Is Gasification More Thermodynamically Efficient Than Direct Combustion?

In summary, the statement is true because gasification does remove some of the ashes and produces a higher flame temperature, but there is no increase in available work from the initial fuel.
  • #1
davidgrant23
22
0
Hi there,

I have been thinking about attempting to quantify the statement that:

"Gasification is more efficient thermodynamically than direct combustion of the fuel source as the gas can be combusted at higher temperatures, resulting in higher thermodynamic efficiencies as defined by Carnot's rule"

Now, this statement is true because you can indeed combust the gas at higher temperatures than the fuel directly, meaning that the hot reservoir temperature is greater (η=1-(Tc/Th)).

However, how would you prove this statement in a more fundamental way? How do you calculate or prove that the gas is able to be combusted at higher temperatures than the fuel itself?

In addition, how would you prove mathematically that one fuel (say a syngas rich in H2 compared to a poorer heating quality syngas) can be combusted at higher temperatures than another, resulting in an improved thermodynamic upper limit efficiency defined by Carnot? Is it simply a case of calculating the heat of combustion?

Thanks,
David
 
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  • #2
Gasification removes the ashes out of the fuel.
Therefore the produced gas ought to have a higher flame temperature.

This is likely what happens, but it does not need to always be the case.
It could also happen that a solid combustible part of the fuel remains ungasified and that the gasified part has a low heat value. For example, if CO is the gas produced it might well have a lower heat value than the un-converted carbon.

But generally, it can happen that gasification increases heat value.
This is then just a purification of the fuel.
 
  • #3
Ask yourself, how does a solid, such as wood burn?
is it the solid burning or is it really the wood being heated up and being "gasified", and the gases burning. To gasify the wood, heat has to be added to the solid fuel to raise its temperature. One can do that previous to the combustion process or during, in which case the flame temperature will be higher.

"Gasification is more efficient thermodynamically than direct combustion of the fuel source as the gas can be combusted at higher temperatures, resulting in higher thermodynamic efficiencies as defined by Carnot's rule"

So if your statement splits the process into 2 phases - the gasification of the wood and the combustion of the gas, then certainly eliminating the first phase and disregarding the heat value necessary to gasify the solid wood, then sure, it does look more efficient for the end user. Overall, there is no free lunch so to speak though.
 
  • #4
Note that gasification requires energy and also leaves a degraded waste.
Therefore, overall, there cannot be an increase of efficiency.
It is just that the gasified fuel offers a better efficiency.
The available work from the initial fuel is not increased.
 
  • #5


Hello David,

Thank you for bringing up this interesting and important topic. I would like to provide some insights into the question of whether gasification is more thermodynamically efficient than direct combustion.

First, let's define gasification and direct combustion. Gasification is a process in which a solid or liquid fuel is converted into a gas by reacting it with a controlled amount of oxygen. On the other hand, direct combustion is the process of burning a fuel directly with oxygen to produce heat.

Now, to answer your question, we need to consider the thermodynamic efficiency of a process, which is defined as the ratio of output energy to input energy. In the case of gasification and direct combustion, the output energy is the heat generated, and the input energy is the chemical energy of the fuel.

The key difference between gasification and direct combustion lies in the temperature at which the reactions take place. In gasification, the fuel is converted into a gas at relatively low temperatures, while in direct combustion, the fuel is burned at high temperatures. This means that the gas produced in gasification can be combusted at higher temperatures than the fuel itself.

Why is this important in terms of thermodynamic efficiency? This is because the efficiency of a heat engine, such as a gas turbine, is directly proportional to the temperature at which the heat is added. In other words, the higher the temperature, the higher the efficiency. Therefore, gasification, which allows for the production of a gas that can be combusted at higher temperatures, is more thermodynamically efficient than direct combustion.

To prove this statement in a more fundamental way, we can look at the heat of combustion of the fuel. The heat of combustion is the amount of heat released when a fuel is completely burned. In gasification, the fuel is converted into a gas with a higher heat of combustion compared to the fuel itself. This means that the gas produced in gasification has a higher potential to release more heat and therefore achieve a higher thermodynamic efficiency.

In conclusion, gasification is indeed more thermodynamically efficient than direct combustion due to the ability to combust the gas at higher temperatures. This is because the higher temperature allows for a higher efficiency of the heat engine. Additionally, the higher heat of combustion of the gas produced in gasification also contributes to its higher efficiency. I hope this explanation helps to clarify the concept.

Best,
 

1. What is thermodynamic efficiency?

Thermodynamic efficiency is a measure of how effectively a thermodynamic system converts energy into work. It is the ratio of the work output to the energy input, expressed as a percentage.

2. How is thermodynamic efficiency calculated?

Thermodynamic efficiency is calculated by dividing the work output by the energy input and multiplying by 100 to get a percentage. The higher the percentage, the more efficient the system is at converting energy into work.

3. What factors affect thermodynamic efficiency?

Several factors can affect thermodynamic efficiency, including the design and materials of the system, the temperature and pressure of the working fluid, and the losses due to friction and heat transfer.

4. How does thermodynamic efficiency relate to the laws of thermodynamics?

Thermodynamic efficiency is directly related to the second law of thermodynamics, which states that the total entropy of a closed system will either remain constant or increase over time. This means that no system can have 100% efficiency, as some energy will always be lost as heat due to the increase in entropy.

5. Why is thermodynamic efficiency important?

Thermodynamic efficiency is important because it allows us to evaluate the performance of a system and make improvements to increase its efficiency. It also helps us understand the limitations of a system and the impact of energy losses, which is crucial in designing more efficient and sustainable technologies.

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