Magnetohydrodynamic (MHD) generators extract electricity directly from a flowing ionized gas — no turbines, no moving parts. You shove hot conductive plasma through a magnetic field, and the Lorentz force separates positive and negative charges onto electrodes. The Soviets actually ran a 25 MW MHD topping cycle (U-25) in Moscow in the 1970s. The problem has always been that you need 3,000 K exhaust to get meaningful ionization, and at those temperatures your channel walls vaporize in hours.

So: what if we built a single colossal MHD channel — a 500-meter-tall chimney, 30 m in diameter, sitting atop a 1 GW coal plant — and used the entire flue-gas stream as our working fluid? Could we recover the 35% of fuel energy currently dumped as heat?

The seeding problem

Flue gas at 1,800 K is essentially non-conductive. To get usable conductivity (~10 S/m), you seed it with cesium or potassium carbonate, which ionize easily. Cesium gives the best yield but costs ~$60,000/kg. A 1 GW plant pushes ~1,200 kg/s of flue gas; seeding at 1% mass fraction means 12 kg/s of cesium. That's $43 million per hour. Potassium at 0.5% is more realistic: ~$200/hour in seed, plus a downstream electrostatic precipitator to recover 99% of it (the Soviets managed 95%).

The chimney as a generator

Power density in an MHD channel scales as P = σ·u²·B²·(1-K)·K, where σ is conductivity, u is gas velocity, B is magnetic field, and K is the load factor. Plug in σ = 10 S/m, u = 800 m/s (subsonic flue gas), B = 5 T (a serious superconducting magnet wrapping the chimney), K = 0.5:

P = 10 × 800² × 5² × 0.25 = 40 MW/m³

The channel volume is roughly π × 15² × 500 = 350,000 m³, but only the first ~50 meters runs hot enough for real conductivity. That gives an active volume of ~35,000 m³ and a theoretical output of ~1.4 TW — which is absurd. In practice you're limited by the gas's thermal energy: 1,200 kg/s × 1,200 J/kg·K × 800 K cooling ≈ 1.15 GW of available heat. At MHD efficiency of ~25%, that's 290 MW of bonus electricity stacked on top of your existing steam cycle.

The 5-tesla problem

Wrapping a 30-meter-diameter superconducting solenoid is, frankly, unhinged. ITER's 13 m bore central solenoid stores 6.4 GJ. Scaling roughly with B²·V, our chimney coil stores ~150 GJ — equivalent to 36 tons of TNT in magnetic field energy. A quench would vaporize the chimney. You'd need a segmented array of warm-bore resistive magnets instead, eating ~50 MW just to run, dropping net gain to ~240 MW.

The wall problem nobody solved

Slag from coal ash deposits on electrodes, shorting them out. The Soviets never ran U-25 continuously for more than 250 hours. With modern ceramic-matrix composites (HfC coatings, melting point 4,200 K) you might reach 5,000 hours — still a maintenance nightmare versus a steam turbine's decade-long runs.

Plant efficiency would jump from 35% to roughly 47% — comparable to a modern combined-cycle gas plant, but applied to coal. Which is the real irony: we'd be making the dirtiest fuel almost as efficient as the cleanest, just in time for nobody to want it.