Scavenging heat from exhaust flues

Question

I am building a set of greenhouses. they will be 2700 sq.ft. each. I intend to grow year round in one and extend the season in the other two with them being dormant for 6 weeks mid winter. We will be using underbed pex tubing under the grow beds and perhaps may run two separate layers, one deep and the other below it to supercharge the soil with increased temperature, beginning in late summer.the goal is to maintain growing bed temperature at 5 - 15 degrees above the air temperature in the greenhouse to facilitate continued growth and production.

I see three individual systems working for each greenhouse with an optional "super tank" filled and heated if feasible and needed to satisfy demand. I realize that heat stored in tanks leads to stratification so I am hoping to position a set of heat exchangers in the tanks to facilitate temperature rise from various sources.

there will be a long solar hot water collector, about 600 sqft oriented S. there will be a number of biochar retorts in a row using common flues for exhaust with a heat exchanger in the flue. there will be wood stoves with water jackets and air heat exchangers for quick warm up and use of available resources. and there will be a ceiling header running the length of each green house to collect and preheat water in this heat cycle.

With multiple retort biomass barrel type processors, and the wide range of temperatures that a heat exchanger will experience inside of the exhaust flue from pyrolysis to wood degassing and combusting those products, what would be a simple way to capture these product of combustion temperatures and put them to good use?

I intend to use 3/4" pex in the under bed systems of less than 400 feet per run. I intend to run the return from each return header in each house to its own tank for pumping. I am at 37 Degrees North Latitude in Missouri. thank you for your thoughtful consideration.

Answer 1

You may be barking up the wrong tree. You are making life way too complicated.

Google rocket stove greenhouse.

A rocket stove is a high efficiency stove that burns very hot for a short period of time, and heats up a large mass (tons) of clay by a small amount.

Secondly: A better way is described here:

http://www.sunnyjohn.com/indexpages/shcs.htm

Briefly, you bury 3 layers of drain tile below your greenhouse space. During the day warm humid air is pulled out of hte greenhosue and blown through the drain tile. half the water condenses releasing it's latent heat. The net effect is to warm the top 3 feet of soil by about 15-20 degrees. Works in zone 4, from what I've heard.

Answer 2

Pyrolysis gas is mostly carbon monoxide and very, very deadly. Of course you have thought about gas warning and ventilation around your kilns and will review the safety with someone qualified, because you are not a total idiot. It is very, very unlikely that you are qualified to do this alone and even then you should get someone to check your work.

You have a complex system with several heat sources, and you should spend some time in properly designing this system (or pay someone to do it). I'll concentrate on this aspect of your question:

With multiple retort biomass barrel type processors, and the wide range of temperatures that a heat exchanger will experience inside of the exhaust flue from pyrolysis to wood degassing and combusting those products, what would be a simple way to capture these product of combustion temperatures and put them to good use?

First, ist this a good idea? Pyrolysis creates tar in the exhaust and you will spend a lot of time cleaning your HX. Pyrolysis gas is energy rich but not trivial to burn (the kinetics of burning tar beeing what they are), using this energy is out of the scope of my answer.

Second, the heat exchanger: In my experience, for exhaust applications a shell and tube type HX is a good choice. You guide the exhaust through the tubes and the water through the shell. Make sure you get one with flanges at either end of the whole assemply so you can access the tubes for cleaning. HX are a whole bunch of steel and not cheap yo you want to get one and use it for all your kilns. You should adopt a 'staggered' operation of your kilns so the flue gas flow and temp. is high over a prolonged time. The exhaust HX will change the draft of your kiln and it's likely that you need some forced ventialtion to overcome.

Third, the buffer and the heat management: You are looking for a stratification buffer tank, a system with a water tank where a smart automation feeds water at different levels depending on temperature. You should still maximise the temp. you get from you HX but as you correctly wrote, the flue gas temp. will be ununiform.

Fourth, Sizing components: You need to make a decent model of your kilns with HX so for different HX sizes you can estimate what heat at what temp you will get. Then look at buffer tanks, what size you need to store your pyrolysis waste heat.

At the end of the day your design will be a tradeoff between energy recovery, cost and a host of other considerations. It is entirely possible that heat recovery from pyrolysis kilns is not worth it in your case.