Heated and cooled air effects on combustion

Question

In a turbocharged engine, an intercooler is installed to cool down the compressed air (apparently for better combustion). In contrast, in the power plant they use air preheater to heat up the air before it mixes with the fuel to fire up. What is the difference?

Answer 1

This has to do with energy efficiency. It does not have to much to do with the "density of oxygen" (the air density does not change significantly from $20^\circ\mathrm C$ to $0^\circ\mathrm C$. The reason is because

$PV =nRT$ and $DV= {m} $ and $n\mathcal{M}=m$ rearrange to form:

$$P= {DRT \over \mathcal{M}}$$ OR $${P\mathcal{M} \over RT}=D $$ And when T is in Kelvins the change of $-20$ from $393 \rightarrow 373 $ is pretty small. So more Oxygen is not the reason (you would get more of other air molecules as well)

What is actually happening here is that the engine is being losing less energy. The cold air outside acts a heat sink. Recall that the canrot efficiency of an engine is : $$e=1 - {T_{\textrm{cold}} \over T_{\textrm{hot}}} $$ while the efficiency for any normal engine is defined, for any heat cycle as $$e = {Q_{\textrm{hot}} - Q_{\textrm{cold}} \over Q_{\textrm{hot}}}$$

but being colder outside, it enables the engine to release excess heat in to the environment.

The $Q_{\textrm{hot}}$ comes from the explosion of gas. $Q_{\textrm{cold}}$ is from the outside in the first example. As to your second question it probably helps with the combustion of the plant material for the air to be hot. The only thing that does matter is the $Q_{\textrm{hot}}-Q_{\textrm{cold}}$ or how much heat you get rid of. So if in each cycle they get rid of a lot of heat, then they are being efficient. If they do not, then they will be less efficient. There is no contradiction. It is incredibly hard to get high efficiencies.

Answer 2

For the turbocharged engine, the first stage of the turbocharger causes a significant increase in the inlet air temperature as the air is compressed. The temperature increase leads to a larger volume of air than what would be expected if the air remained at ambient temperature, based on the ideal gas law. To avoid designing a large second stage on the turbocharger, an intercooler is used to cool the first stage air, which decreases its volume at the given pressure, which leads to a smaller second stage on the turbocharger. This allows for a better matching of the sizes of the first and second stage of the turbocharger, and it is more cost effective to design the turbocharger/intercooler combination this way. Thus, the intercooler is used for cost considerations and ease of manufacture considerations, not combustion efficiency considerations.

For the inlet air on a furnace, ambient air is composed of 78% nitrogen, which doesn't burn. All of this nitrogen gets exposed to flame temperatures, and it exits the stack of the furnace at a fairly high temperature (300-400 deg F). Heating all of this nitrogen from ambient temperatures to stack temperatures represents a big loss of combustion efficiency, so it is desirable to recover as much heat from the stack gases as possible. The most practical way to do this is to (where practical) generate steam with the higher temperature stack gases, and cool those gases down to 500-600 deg F. In addition, the "somewhat cooled" stack gases can be used to preheat the air going into the furnace, thereby recovering some of the heat that would have been "dumped" to the atmosphere. Thus, for a furnace application, the idea is to improve combustion efficiency by using air preheat.

Note that it is difficult to cool the stack gases below approximately 300 deg F, because there is usually a small amount of sulfur in furnace fuel (e.g., natural gas), and acidic materials condense on furnace surfaces at low temperatures, leading to severe corrosion.