100% renewable geothermal methanol

A geothermal powerplant extracts heat from Earth’s core, either exploiting natural vapour and hot water circulation, or through water injection and steam collection. The water/vapour stream is however not made of pure water but contains contaminants, in large proportions CO₂. Therefore, even if fully renewable, power generation in geothermal plants releases CO₂, on average about 45 g_CO2/kWh.

However, CO₂ concentration in flue gas is quite high, facilitating its capture. CO₂ and electricity availability creates the perfect conditions for a methanol plant, as electricity allows hydrogen production through electrolysis. Carbon Recycling International (CRI) built such a unit in Grindavik, Iceland and branded its renewable methanol as vulcanol.

The main drawback of relying on a geothermal source for power, heat and CO₂ is its low temperature (60-180°C), which is below the optimum temperature for the methanol reactor as well as (too) low for the separation and distillation steps. Management of excess heat from the reactor becomes critical, while compression (running on electricity) must be favoured over heating when possible (CO₂ and H₂ compression upstream reactor, pressurised lower distillation column).

CRI’s plant annually produces 4,000 t_CH3OH, avoiding the release of 5,600 t_CO2 and relying on an industrial-scale electrolysis unit fed with 100% renewable electricity (800 t_H2). As a scaling perspective, the annual Icelandic consumption of crude-based fuels is ~750,000 t, mostly in cars and fishing boats, vehicles that can be converted to CH₃OH use. Yet, as methanol energy content is about half that of conventional crude-based fuels, a production of ~1,500,000 t_CH3OH would be required.

According to CRI’s own published figures, producing 1 ton of methanol requires:

• Capturing 1.4 t_CO2 from geothermal flue gas, itself requiring 0.8 MWh
• Producing 0.193 t_H2 through electrolysis, consuming 8.25 MWh (efficiency of ~78%)
• Compressing CO₂ and H₂ before injection into the reactor, consuming 0.45 MWh of electricity
• Distillating the reactor products (recycling heat from the methanol reactor) to extract 0.59 t_H2O

Total energy consumption is therefore 9.5 MWh, while the energy content of 1 t_CH3OH is 5.58 MWh, leading to an energy to methanol efficiency of 58.7%.

By far, the main energy consumption item is the hydrogen production with electrolysis. Applying the same general approach to a CO₂ source that can (at least partially) also provide some H₂ would largely improve the overall efficiency. Such source exists, notably in blast furnaces in steel plants.