Targeted methane removal

Methane is a potent enough greenhouse gas (~20-80x CO₂) that it could be worth spending additional energy either to remove it or to turn it into CO₂. But it’s tough to purge dilute methane from air efficiently: most techniques have to spend energy on all of the air molecules, which is a losing proposition. If we couple intense laser light into dilute methane mixtures, we might be able to pump several eV of energy into the vibrational modes of the methane molecules alone, possibly triggering efficient decomposition or combustion. My crude analysis suggests that if the science works perfectly, there’s perhaps a marginal business case here, assuming that carbon credits reach $100/ton CO₂ and we can build a suitable mid-infrared laser for $20/W. That’s not substantially crazier than iron-chloride or other hydroxyl-radical proposals for destroying atmospheric methane, but it’s not a slam dunk.

The same technical approach seems much more likely to give cost-effective (order $1/t CO₂e) methane abatement in higher-concentration (>1% CH₄) settings such as cattle barns or some combustion exhaust. While it looks tractable, the smaller addressable market and permanent dependence on carbon subsidies mean it’s not for us.

(A broader look at what cheap lasers could do for chemistry, including a version of this analysis, is in New frontiers for bright radiation.)

Oxidizing methane to CO₂ eliminates 97% of its warming impact when assessed over 20 years. Is it worth spending energy to burn methane that would otherwise enter the atmosphere? We can estimate the break-even energy for electrified combustion. Over a 20-year window, converting 1 kg of CH₄ to CO₂ is equivalent to removing 83 kg of CO₂ from the atmosphere, while electricity from natural gas emits 0.5 kg CO₂e/kWh. We break even at 83 kg ÷ 0.5 kg/kWh = 167 kWh per kg of CH₄removed, or about 100 eV per CH₄ molecule. (Solar electricity’s CO₂e intensity is supposedly 10× lower, giving 1000 eV/molecule; wind is allegedly 5000 eV/molecule.^[1]) To do obvious good, and to make any money from carbon credits or similar, I suggest that our target energy budget should be less than 10% of break-even. With electricity from natural gas, that’s a budget of 10 eV/molecule.

A thermally activated oxidation scheme would fit within these budgets only at relatively high methane concentrations. Consider a large methane leak where the air is 0.1% (1000 ppm) methane. Using baseload electricity from natural gas to heat the bulk gas, we’d have 10 eV × 10⁻³ = 0.01 eV to distribute per air molecule. That would heat the air by only ~30 K. Considerably higher temperatures could be reached with efficient counterflow heat exchange, but this is not much of an energy margin: 0.1% methane looks right on the edge of what’s possible to combust, even with a nice catalyst. Lower concentrations look basically out of the question, because the available energy is spread even thinner. With renewable electricity we might go another ~10× lower in concentration (to 0.01%, or 100 ppm), but that’s it.

One way around this unfavorable concentration scaling might be to deliver energy directly to CH₄molecules using laser light. Methane has strong vibrational absorption bands in the mid-infrared (MIR) and considerably weaker bands in the near-infrared (NIR).^[2] By coupling resonantly into one of these bands, we can bring methane molecules to a very high vibrational temperature, far out of equilibrium with their translational and rotational temperatures. The hope would be to pump enough energy into the vibrational modes that the methane molecules become extremely reactive, and either pyrolyze or react with oxygen to form CO₂.

Whether this strategy can succeed depends on timescales. The first is the vibrational–vibrational (V-V) relaxation time: the time it takes optically excited molecules to redistribute their energy among their various vibrational modes into a thermal distribution. I suspect this is ~1 ns.^[3] The second, and perhaps most relevant, is the vibrational–translational (V-T) relaxation time: the time it takes for the energy pumped into CH₄’s vibrational modes to bleed out into the translational energy of the gas—i.e., how long before you’ve effectively just heated the bulk gas. The V-T time appears to be ~10 µs for dilute CH₄ in a diatomic gas at standard conditions.^[4] To get something interesting to happen, we should pump few-eV energies into each molecule within the V-T time, with bonus points for doing it within the V-V time. (Of course, there’s no guarantee that highly vibrationally excited methane becomes reactive enough for our purposes—this part is still a leap of faith. But V-T non-equilibrium seems to be quite enabling in plasma chemistry.)

What laser setup could pump ~3 eV into methane’s vibrational modes within 10 µs? First consider exciting the NIR overtones, since this is where cheap, efficient solid-state lasers (and somewhat less cheap and efficient fiber lasers) are available. Here the optical cross-sections are ~10⁻²³ cm², so we’d need an intensity of at least 3 eV ÷ (10 µs × 10⁻²³ cm²) ≈ 5 GW/cm². That’s a huge intensity, but it’s at least imaginable to reach by focusing a large array of diode bars.^[5] The main trouble is that we also need most of the light to be absorbed by the gas, or else we’ll blow through our energy budget, and even at 0.1% methane in air, that takes 10–100 km of propagation.^[6] We could try to build a high-Q resonator to help with this, but even if we could, it would be a bad idea for an environmental application: the optical surfaces would quickly get dirty and ruin the resonator.^[7]

The MIR modes (at 3.3 µm and 7.6 µm) are more interesting targets, because their optical cross-sections are a hundred thousand times larger: ~10⁻¹⁸ cm². The required intensity is ~50 kW/cm², and if the laser is tuned exactly on resonance,^[8] the light will be absorbed within about a meter at 0.1% methane, 10 m at 0.01%, and so on. These are much more reasonable figures. The challenge in this wavelength range is the lack of a suitable light source. Quantum cascade lasers can operate around 7.6 µm, but they are typically only 5–10% efficient, cost something like $1000/W, and aren’t particularly bright. Another option might be an optical parametric oscillator (OPO) pumped by a (possibly pulsed) fiber laser.^[9] The fiber laser would probably set the cost at around $20/W in volume, and I suspect the efficiency again wouldn’t exceed 10%. In any of these cases we’d have to be clever with the optics to keep the beam from diverging too quickly, etc. I’ll ignore these very important practical issues for now.

Say we have a 10% efficient MIR light source, and the science works so well that 3 eV supplied per molecule leads to 100% destruction of CH₄. We’re already at an electricity consumption of 30 eV/molecule, or 50 kWh per kg of CH₄ removed. That slightly exceeds our budget for electricity from natural gas, but it’s within our budget for solar electricity. Could we make money doing this? Assume we use solar electricity at $0.04/kWh and carbon credits are $100/ton CO₂. Accounting for our electricity emissions at 0.05 kg CO₂e/kWh, removing 1 kg of CH₄ is equivalent to removing 80 kg of CO₂. The electricity costs 50 kWh/kg CH₄ × $0.04/kWh = $2/kg CH₄, or $25/ton CO₂, which leaves some room for CapEx. If our equipment lasts 20 years at a 25% duty cycle (because we’re using solar), it consumes 440 kWh per rated optical watt over its lifetime (remember, it’s 10% wall-plug efficient). If we want the CapEx to roughly equal the electricity cost (leaving about $50/ton CO₂ for other OpEx and profit), each optical watt should cost $0.04/kWh × 440 kWh ≈ $18.

Looking back at our laser estimates: if we can really build a suitable MIR OPO from a fiber laser for $20/W, and if carbon credits really reach $100/ton CO₂, and if all the science works out perfectly, and if all the other little issues I swept under the rug are resolved, this very crude analysis suggests we’d be in business.

Of course, things would look a lot better at higher methane concentrations. In places like cattle barns, exhaust streams, or LNG handling areas with >1% CH₄, this laser gambit looks like it would be well in the money even with a subsidy under $10/t CO₂e. But you’d still be talking about a small, subsidy-dependent business with a challenging measurement, reporting, and verification (MRV) situation. A cool opportunity, perhaps, but clearly not the startup idea we’ve been looking for.

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