The Azimuth Project
Carbon negative energy (Rev #11)


The idea

Carbon negative energy refers to any form of usable energy whose production reduces the amount of carbon dioxide in the Earth’s atmosphere.

If you can think of other sources of carbon negative energy, enter information about them here! — John Baez

Biofuel versus biochar

Photosynthesis can take carbon out of the atmosphere and provide useful fuel. If all the carbon taken out is returned to the atmosphere when this fuel is burnt, this process is roughly carbon-neutral. This is an oversimplification of the actual story. On the one hand, it takes energy to grow and process vegetation into usable fuel; if this energy comes from fossil fuels there may be a net gain of CO2 in the atmosphere. On the other hand, not all the carbon is returned to the atmosphere: some stays in the ground, at least for a while, in the form of roots, humus, etcetera.

We need detailed figures here! What is the overall carbon footprint of biofuels? People have done research on this already, and the results should probably go in a page on biofuel.

Ignoring these subtleties, we may very roughly say that fossil fuels are carbon-positive while biofuels are, at best, approximately carbon-neutral. On the other hand, biochar offers the possibility of strongly carbon-negative energy production: significant long-term reduction in atmospheric CO2 combined with the production of significant amounts of energy. The idea here is to create some usable fuel from plant material, while also creating large amounts of charcoal which can be buried to sequester carbon. Unlike rotting vegetable matter, buried charcoal can sequester carbon for centuries or even millennia.

For more, see biochar.

Olivine weathering

One of the main long-term mechanism that removes carbon dioxide from the ocean and atmosphere is the weathering of rocks. During major periods of tectonic uplift in the Earth’s past, huge slabs of rock rich in the mineral olivine (mostly peridotite) were pushed up through the Earth’s crust, with some of it being exposed at the surface. The resulting chemical weathering caused or contributed to a significant drop in CO2 levels leading to global cooling.

We can do this ourselves: we can dig up olivine, crush it into powder and spread it around. It then ‘weathers’ by reacting with carbon dioxide. It weathers completely within a few years, depending on the grain size.

Gem-quality olivine

Olivine is a magnesium iron silicate whose chemical formula is (Mg,Fe)2SiO4, meaning that either magnesium or iron can appear at the same lattice site of this crystal, in variable amounts. For simplicity let us consider the extreme form of olivine with all magnesium and no iron, known as fosterite. It reacts with carbon dioxide as follows:

CO2 + 12\frac{1}{2} Mg2SiO4 \to MgCO3 + 12\frac{1}{2} SiO2

or in words:

carbon dioxide + fosterite \to dolomite + silica

This reaction is exothermic and produces about 90 kilojoules per mole.

A related reaction is:

Mg2SiO4 + 4 CO2 + 4 H2O \to 2 Mg2+ + 4 HCO3- + H4SiO4

So, 140 grams of olivine will sequester 176 grams of CO2, with the help of 72 grams of water, i.e. rain or seawater. All the CO2 that is produced by burning 1 liter of oil can be sequestered by less than 1 liter of olivine.

Question: what is the relation between the two reactions shown here? Are they alternatives, different ways of viewing what actually happens, or what? It needs clarifying.

Also: the first reaction produces ‘90 kilojoules per mole’ according to Philip Goldberg et al. Is that moles of CO2, as opposed to Mg2SiO4? Is that what the 1/21/2 is for? I.e. if we double the quantities on both sides of this reaction, we get 180 kilojoules per mole of Mg2SiO4?

The weathering of olivine is exothermic but slow. In order to recover the heat produced by the reaction to produce electricity, a large volume of olivine must be thermally well isolated. This process is not yet practical at sufficiently large scales to help prevent global warming, but it may be one day. Some work on this has been done by Olaf Schuiling, a professor in geochemistry at the University of Utrecht:

See also:

Other forms of mineral weathering

The following paper discusses olivine weathering and also weathering of the mineral serpentine:

Serpentine is about 10 times more abundant than olivine. A ton of serpentine can dispose about two-thirds of a tone of CO2; this reaction is exothermic and yields about 64 kilojoules/mole:

Mg3Si2O5(OH)4 + CO2 \to MgCO3 + 2 SiO2 + 2 H2O

The above paper sketches some technical challenges and also the progress made by the authors, a team of researchers at the Albany Research Center (ARC), the Los Alamos National Laboratory, the Arizona State University, and the National Energy Technology Laboratory. To quote:

Technical Challenges and Program Goals

The major technical challenge now hindering the use of minerals to sequester CO2 is their slow reaction rate. Weathering of rock is extremely slow. The highest priority is given to identifying faster reaction pathways. Second, the optimized process has to be economical. Although many carbonation reactions are exothermic, it is generally very difficult to recover the low-grade heat while the long reaction time and demanding reaction conditions contribute to process expense. Clearly, the environmental impact from mining minerals and carbonation processes must be considered. The program goals are specifically designed to address these challenges, including

i. identifying favored technical processes,

ii. determining the economic feasibility of each sequestration process identified, and

iii. determining the potential environmental impacts of each process.

Rapid Progress

Although the program only has about two years of history, the working team consisting of Albany Research Center (ARC), the Los Alamos National Laboratory, the Arizona State University, and the National Energy Technology Laboratory has made significant progress.

In striving to accelerate overall reaction rates, the team has identified one very promising reaction pathway and succeeded in achieving dramatically shortened carbonation reaction times employing magnesium silicates such as olivine and serpentine. For example, research at the Albany Research Center (10,13) has focused upon the direct carbonation of olivine. When the program first started, it took 24 hours to reach 40-50% completion of carbonation of olivine. The reaction required temperatures of 150-250 C, pressures of 85-125 bar, and mineral particles in the 75-100 micron size range. Careful control of solution chemistry yielded olivine conversions of 90% in 24 hrs and 83% within 6 hrs. The most recent results show further modifications of the same basic reaction can achieve 65% conversion in 1 hour and 83% conversion in 3 hours.

While the potential to utilize olivine to sequester CO2 is clearly significant, there is approximately an order of magnitude more serpentine than olivine. Consequently, finding a way to use serpentine to scrub CO2 will have greater practical impact than using olivine. Both minerals are valuable feedstocks and progress has been made in direct carbonation using serpentine also. When the program started, tests conducted at Los Alamos National Laboratory only achieved 25% conversion using 100 micron serpentine particles with CO2 even at a very high pressure of 340 bars. Independently, researchers at ARC developed a successful carbonation process for serpentine that utilizes mineral heat pretreatment and carbonation in carbonic acid in aqueous solution. A recent literature review indicated that weak carbonic acid treatments had also been suggested for Mg extraction in the prior literature (12). Carbonation tests performed at ARC employing heat pretreated serpentine have resulted in up to 83 % conversion in 30 minutes under 115 bars (13).

Because the high pressure requirement of the carbonation reaction will certainly lead to high process costs, the team is modifying solution chemistry to allow reaction to proceed at a lower pressure and temperature. The research is guided by the idea that the concentration of HCO3 - in the solution is critical to the reaction rate. The high CO2 pressure will lead increased CO2 absorption in the solution and thus enhance the HCO3 concentration. Adding bicarbonate such as sodium bicarbonate in the solution will significantly increase the HCO3-concentration even at a relatively lower CO2 pressure. Indeed, by increasing sodium bicarbonate concentration the carbonation reaction of serpentine can reach 62% completion under 50 bars.

To support laboratory carbonation tests, researchers at Arizona State are employing an environmental-cell dynamic high-resolution transmission electron microscope to directly image dehydroxylation of Mg(OH)2, an important step in Mg(OH)2 carbonation reactions. They are extending this technique to study the solid gas reaction path using serpentine to provide insights into pretreatment and reaction issues.

In the process development area, the team has completed a feasibility study of a process originally proposed by Los Alamos National Laboratory (9, 11). This process uses HCl solution reacting with serpentine to produce Mg(OH)2 which is subsequently used to sequester CO2. Although the study found the process energy intensive and inappropriate for CO2 sequestration, the analyses of individual steps were useful for developing new processes. Los Alamos National Laboratory is currently pursuing reaction mechanisms that may allow the heat treatment step for serpentine to be bypassed. Progress has also been made in identifying sources of alternative minerals that can be used for CO2 sequestration. In addition to natural olivine and serpentine deposits, researchers at NETL are engaged in a study of using waste streams such as coal ash rich in calcium and magnesium as a potential mineral source to sequester CO2.

Energy storage and load balancing

Energy storage and load balancing both have carbon negative effects. But do these count as carbon negative energy?