calorimeter consistently measures the heat that is released by a reaction.
A bomb calorimeter is a type of constant volume calorimeter used to measure the heat of combustion of a specific reaction. Bomb calorimeters need to withstand the high pressure inside the calorimeter as the reaction is being measured.
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Electrical energy is used to ignite the fuel; as the fuel burns, it will heat up the surrounding air, which expands and exits through a tube that leads the air out of the calorimeter. When the air is escaping through the copper tube, it also heats up the water outside the tube. The change in temperature of the water allows for calculating the calorie content of the fuel.
Essentially, a bomb calorimeter consists of a small cup to contain the sample, oxygen, a stainless steel bomb, water, a stirrer, a thermometer, the dewar or insulating container (to prevent heat flow from the calorimeter to the surroundings), and an ignition circuit connected to the bomb.
By using stainless steel for the bomb, the reaction will occur with no volume change observed.
In recent calorimeter designs, the entire bomb, pressurized with excess pure oxygen (often at 30 atm) and containing a measured mass of a sample (usually 1–1.5 g) and a small fixed amount of water (to saturate the internal atmosphere, thus ensuring all water produced is liquid, and removing the need to account for enthalpy of vaporization in calculations), is submerged under a known volume of water (approximately 2000 ml) before the charge is electrically ignited. The bomb, with the known mass of the sample and oxygen, forms a closed system—no gases escape during the reaction. The measured reactant placed inside the steel container is then ignited. Energy is released by the combustion, and heat flows from this crosses the stainless steel wall, thereby raising the temperature of the steel bomb, its contents, and the surrounding water jacket. The temperature change in the water is then precisely measured with a thermometer. This reading, along with a bomb factor (which depends on the heat capacity of the metal bomb components), is used to calculate the energy given out by the sample burn. A small correction is made to account for the electrical energy input, the igniting wire, and acid production (by titration of the remaining liquid). After the temperature rise has been measured, the excess pressure in the bomb is released.
Q: What is Calorie?
A: A calorie is the measure of heat required to raise the temperature of 1 kg of water by 1 degree Celsius.
Q: What is Calorific value?
A: The calorific value or heat of combustion of a fuel oil is a measure of the amount of heat released during complete combustion of a unit mass of the fuel, expressed in kilojoules per kilogram. The SI unit of calorific value is Cal/g or KCal/kg. The calorific value determined by a bomb calorimeter is the gross or 'higher' value, which includes the latent heat of water vapor formed by the combustion of the hydrogen. The net or 'lower' calorific value is that obtained by subtracting this latent heat. The difference between the gross and net values is usually around 600–700 kcal/kg, depending on the hydrogen content.
Determination of HCV and LCV
HCV (Higher Calorific Value):
The total amount of heat liberated when a unit mass of fuel burns completely and the products formed are cooled down to their initial temperature is calculated value is found high.
HCV = 1/100 [8080C + 34500 (H – O/8) + 2240S]
LCV (Lower Calorific Value):
The total amount of heat liberated when a unit mass of fuel is burned completely and the products of combustion are allowed to escape.
LCV = HCV – 9/100H x 587 (latent heat of steam)
A bomb calorimeter consists of several key components:
Q: Why is high-pressure oxygen used in a bomb calorimeter?
A: Oxygen Bomb Calorimeters (also known as Constant Volume Calorimeters) are used for various applications in different industries to calculate the heat released from a combustion reaction (also called an ignition calorimeter), and the calorific value of the sample, be it any solid or liquid substance like coal and oil, is then measured. It is known as a constant volume calorimeter as it maintains a constant volume and the oxygen at a specific pressure (in DDS Calorimeters, we use approximately 30 bar). The sample used for calorific value determination is prepared and weighed, then placed into the vessel and sealed with the knife cover/screw top. The sample is then ignited with an electrical current in the presence of oxygen. The rise in temperature is then measured to determine the heat released during the combustion of the sample. DDS Calorimeters automatically calculate the calorific value of the sample, eliminating the need for manual calculations by the operator.
| Fuel | Calorific Value (kJ/kg) |
|---|---|
| Cow Dung | 4000-6000 |
| Coal | 25000-33000 |
| Wood | 17000-22000 |
| Sugarcane Fibres | 19259 |
| Sugarcane Sugar | 16747 |
| Fuel Oil | 9520 |
| Gasoline (Petrol) | 45.8MJ/kg |
| Diesel | 45.5MJ/kg |
| Kerosene | 48000 |
In short, the process of a calorimeter involves measuring the heat of a fuel sample when burned under stable temperature conditions to evaluate the heating energy of the fuel sample. The fuel sample can be a solid or liquid but not a gas.
Our calorimeters require approximately 0.5g of sample matter (e.g., food) weighed in a crucible. The weight needs to be entered with four decimal places (e.g., 0.4972g). Place the crucible inside the stainless steel container ('the bomb vessel') and fill the bomb vessel with 30 bar (435psi or 30 atm) of pure oxygen.
Place the filled bomb vessel inside the calorimeter and seal the lid. The bomb vessel is now sealed and isolated from external temperature influences. Once the bomb vessel temperature has stabilized in the bomb well, the sample is then ignited. An electrical ignition charge instantly heats the ignition wire, which in turn ignites the attached igniting cotton. The burning cotton thread falls into the fuel sample below, causing the sample to ignite.
During the combustion of the fuel sample, the crucible can quickly exceed 1000°C with the pressure spiking to three times the initial pressure. Immediately, the heat of the reaction starts to dissipate into the bomb vessel, and the pressure begins to diminish.
To accurately measure the temperature of the vessel, sensitive high-resolution temperature sensors are used, measuring every few seconds for the duration of the determination.
When the determination is complete, typically within 4 minutes (depending on the model), the calorimeter calculates the Calorific Value (CV) of the fuel sample. At this point, the bomb vessel is removed from the bomb well to cool down. Typically, the bomb vessel is now between 8 to 14°C higher in temperature.
Once the bomb vessel has sufficiently cooled in a cooler, it can be reused again for subsequent experiments.
To demonstrate the working of a bomb calorimeter experimentally, the following steps are taken:
Here, the cooling correction accounts for any heat loss to the surroundings during the experiment.
Question 1:
Calculate the gross and net calorific value of fuel containing 84% carbon, sulfur 1.5%, nitrogen 6%, hydrogen 5.5%, oxygen 8.4%. The latent heat of steam is 587.
HCV = 1/100[8080C + 34500(H – O/8) + 2240S]
HCV = 1/100[8080*84 + 34500(5.5 – 8.4/8) + 2240*1.5]
HCV = 1/100[678720 + 153525 + 3360]
HCV = 8356.05 Kcal/kg
LCV = 8356.05 – 9H/100 * 587
LCV = 8356.05 – 5.5*9/100 * 587
LCV = 8356.05 - 290
LCV = 8065.485 Kcal/kg
Question 2:
On burning 0.83g of a solid fuel in a bomb calorimeter, the temperature of 3500g of H2O increased from 25.5°C to 29.2°C. The water equivalent weight of the calorimeter is 385g, and the latent heat of steam is 587. Calculate H.C.V and L.C.V (0.75% - Hydrogen).
HCV = (3500 + 385) * 3.7/0.83
HCV = 17318.67 Kcal/kg
LCV = 17318.67 – 9 * 0.7/100 * 587
LCV = 17281.689 Kcal/kg
Question 3:
Calculate the LCV of a fuel having 8% of hydrogen and HCV is 6500 Kcal/kg.
LCV = 6500 – 8 * 9/100 * 580
LCV = 6082.4 Kcal/kg
Question 4:
A sample of coal contains carbon 98%, hydrogen 8.9%, ash – 3%. The following data was obtained when coal was tested for calorific value in a bomb calorimeter.
HCV = (2500 + 250) * 2.5 – (50 + 10 )
HCV = 6875 – 60
HCV = 6815 Kcal/kg
LCV = 6815 – 9 * 8.9 / 100 * 587
LCV = 6344.813 Kcal/kg
In conclusion, the bomb calorimeter is an essential tool in the field of thermodynamics and energy analysis. It allows for the precise measurement of the heat released during combustion reactions, providing valuable information about the calorific value of various fuels and substances. The operation of a bomb calorimeter involves a careful and controlled process, where a sample is ignited in a sealed container in the presence of pure oxygen. The resulting heat generated from the combustion is measured, and this data is used to calculate the calorific value of the sample.
The bomb calorimeter finds extensive applications in various industries, including coal analysis, explosive testing, thermodynamics studies, fuel testing, waste disposal assessment, and metabolic studies. Its ability to accurately determine the heat content of different materials is crucial for quality control, safety evaluations, and research in these fields.
The numerical examples provided in this report illustrate the practical use of calorimetry equations to calculate both the higher calorific value (HCV) and lower calorific value (LCV) of various fuel samples. These calculations are essential for assessing the energy content and efficiency of different fuels and materials, aiding in decision-making processes across industries.
In summary, the bomb calorimeter is a versatile and reliable instrument that plays a significant role in scientific and industrial applications. Its ability to measure heat with precision ensures accurate data for a wide range of purposes, contributing to advancements in energy analysis and research.
Bomb Calorimeter: Measuring Heat in Combustion Reactions. (2024, Jan 10). Retrieved from https://studymoose.com/document/bomb-calorimeter-measuring-heat-in-combustion-reactions
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