Gas Grill


Step 1: Fuel Source

Gas grills typically use propane gas, an energy-dense fossil fuel. Propane itself is not actively searched for and produced, but rather it is a by-product of distilling natural gas and refining petroleum. Natural gas and petroleum are formed by the organic material of dead plants and animals. Millions of years ago this matter accumulated at the bottom of oceans and other bodies of water, and were subsequently covered by layer upon layer of sediment due to earths natural processes. Subjected to high pressures and temperatures deep beneath the earths surface, the organic material is broken down and forms either natural gas or petroleum based on the amount of hydrogen and carbon present.

                                                                   Part 1                  ----->                Part 2                              ----->  Part 3     

Surprisingly, the propane tank connected to your gas grill contains very little gas! Instead, it contains liquid propane, abbreviated LP. Propane is an easily compressible gas due to its chemical structure, and we have taken advantage of this property.

Propane has a boiling point of around -42 C. This means that unless you live in Fairbanks, Alaska or some other Nordic region with god-awful winter temperatures, your propane will almost always be in gas form. But due to the expansive nature of gases, storing propane in gas form at normal temperatures would be very inefficient for daily grilling use. However, with some rearranging of the ideal gas law:

                                                                   PV=nRt ----> P=nRt/V

We can observe that gas pressure and volume have an inverse relationship. By increasing the pressure, we can drastically reduce the volume a specified amount of gas takes up. Once we reach a pressure high enough, the gas condenses to liquid form, and voila- we now have easily transportable, liquefied propane. Figure 1.1 is a phase diagram that describes the state of propane at different temperatures and pressures. The rectangular area on the left represents the solid phase, the large grey area represent the liquid phase, and the white area represents the gaseous phase. A pressure of about 8.5 (850kPa) bars is required to condense propane at room temperature (~293 K). We are able to apply this pressure to propane by using strong metal tanks to store it. As the gas tries to expand in a highly pressurized environment (in our case, the metal tank which doesn't allow it to), the environment pushes back on the gas with an equal but opposite force. The molecules then are close enough to one another to exert dipole attractions and form the liquid state of matter. However, it is important to not that the tank is not completely full of liquid propane. 

                  figure 1.1

Step 2: Transportation of Fuel to Stable Combustion Environment

When you turn the propane tank "on" what you are actually doing is opening a small whole which allows the propane in the tank to escape through the gas line of the grill to the burners. Even though the vast majority of moles of propane in the tank are in liquid form, there is still some vapor in the tank. The propane vapor is located at the top of the tank due to having a lower density than its liquid form. The vapor is what pushes its way into the opening and travels through the gas line. As the propane vapor leaves the tank, one might think that the pressure will decrease. However, this is not the case. Once a certain amount of moles of the gas leaves, the same moles of propane boil into the vapor phase and replaces the pressure that should be "lost". Thus, we can say that the tanks pressure is in equilibrium. This remains the case until all the liquid is used up, at which point the pressure will start to decrease as gas escapes.

Step 3: Conversion of Propane to Heat
Now that we understand the propane used in our grill, we can feel comfortable using it to grill our steak. However, we still lack some understanding of the process, namely how is the propane used to produce heat?

Once the propane reaches the burner, a predictable process takes place: the burning of propane gas. The propane flows evenly through the burner holes and a small spark is added through the "starter" switch of the grill which in turn ignites all the propane and produces the flames you see. This is an example of a combustion reaction. A combustion reaction is a chemical reaction in which a substance and oxygen are combined to produce heat, light, and the by products of the reaction. The heat and the light components are evident, however the reaction is also producing carbon dioxide and water. The balanced combustion reaction of propane is shown below.

   C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat

We now have turned our propane into heat, and we are only a preheat and 4 minutes on each side away from eating!

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