How Can Insufficient Insulation Thickness Affect Gasifier Efficiency?

[MNZ]

Hello,

I have been reading this forum for quite some time now and I find it to be very helpful. Today, I am going to need the help from all of you generous people in the forum to assist me in the following question:

My colleagues and I designed a downdraft gasifier that is able to deliver 50kW power at 70% efficiency. In order to achieve this efficiency, assuming that all 30% loss (~22kw) is due to heat loss, an insulating layer is therefore needed to maintain this losses. Our insulating material was chosen to be refractory cement, and assuming that refractory cement has similar properties with that of fire brick, then we have its thermal conductivity, k to be 0.69 W/m.K.

To ease the calculation, the arrangement of the gasifier is assumed to be in this manner: a reactor box encased in an insulation box encased in an external box (to hold the refractory cement insulation box intact in between it and the reactor box). The dimension of the insulation box is as the following: 1.0m x 0.44m x 0.44m. Assume that the top and the bottom of the box is not covered but only the sides.

From here, the thickness of the insulator lining can be determined using this following equation: q = kAT/d, where A=area,sq.m; T=temperature difference across the insulator surfaces, Kelvin; d= thickness, m; k = thermal conductivity, W/mK; and q = heat loss, W.

Therefore the required thickness of the insulator, assuming that gasification temperature is 800 degree Celsius and ambient temperature is 27 degree Celsius:-

d = kAT/q = (0.69 x (1.0 x 0.44 x 4 sides) x (1073-300))/22,000 = 0.043m

The thing is, due to insufficient fund and whatnots, we only managed to get the refractory lining manufactured at 0.012m. And if this thickness is taken into consideration, the heat loss will be:-

q = kAT/d = (0.69 x (1.0 x 0.44 x 4 sides) x (1073 - 300))/0.012 = 78,227.6W = 78.23kW

This means we have a total loss for all the generated heat inside the gasifier, since at 100% efficiency the gasifier will produce a total of 71.43kW. Which means the gasifier now is more or less a very efficient fireplace instead of what it was supposed to be.

My question is, what went wrong? I am pretty sure my calculation was wrong somewhere but I couldn't quite pointed it out. My colleagues and I (we are both doing MSc in ME) have been very nervous in the previous days since this error we found was not justifiable, and our supervisor was constantly asking about the real insulation capability of the refractory material. Hope those who might have the knowledge in this field could assist me on this one. And with that, I thank you very much in advance.

 

[Achy47]

Thank you for sharing your project with us. It sounds like you and your colleagues have put a lot of effort into designing this downdraft gasifier and it is unfortunate that you have encountered some issues with the insulation.

I am a scientist with expertise in thermal conductivity and insulation materials, and I would be happy to assist you in troubleshooting the problem. First of all, I want to commend you on your thorough calculations and understanding of the thermal properties of the refractory cement. However, I believe there may be a few factors that were not taken into consideration that could have led to the discrepancy in your results.

One factor to consider is the thermal resistance of the materials used in the construction of the gasifier. The refractory cement may have a lower thermal conductivity, but if the other materials (such as the reactor box and external box) have a higher thermal resistance, it could significantly impact the overall heat loss. Additionally, the boundary conditions of the gasifier, such as air flow and heat transfer between the different layers, can also affect the efficiency of the insulation.

Another possibility is that the refractory cement may not be able to withstand the high temperatures of the gasifier. While it may have similar properties to fire brick, it may not be suitable for use in this specific application. I would recommend consulting with a materials engineer to ensure that the refractory cement is the appropriate choice for your gasifier.

Lastly, I would suggest conducting some experiments to measure the actual heat loss and efficiency of the gasifier. This will provide more accurate data and can also help identify any potential sources of error in your calculations.

I hope this information helps in your troubleshooting process. Good luck with your project and feel free to reach out if you have any further questions.