Biomass Fueled Brayton Cycle

Large biomass fueled Brayton Cycle engine. http://www.proepowersystems.com/Engine.htm

Looks interesting. My criticism is in determining if heated air (which is compressible) can expand to exhort enough force to drive a piston and supply enough oxygen to feed the cycle at the same time. Also fouling will eventually limit the heat transfer creating large variances in operating conditions. In other words the fouling may choke the expansion of gas and stop the system by gradual cooling.

Neat idea though!

Just gave the idea another thought and believe that a system driven by steam will be much more efficient even given a compressed gas.

Amedean Messan wrote:Just gave the idea another thought and believe that a system driven by steam will be much more efficient even given a compressed gas.

What leads you to this conclusion?

http://www.proepowersystems.com/Engine.htm wrote:Air Heater:

The Air Heater is a bank of simple tubular, counterflow heat exchangers in parallel. In each tube assembly, compressed air from the compressor travels through an annulus bounded by a tube on the outside and a tube containing hot flue gas from the combustor on the inside. As the compressed air from the compressor passes through, it is heated to about 1450 degrees F while the flue gas is cooled from about 1500 F to 500 F.

The high temperature requirements make an air driven design more difficult to sustain. Also given that water expands to a density of 1700 times with much less heat.

Because your versed well in mechanics, I guess if I were to use an analogy it would be like trying to run a refrigerator on air instead of freon - requires more work with less output basically. I could have made a wrong assumption though...

Amedean Messan wrote:Looks interesting. My criticism is in determining if heated air (which is compressible) can expand to exhort enough force to drive a piston and supply enough oxygen to feed the cycle at the same time. Also fouling will eventually limit the heat transfer creating large variances in operating conditions. In other words the fouling may choke the expansion of gas and stop the system by gradual cooling.

I thought the same. I then considered that 1 pound of dry wood requires about 6.5 pounds of dry air for full combustion. The air heater in this system takes the air temperature from 330F to 1450F according to the web site, and this represents about 1750 btu of heat absorbed by 6.5 pounds of air. Well, the combustion of one pound of dry wood provides more than 8000 btu of heat. So, it seems the system provides a lot more air than is required to support the combustion necessary to heat the air. Wood furnaces normally provide about 50% excess air (it’s not normally even close to stoichiometric), but the specific amount of air delivered to the furnace i this case is a lot higher. However, since the air exhausted from the expander is at 900F when supplied to the furnace, then the furnace can support a lot more excess air without driving temperatures too low. Also, the flue gases are noted to be at 500F. This represents a lot of heat not transferred to the air. While this seems like a large loss, and I suppose it is, it seems it would be difficult to do better when considering the higher mass flow rate of flue gases compared to the air. As a practical matter, the hot flue gases can be used to dry the fuel. Note that the excess air should also support full combustion and a very clean exhaust gas which should minimize fouling of the heat exchanger.

My thoughts are that the efficiency of the system is not optimal from a thermodynamic standpoint because of adiabatic expansion (no heat added during expansion). This could be overcome by achieving expansion in stages and adding reheaters. However, the system would be too complicated. A lot of heat is regenerated when the hot expander exhaust air is sent to the furnace, so this makes up for a lot. Also, the system would do better to approach isothermal compression, and the patent discusses this with a water cooled compressor. It also mentions taking the air temperature to 1500F in the heater. These changes should catch more heat from the furnace by providing cooler air to the air heater. Of course, the heat of compression is lost, but this also represents less work required in compression. Under these conditions the patent claims 36-40% thermal efficiency depending on air pressure of 3-10 bar... the higher pressure seeing the greater efficiency.

Amedean Messan wrote:

http://www.proepowersystems.com/Engine.htm wrote:Air Heater:

The Air Heater is a bank of simple tubular, counterflow heat exchangers in parallel. In each tube assembly, compressed air from the compressor travels through an annulus bounded by a tube on the outside and a tube containing hot flue gas from the combustor on the inside. As the compressed air from the compressor passes through, it is heated to about 1450 degrees F while the flue gas is cooled from about 1500 F to 500 F.

The high temperature requirements make an air driven design more difficult to sustain. Also given that water expands to a density of 1700 times with much less heat.

Because your versed well in mechanics, I guess if I were to use an analogy it would be like trying to run a refrigerator on air instead of freon - requires more work with less output basically. I could have made a wrong assumption though...

Steam at a similar pressure (or a bit higher) and temperature, and with heat regeneration in the form of combustion air preheating, might see better efficiency through lower mechanical losses (higher expander mean effective pressure, and a lot less work in recompression). However, it would be very difficult to get full expansion of the steam at a significantly higher pressure without compounding. Also, 1500 steam sure is scary. I think a piston steam engine could do better than this Brayton engine, but it would require a radically different form. The simplicity of this engine is compelling.

ADDENDUM: I considered a system fundamentally identical to this a couple years back. It called for using existing components like small compressors and small engines converted to air expanders. I recognized the limitations in getting good efficiency (beyond temperature limits) as the difficulties in getting sufficiently high mean effective pressure in the expander while simultaneously achieving high expansion. A closed system might do it (to get higher pressures), but then that introduces other problems. It seems this Brayton cycle engine requires a purpose built expander. I think a smaller system operating on this principle could do well with a compounded expander not unlike a compound steam engine, but using hot compressed air, and I know a particular design that should be ideal. The ideal kind of configuration (to optimize efficiency and NOT simplicity) it seems would be a multi stage compressor with water cooled cylinders and intercoolers to approach isothermal compression, get the highest practical pressure in the compressor, then the highest practical temperature in the air heater, then a multi stage expander with reheat stages to approach isothermal expansion. Send the high temperature exhaust air to the furnace as before.

Its a compelling external combustion engine idea. Trying to think out how it'd stack up against a similar steam engine system.

Pros:
-Safer than steam, no boiler explosions to worry about.
-Less mechanically complex (no feedpump, draft fans, condenser and simpler control strategy)

Cons:
-Likely to be less efficient without regeneration and multiple stages of expansion, both of which detracting from mechanical simplicity.
-Less energy density per unit volume due to lower mean cylinder pressure than steam. (probably more critical for mobile power applications).

Unknowns:
-Engine cost per unit power (does higher volume increase material and machining costs?)
-Engine life of brayton versus steam


At first glance it looks more attractive than steam as I think reasonable efficiencies (particularly with smart combined power and heat strategies) can be achieved, but it does beg the question of why did it not win out during the first round of the industrial revolution as an engine?? Was it simply that the Brayton came too late in the game to establish itself or was it a technical barrier?? Very interesting concept, would love to see one in action.

Hello guys, I'm new here but I see you are discussing Brayton and I know some things about the engine posted above... It was proposed by Richard Proschell of Pro power systems...  He was never able to make a working version of the engine. There are some early engines that have a piston and expander with heat added in between but they are usually large engines and they make little power...  true Brayton engines on the other hand are actually combusting in the expansion cylinder. There is a big difference between a Brayton engine and engines of this type. I have made several Brayton type engines over the last 15 years .. here is one of my latest engines. https://www.youtube.com/watch?v=wlTfwrLiJ0E here is an engine which adds heat between a compressor and expander... https://www.youtube.com/watch?v=lbYvlBrRjmQ