#barmyidea4climate

An idea for capturing CO₂ & storing
heat under your new house

Question: “Is it a good idea or barmy?”

A heat store to capture CO₂

P1. Store 50+ tonnes of rock that absorbs CO₂ under (or near) a new house.

The mineral olivine is taken as the example for this note. It absorbs CO₂ slowly at ambient temperature (25°C) but much faster at a temperature of 186°C.

P2. Pass flue gasses from biomass boiler through the rock store to catch CO₂.

Suggested target is to use 2 tonnes of biomass per year, to sequester a substantial proportion of the 2.75 tonnes CO₂ produced at a high(ish) temperature but a speed of reaction much slower than in research projects, which envisage pressures of many atmospheres.

P3. Divide the rock store into compartments separated by thermal barriers.

The  barriers let the gas through the compartments in sequence, giving up heat to the rock store. The insulation between compartments is to keep the entry compartment as hot as possible, with each compartment in the sequence becoming a bit cooler. The exit compartment will be the coolest.

P4. Heat recovery is via a reverse flow through separate compartments on top of the rock store.

The flow uses a fluid to transport the heat back to the house – possibly water but probably air from the outside (after heat exchange with stale air from the house). The reverse flow starts at the exit compartment of the flue gasses and proceeds towards the entry compartment but is drawn off at a stage when it needs no eextra heat. The flow can be on top of the heat store as convection will keep the top of the rock compartments warm.

P5. At intervals a new entry chamber is chosen so flue gasses react with fresher olivine.

And in a decade the 50 tonnes of rock may need replacing.

P6. Spoiler: This probably won’t work but do look. It may have implications.

It shows some of the difficulties of one form of carbon sequestration.

Notes on advantages and difficulties

Olivine

Olivine is one of the rocks that can absorb CO2.  In the journal Nature,  Rock’s power to mop up carbon revisited, it says:

In theory, one kilogram of olivine sequesters about one kilogram of CO₂, but the rate at which this happens can be slow. And the actual efficiency of sequestration will be much lower than 100%, because of the energy used — and emissions released — in grinding and transporting the rock. In some cases, this could emit more carbon than would be sequestered.

Olivine can contain toxic heavy metals such as nickel that could accumulate in the environment. Grinding rocks would produce dust, which might harm human health.

Phil Renforth studies carbon sequestration and minerals at the University of Oxford, UK, and attended the Hamburg meeting. He says that there is a pressing need to conduct more work on enhanced weathering given that carbon emissions are likely to continue to rise, and because of the current focus on dealing with emissions by capturing them from power stations and storing them underground.

Flue temperature

 Assessment of wood pellet combustion in a domestic stove by Changkook Ryu suggests that flue gas temperature could reach the entry compartment some 200°C above ambient temperature. The maximum temperature should be restricted to 186°C

Thermal capacity of a 50 tonne store

According to Robie et al.  Heat capacities and entropies of … between 5 and 380°K the heat capacity of  Mg₂SiO₄ (forsterite, an olivine) is 0.8343 (joules/gm/°C) at 20°C and 0.9575 at 100°C.  Postulate that the heat store reached an average temperature of 70°C (significantly hotter in the entry compartment but much lower in the exit compartment). At 50°C above ambient temperature (20°C), the heat stored in a 50 tonne store would be 2250 MJ (625 kWh) .

The stored heat is about a fortnight’s heating load for a well insulated house.

CO₂ and energy from woodchip burning

Forest Research gives the carbon content of woodchips, with moisture content 20%,  as 37.5%. The CO₂ from burning 2 tonnes of woodchips is 2.75 tonnes of CO₂ (2*0.37*44/12). However,  other CO₂ emissions must be accounted for in a complete life cycle analysis. These other CO₂ emissions are not in the 2.75 tonnes of CO₂ from burning 2 tonnes of woodchips – for example by transport and the chipping process.

Wight Heat give the energy density of woodchips (30% moisture) as 3500 kWh per tonne. For woodchips with 20% moisture that’s 4057 kWh per tonne.

Burning 2 tonnes of woodchips (20% moisture) creates 8114 kWh of heat energy and emits 2.75 tonnes CO₂.

Volume of the heat store and its CO₂ capacity

WebMineral gives an average density of olivine as 3.32. At a packing ratio of 60% that’s about 24 m³. If the olivine store were 1.5m deep it would have an area of 18 m² . That could be 3m by 6m.

The theoretical maximum amount of  CO₂ that can be absorbed by 50 tonnes of olivine is as follows: 153.31 gms (1 mole) reacts with 88 gm of CO₂ (2 moles). That means 50 tonnes of olivine can sequester 28.7 tonnes  CO₂.

As 2.75 tonnes of CO₂ are produced by burning 2 tonnes of woodchips each year. If the whole of the 50 tonnes of olivine were to be consumed by the reaction, the rock should be replaced every 10.4 years.

Heating load for houses

In How much heating does your house use, Ovo Energy say

A ‘typical’ home in a mild climate uses between 5,000 kWh and 30,000 kWh of energy a year for its heating. So how much does your home use?

In Energy consumption and Passiv Houses, Matthias Grätz of the Baltic Environmental Forum reports that Passiv houses have heating demand of less than 15 kWh/m²a. That is 1275 kWh per year  for an 85m² house. He also says

Low-energy houses — that is, houses that have an annual heating demand of between 50 and 70 kWh/m²a — are becoming the norm in new constructions.

For an 85m2 low-energy house that is 5950 kWh per year.

d1) CO₂ from transport

Transporting 50 tones of CO₂ absorbing rock to has several problems. Emissions cause by transport is one. TheGreenRationBook gives the CO₂ cost of freight transporting a tonne of material by different modes as

₁₂₃₄ ¹²³⁴

The theoretical calculation also suggests that only 7.20kJ of energy would be generated for every kilogram of Olivine reacted. If 50 tonnes of olivine were to react fully with the CO2 from flue gasses 100 kWh of extra heat would be generated (50000* 7.2kj = 360 Mj = 100 kWh). Over 10.4 years mentioned above for olivine renewal that’s just 10kWh a year and is insignificant compared to the 8114 kWh generated by burning 2 tonnes chipwood a year.

Conclusion

This idea may be barmy.