Combining BECCS and Enhanced Weathering A hybrid technique for removing carbon dioxide from the atmosphere Tim Kruger 9 May 2011

Overview Description of a proposed new method to reduce atmospheric carbon dioxide levels Combines two existing proposed geoengineering techniques to produce a hybrid solution: BECCS (Biomass Energy and Carbon capture and Storage) Enhanced weathering

BECCS - Description BECCS – biomass energy and carbon capture and storage – involves harvesting biomass, combusting it to generate process heat and/or electricity and sequestering the resulting CO2 in geological formations This is a carbon-negative process – one that can be employed to draw CO2 out of the air The method uses the natural ability of plants to capture CO2 from ambient air One of a number of techniques (e.g. gasification, oxy-fuel combustion or post-combustion separation by amine scrubbing) are used to generate pure CO2 This pure CO2 is then compressed and injected into suitable geological formations for long-term storage.

BECCS – Pros and Cons Advantages Disadvantages ‘Natural’ process The de-mixing of CO2 from ambient air is performed for ‘free’ by plants Two revenue streams – (i) process heat and/or electricity and (ii) carbon credits Financial, energy and carbon costs associated with harvesting and transport of biomass Growing biomass as an energy crop puts pressure on food crops and biodiversity Geographical limitation – requirement to be close to a market for electricity and to a site for geological storage

Enhanced Weathering - Description Weathering of rocks is a natural means by which carbon dioxide is drawn down from the atmosphere Carbon dioxide in the air dissolves in raindrops as they fall, producing a weak carbonic acid. This reacts with silicate and carbonate rocks to form the ‘hardness’ in water (calcium and magnesium bicarbonate solutions) locking the carbon dioxide away from the atmosphere Weathering reactions tend to be a slow method of drawing down CO2, but there are a number of techniques that have been proposed to enhance the rate of the reactions – one such method is detailed in the next slide

CO2 Mitigation via Capture and Chemical Conversion in Seawater Recent paper by Greg Rau1. Builds on a previous paper in 19992. 10% concentration CO2 flue gas is passed through seawater. The pH drops, the dissolved inorganic carbon (DIC) increases and the saturation state of calcite falls from ~4.5 times supersaturated to undersaturated. Finely ground limestone is added to the treated seawater. It dissolves raising the pH and Ca2+ ion concentration The seawater is now allowed to re-equalibrate with ambient air. Some of the CO2 previously dissolved is lost to the air, but much remains in solution as the saturation state of calcite is now much higher (~9 times supersaturated) Rau, G.H. CO2 Mitigation via Capture and Chemical Conversion in Seawater. Environ. Sci. Technol. 2011, 45, 1088-1092. Rau, G.H.; Caldeira, K. Enhanced carbonate dissolution: A means of sequestering waste CO2 as ocean bicarbonate. Energy Convers. Manage. 1999, 40, 1803-1813.

Rau’s Process – Pros and Cons Advantages Disadvantages ‘Natural’ process – mimics the natural process of weathering Can utilise a 10% CO2 flue gas – i.e. Does not need expensive purification, compression, transportation, injection and monitoring associated with CCS Far less geographically constrained than CCS – access to oceans and limestone On its own this is not a method for drawing carbon dioxide from the air – it is simply (a much better) alternative to CCS

A hybrid solution - Description Combining the two approaches together creates a more powerful solution: Biomass is combusted in air to produce a low CO2 concentration flue gas This flue gas is used to drive Rau’s process, leading to the CO2 being securely sequestered

Advantages and Challenges ‘Natural’ process – growing plants and accelerating weathering Obviates need to produce pure CO2, thereby substantially reducing the costs of capturing carbon when producing energy from biomass In combination with BECCS, Rau’s method becomes a means by which to draw down CO2 from the atmosphere Challenge of adverse effects on food prices and biodiversity remain Work needs to be undertaken to assess the terrestrial and oceanic impacts Would this require an amendment to the London Convention? Creates an existential threat to those seeking to develop CCS technologies. Expect opposition from vested interests

Discussion It may not be necessary to generate process heat and/or electricity from the combustion of biomass to be economically feasible. BECCS is geographically constrained by the need for a market for the process heat and/or electricity The capital and operational costs would be much smaller if process heat, electricity and pure CO2 were not required. This opens up the possibility of setting up operations with carbon credits as the sole revenue stream and would allow implementation in remote and currently economically impoverished regions While this would create its own challenges, it would also potentially create opportunities for development

Further work Rau details experiments using limestone, but the method should also work using magnesite and dolomite – this should be investigated Detailed economic costing required, along with carbon and energy budgets Scalability of biomass methods (BECCS, biochar, ocean fertilisation and afforestation) to draw down atmospheric CO2 is hotly contested