Waste-Heat Recovery for a Small Intermittent Brick Kiln: Air Preheat and Brick Drying, with Costs, Failure Modes and the Cases Where It Is Not Worth Building

SIMULATED, NOT MEASURED. A first-principles design for waste-heat recovery on a small intermittent brick kiln firing 10,000 bricks per batch on a 48-hour schedule. The kiln burns 3,800 kg of firewood per batch and its exhaust carries 33.2% of the fuel energy. A steel-tube recuperator, bypassed during the tar-heavy smoking phase, preheats combustion air to 314 C and recovers 8.93 GJ per batch; the flue gas then dries the next batch of bricks, adding 2.28 GJ of useful heat. Total fuel saving is 19.7%, payback 2.12 years at 70 batches per year and 75 USD per tonne of wood, with tar fouling, acid-dew-point corrosion and draft interference each answered and priced. A thermoelectric tail (702 batches to repay its modules) and a small organic Rankine tail (a loss per batch before capital) are computed and rejected. Recovery is not worth building below about 31 batches per year or below about 36 USD per tonne fuel price. Nothing has been built or hardware-tested; every number comes from the accompanying computation script.