For decades chemical engineers have known that the particles in a fluidized bed "scour" through the thermal boundary layer resulting in heat transfer coefficients an order of magnitude higher that with the gas alone.
Little to nothing has been done with this phenomenon in mechanical engineering. "Fluidization" or "fluidized beds" is never mentioned in any textbooks or handbooks on heat transfer.
Like I said, absolutely nothing on _fluidization_ where, at a threshold flow rate, the bed expands from 50% to over 500% and the particles are in constant motion and the two phase flow looks like a liquid.
In that case the fluidised bed is usually limestone, which removes the sulphur oxides released by combustion of the coal in the fluidised bed. The heat transfer uses the exhaust gases coming off the bed, the input being pulverised coal and air. There could also be some steam in the air to regulate the temperature and avoid slag formation. The exhaust gases will be a mixture of Nitrogen, Carbon dioxide, Carbon monoxide, Hydrogen and various hydrocarbons, which will burn in secondary air. That's when the heat transfer to the boiler or heat exchanger takes place.
some fluidized bed combustors do indeed work as you describe, extracting heat from the relatively clean gas after it leaves the bed, but it also describes 1st generation pressurized fluidized bed technology which has heat exchangers (boiler tube bundle) immersed in the bed.
All the HX done with fluidization so far has always been directly in combination with combustion or other chemical reactions.
No one has exploited the high heat transfer rates of _inert_ fluidizing particles to build and optimize a cheap, compact, high effectiveness, low pumping loss gas-gas heat exchanger that could be used, for example, to "breath" highly insulated homes and buildings in cold climates. The air in a 3,000 ft^2 house could be exchanged several times a day with very little additional heat or mechanical power consumption. This would be an easy home brew project -- less than $150 - $200 in parts and a few nickles a day to operate.
Low pressure ratio regenerated engines have very high efficiencies but the regenerator is always too big and expensive, leaks because of mechanical/dynamic seals and has high pumping losses.
A 20 kW 2.2 : 1 p. r. regenerated gas turbine would boost mpg in a series hybrid to 70 with almost as little smog as external combustion.
Locomotives need to get away from the smoggy 6,000 degree 4,000 psi diesel and go to low pressure ratio regenerated GT.