HOC Experiment and Procedures

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Fuel Combustion Experiment

As the world economies struggle to cope with the rising cost of fossil fuels, it is imperative to

understand combustion processes with the goal of extracting as much energy as possible from

burning the fuel with oxygen. One abundant source of energy is coal; however, burning it hassome deleterious consequences, such as production of C02, a greenhouse gas. If the fuelcontains sulfur, nitrogen, or metal compounds, some oxides of these elements, such as sulfur

dioxide, are also produced as well. Therefore, a worldwide effort is underway to develop so-

called clean coal technologies.

The energy of a fuel can be quantified as the amount of heat released upon combustion with

oxygen in an instrument called a Bombcalorimeter. What happens inside such a calorimeter is

that a material of interest (i.e., a fuel) is combusted with oxygen and the heat released by thereaction is absorbed by a surrounding water bath which rises in temperature.The total amount ofenergy released during the combustion of a sample (kcal/g) can be determined simply by

multiplying the change in temperature of the calorimeter system (C) times the heat capacity ofthe calorimeter system (kcal/C) and dividing by the number of grams of sample used.

( T)(heat capacity of system)

Heat of combustion of a fuel (c

H ) = (Equation 1)

(wt of sample)

The heat capacity of the calorimeter (units of kcal/C) is the combination of the heat capacity ofthe bomb, the water surrounding the bomb, the stirrer, the thermometer and the water container.

However, the heat capacity is determined quite simply by running a standard (benzoic acid)

which has a known heat of combustion of 6.32 kcal/g and rearranging equation I to produce

equation II.

(6.32 kcal/g)(wt. of benzoic acid)

(Heat capacity of system)= (Equation 2)

( T)

Recall from your ChNE 251 that, with complete conversion, a combustion chemical

reaction (unbalanced) can be written as follows,

OHCOOHC yx 222 . (Equation 3)

This can be an exothermic reaction with a heat of combustion,c

H . Perhaps the easiest method

to experimentally measurec

H is with a Bomb Calorimeter which is shown schematically in

Figure 1. With this method, a sample to be combusted is placed in a crucible with a fuseimbedded within the powdered (or liquid) material. The crucible is surrounded by a constant


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temperature, small volume, water bath. The chamber is charged with pure, high pressure

oxygen. The sample is then ignited using a Pt fuse wire with a given heat capacity. Upon

ignition, we assume that the material burns to completion. The released heat from the combustedmaterial (including the fuse wire) is absorbed by both the stainless steel Bomb and the

surrounding water bath causing their temperatures to rise by a small amount, T. (Note: this is

an adiabatic calorimeter set-up since no heat is transferred to the surroundings, i.e., the labroom.) The first part of this experiment is to experimentally determine the heat capacity of theBomb with a known compound, benzoic acid. Then the student will measure the heat of

combustion of a coal sample. More information can also be found in the operators manual for

the calorimeter, which is available upon request.

Pre-Lab Tasks

1) Write the balanced combustion reaction for benzoic acid. Calculate how much heat would be

released if 1.0 grams were combusted. Then calculate the corresponding heat capacity of thesystem if the heat released by the combustion raised the temperature of the system, comprised of

2000 mL of water, by 1C.

2) Look up two types of coal that are commercially available. List their heats of combustion and

comment on their sulfur content. More info on this can be found in the manual for the

calorimeter.

Post-Lab Tasks

1) Use your experimental data to estimate the heat capacity of the system.

2) Use your experimental data to estimate the heat of combustion of coal. Compare with

literature values. What is/are the biggest source(s) of error in this experiment?


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Figure 1: Schematic of Bomb Calorimeter


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Procedures for Operating the Bomb Calorimeter

NO EXTERNAL MEMORY DEVICES ARE TO BE USED ON

LABORATORY COMPUTERS. ALL DATA FILES MUST BE E-MAILEDTO YOUR GROUP.

Safety Considerations

1. Wear safety glasses at all times.2. Wear latex or nitrile gloves when handling the chemicals used in this experiment.3. No shorts or open-toed shoes.4. Report any equipment problems or safety issues to the lab supervisor immediately

Additional Equipment Required1. Platinum Wire2. Oxygen3. Ruler4. Gloves and Safety Glasses5. Balance6. Coal7. Scissorsdont run with these. You might fall and poke your eye out.8. Benzoic Acid

Characteristics of Coal

As Received Dry Basis

% Moisture 2.64 0

% Ash 6.33 6.5% Volatile 32.16 33.03

% Fixed Carbon 58.87 60.47

Btu/lb 14080 14462

% Sulfur .69 .71

Operating the Calorimeter

1. Start the LabView program.a. You start the program by pressing thebutton that reads Start the Program.b. Choose an appropriate name for your data spreadsheet.c. Save your file to the desktop.d. Choose a sample name.e. Choose a data collection time interval.

2. Remove the RTD that is in the lid of the calorimeter .3. Lift the stirrer out of the way.4. Open the lid of the calorimeter.5. Run the hot water on the calorimeter until the supply line becomes hot.6. Measure 2000 mL of deionized water and add it to the bucket


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7. Attach a 10-20 cm length of platinum wire to the electrodes, as shown in the figureabove.

8. Place the combustion capsule into the round holder attached to the top of the bomb.9. Weight one tablet of benzoic acid and add it to the combustion capsule. Assure that the

wire touches the benzoic acid, but that it doesnt touch anything else.

10.Put 1 mL of distilled water into the bombnot in the combustion capsule.11.Screw the top of the bomb on tightly.

12.Assure that the purge valve is closed.13.Charge the bomb with 30 atmospheres of oxygen.14.Place the bomb into the bucket containing the water.15.Connect the electrical leads to the bomb.16.Close the lid of the calorimeter.17.Replace the RTD.18.Lower the stirrer and turn it on.19.Use the hot and cold water control switches to equalize the water temperature between

the bucket and the cooling/heating jacket.

20.Bring the two temperatures within 0.1 F of each other.21.Start recording databy pressing the Acquire Data button on the LabView program.Note: the Acquire Databutton will turn bright green, indicating that the program

is recording your data.22.Ignite the sample by pressing and holding the Ignite button. The red led will light up,

indicating that the sample has ignited. When the light goes out, release the button.23.The temperature of the water in the bucket will start to rise within 10 seconds. Start

running the hot water on the calorimeter immediately.

24.Use the hot and cold water control switches to equalize the water temperature betweenthe bucket and the cooling/heating jacket.

25.When the temperature of the water in the bucket stops increasing (about ten minutes),press the Acquire Data button on the LabView program.

26.Remove the RTD that is in the lid of the calorimeter.27.Lift the stirrer out of the way.28.Open the lid of the calorimeter.29.Vent the bomb under a hood by slowly opening the purge valve on the top of the bomb.30.Disassemble the bomb and clean the inside of the bomb and the combustion capsule.31.Measure and record the length of any remaining wire.32.Perform another experimental run on the benzoic acid by repeating steps 1 through 30.33.Perform three experimental runs on the coal, using one gram of coal per run, by repeating

steps 1 through 30.

34.Stop the LabView program by pressing the STOP button.35.E-mail your data file to yourself and your teammates.

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HOC Experiment and Procedures