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The Carbon Culprit

Carbon is the basis of all life. Molecules which contain carbon as a basis, or have carbon in a chain, or polymers, are known as organic molecules.

Carbon is essential for life on the planet, and is circulated through the biosphere and lithosphere by the mechanisms of the carbon cycle. Carbon enters the life cycle through photosynthesis, where plants absorb $CO_2$, unite it with water, under the energy of sunlight, to produce cellulose, which is $C_6H_{12}O_6$. This large molecule is then converted to sugar, and consequently amino acids, proteins, and all the substances which comprise organisms.

The main pathway of carbon is the atmosphere. There is a long-term balance of $CO_2$ in the atmosphere of around 300 parts per million (0.3%). You are right to think this is not very much. For every carbon dioxide molecule floating around in the atmosphere, on average there are 70 oxygen molecules, and 240 nitrogen molecules!

GISS images
Images courtesy of NASA’s Goddard Institute for Space Studies (GISS)

So, if carbon is in such low quantities, and so essential to life, it may be tempting to think that problems would arise if there were too little carbon, not too much?

The problem lies with fossil fuels. Although there is a rough balance between plant transpiration (converting atmospheric carbon to solid cellulose) and respiration (the return of carbon to the atmosphere as $CO_2$, when we breathe out, or burn something), there is also a 'big picture' part of the carbon cycle. Carbon is 'lost' from the biosphere, as peat is pushed underground, or sealife dies and drifts into sea sediment. And carbon is brought back into the atmosphere through the disintegration of carbonate rocks, which becomes magma, which is fired out of volcanoes.

These lithosphere (underground) sinks and sources of carbon should also remain in a rough long-term equilibrium. Fossil fuels (coal, oil, gas) are mid-stages of carbon in sinks (dead organic matter being squeezed by the Earth's weight as it sinks deeper into the crust). This carbon is many millions of years old, and is supposed to stay out of the atmosphere for a long time still.

Solar and Earth radiation spectra
Solar and Earth radiation spectra: the Sun's incident radiation is short-wavelength, and the Earth's is long-wavelength

However, since the Industrial Revolution began to burn coal at ever-increasing rates, and motor vehicles have been burning millions of tonnes of oil per year, this carbon has been brought back into the biosphere far too early, and has caused an sharp increase in the amount of $CO_2$ in the atmosphere. In fact, instead of 0.3%, there is now 0.4% CO2 concentration in the troposphere, the first 15 km of the atmosphere. That's how busy we have been generating energy from fossil fuels for two hundred years!

This excess of carbon acts like a blanket around the Earth. The incident solar radiation has a shorter wavelength than thermal radiation. This means that most of the sunlight can pass through the atmosphere, to be absorbed by the Earth's surface, which warms. The Earth then re-radiates this energy with a longer wavelength, in the infrared portion of the electromagnetic spectrum. CO2, methane and water in the atmosphere can absorb this longer wavelength, trapping the energy in the atmosphere. Instead of re-radiating the right proportion of the Sun's radiation energy each day, the Earth retains more and more of it, gradually heating the land, sea and atmosphere.

Carbon dioxide is the major greenhouse gas that is produced by human activities. That is why carbon is the no. 1 culprit in climate change, and the only solution is to stop using fossil fuels. The percent reductions discussed by the Paris Conference, December 2015, the Kyoto Protocol, and the UNFCCC, are political targets. There is no 'safe level' of excess carbon emissions, at which we can be sure we do not cause an irreversible change to the biosphere.

We are a large mammal, enjoying a temporary niche in a dynamic system. Any alteration to the baseline conditions will not be to our benefit.

Industrial Sources of Carbon Emissions

Cement and Concrete

Limestone quarry
3.5 GJ of energy is needed to convert limestone to one tonne of clinker for cement production

Road vehicles are responsible for around 13% of global CO2 emissions. However, the production and use of cement, mainly for construction concrete, is alone producing 5% of the world's total CO2 emissions! This comes from two sources: a little less than half of the carbon dioxide is produced during the consumption of fossil fuel in the cement production process, and most of the rest is released while calcium carbonate undergoes thermal decomposition as the concrete sets in-situ. 900kg of CO2 is released for each tonne of cement.

Each year, more than one cubic metre of concrete is made for every person on Earth.

Cement
Portland cement production involves high temperatures, 1500°C, to produce clinker from limestone, which accounts for 40% of the CO2 emissions of concrete

Limestone is burnt at temperatures as high as 1500°C for a few hours to form clinker. This is due to the fact that alite (Ca3SiO5), the mineral component which provides concrete with its initial strength, needs to be heated to 1500°C during the clinker-forming process. The substitution of this function by belite (Ca3SiO4) would save on fuel, since this requires a lower temperature of 1200°C. Belite is even stronger than alite in fully cured concrete, although this takes up to months to reach maximum, creating the difficulty of weaker concrete for the intervening period, which may make it unfeasible for certain applications. Additives are being investigated, may be able to speed up the curing time. A final problem is that belite requires more energy in processing, mainly grinding, so this offsets the energy saved in heating fuel.

Concrete consists of c. 14% cement, a serious greenhouse gas emitter
Concrete consists of c. 14% cement, a serious greenhouse gas emitter

Concrete consists of around 15-20% cement, the rest being variable proportions of fly ash, aggregate, and pozzolan. Normally, only the cement needs to be imported from remote suppliers, making the embodied energy (the energy budget for providing and using the materials, also known as grey energy) of concrete favourable in comparison to other materials, such as wood and steel. The embodied energy of concrete is typically 7% transportation and 70% for cement production. The admixture of fly ash reduces the cement production embodied (grey) energy account proportionally by 70% of the replaced weight.

Equation: $2Ca_3SiO_5 + 7H_2O → 3(CaO)·2(SiO_2)·4(H_2O) + 3Ca(OH)_2$. During curing, cement powder is hydrated to produce a cementiferous paste, or calcium-silicate hydrate, and calcium hydroxide.

Concrete has a natural tendency to absorb some CO2 from the atmosphere over time. And concrete can be made CO2 emission neutral by the addition dicalcium silicate during the curing phase, which absorbs CO2 and offsets up to 100% (or more) of the CO2 emissions from heating fuel. This could save the 400kg/m3 in the final concrete, which is the normal CO2 cost of the curing phase.

Calcium Carbonate

Calcium carbonate is a naturally occurring chemical compound, with formula $CaCO_3$. It is the residue of marine organism shells, which form limestone, and is used in a broad range of applications, including cement, chalk and agricultural products.

Calcium carbonate is the main chemical component of limestone, calcite, scale (white residue in kettles), aragonite, chalk, marble, pearl, and oyster.

$CaCO_3$ will react with acid, and thermally decompose, to release $CO_2$. This makes it particularly susceptible to acid rain attack.

$CaCO_3(s) + 2HCl(aq) → CaCl_2(aq) + CO_2(g) + H_2O(l)$

$CaCO_3(s) + 2H_2SO_4(aq) → CaSO_4(aq) + CO_2(g) + H_2O(l)$

heat + $CaCO_3 → CaO(s) + CO_2(g)$

Formation of bicarbonate: $CaCO_3(s) + CO_2(aq) + H_2O → Ca(HCO_3)_2(s)$

The bicarbonate reaction occurs when rain water saturated with $CO_2$ erodes limestone rock to form caverns. It also creates hard water.

Evidence of calcium carbonate on Mars may indicate that liquid water was once prevalent on the planet.

Content © Andrew Bone. All rights reserved. Created : June 17, 2015 Last updated :March 26, 2016

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