Sustainability and Cement CO2 emissions: US cement outlook

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Approximately ~1.3 T CO2 are produced per T of cement. Globally, the cement industry contributes to 5% of the anthropogenic CO2 emissions (1.34 giga tons of CO2).

Comparisons between cement production and pulverized-coal combustion:
There are some commonalities and contrasts between CO2 emissions from coal-fired power plants and cement plants. Both "commonly" consume coal as a fuel,and flue gas emitted from both processes is relatively dilute in CO2. However, a greater share of CO2 emissions from cement are a result not of coal burning, but due to the calcination of the raw material (calcium carbonate, CaCO3). The operating temperature of a cement kiln affects the quality of the clinker and the cement produced, and therefore controlling it is relatively more complex compared to coal-fired combustion. Additionally, cement production is essentially a hot gas-solid heat exchange process (with simultaneous mass transfer, chemical reaction and material flows) whereas in coal combustion, the objective is to produce a hot flue gas stream to generate steam.

What has the cement industry done to reduce its CO2 emissions?
In cement production (similar to heavy-metal industries such as bauxite and steel) energy efficiency (lower CO2 emissions) directly translate to cost savings. Therefore, the industry has an incentive to maximize the tons of product produced/unit of energy spent. Innovations in kiln design (preheaters, precalciners) as well as a move away from wet process kiln technology have resulted in considerable energy savings. Because a greater portion of the CO2 emissions from cement plants are from the raw material calcination itself, any process that requires lesser raw material to be calcined significantly reduces the CO2 emissions from (and energy requirements of) cement production. Examples of this are blending pozzolonas (fly ash) to cement and blending ground limestone into cement. The ASTM standard limits fly ash blending at 25% w/w in cement for reinforced cement concrete applications, whereas the relevant standard for limestone currently allows 5% blending. Because both of them displace an equivalent amount of clinker, blending limestone or fly ash results in reduction of CO2 emissions and is also very cost-effective. However, recent NOx regulations have forced powerplants to reduce flame combustion temperatures to avoid high-temperature NOx, and this results in residual carbon in flyash. High-carbon fly ash is not suited to be blended into Portland cement. Fly ash blending gives durability to cement. Overall, in my opinion, cement manufacture is a nice example for industrial ecology, because fly ash (a byproduct of coal combustion) and gypsum (a by-product of phosphoric acid manufacture) are used to make cement, which in turn is used to make concrete.

What can the industry do in the near future?
A few of the many innovations in reducing GHG emissions from cement production include:

• Oxyfuel combustion: burning the fuel in an atmosphere rich in oxygen instead of air, to result in a concentrated stream of CO2 which can be separated easily. This process requires efficient N2-O2 separation from air, and often the operation of the cement plant will be coupled to the operation of the air-separation unit (ASU). Additionally, this separation costs energy, and this has to be balanced with lower downstream-separation costs (ex: elimination of methanol amine scrubbing). Improvements in gas separation processes, such as ion transport membranes (ITM) will be required to lower the costs of using this technology.

• CO2 capture using algae and producing a combustible liquid fuel: In previous posts on this blog, I have analyzed this process, using a simple process model. Essentially, the process economics are dictated by what one can do with the algae. As someone in the cement industry recently told me, cement plants do not necessarily require diesel as fuel. Choosing strains of algae which can be easily separated, dried and converted to a stable solid/gaseous fuel could significantly affect the economics of this process.

• Forming mineral carbonates from CO2: Examples of this include Calera. Essentially, the idea is to form carbonates using sea water and CO2, and blend the resulting carbonate into cement. The idea makes economic sense if the produced calcium carbonate is cheaper than the cement clinker (or cheaper than the limestone itself). In a previous post on this blog, I partially examined this process. However, keep in mind that details are not fully known and therefore any analysis will be only preliminary.
  From a general perspective, the process of precipitating calcium carbonate from CO2-saturated solutions requires high-pH conditions. This scenario is a catch-22 situation, because increased CO2 concentrations in water (under high pressures) which is essential for precipitating CaCO3 (for example), also results in decreased pH, which disfavors carbonate precipitation. On the other hand, many marine organisms such as corals, mollusks and algae form CaCO3 either within their bodies or externally, under relatively dilute-calcium concentrations. Some alkaline materials that could be added to water to increase its pH economically are: alkaline fly ashes, and cement kiln dust.
  
  Another example of a company involved in making carbonates is Carbon Sense Solutions Their process involves accelerated CO2-curing of concrete, which is essentially a reverse of the calcination step. I partly commented on this process on the peakoil forum At best, thsi process would make concrete (and cement production) carbon-neutral. However, getting a CO2-source close to the curing plant would require additional infrastructure to transport CO2. Compared to this, the use of external cations (either from sea water (Calera) or an added alkaline material) has the potential to result in a net-reduction of CO2 emissions

• Enzymatic processes to capture CO2 from flue gases: Carbonic anhydrase (CA) is a well known biocatalyst mediating the reversible hydration of CO2. In cases where CO2 dissolution in water is the rate-limiting step, the use of this enzyme would speed up the kinetics (rates) of CO2 dissolution and CO2 stripping. Note that the use of an enzyme DOES NOT change the thermodynamics of the reaction. In other words, the process would still require the same amount of energy/mole of CO2, but the rate (moles of CO2/unit time) would be significantly changed due to the enzyme. A company called CO2 Solutions has a CA-based process for capturing post-combustion CO2 from point sources such as power plants and cement plants.


• Cleaner concrete processes: Because the ultimate purpose of making cement is to make a building material, one can test different mixes of non-clinker-based raw materials which result in the same strength and durability as Portlan cement concrete, for various applications. For example, Cal Star cement likely has a process for making fly ash-based bricks as replacement for concrete.
  

Summary and Outlook:

The cement industry has modified its processes to be more energy-efficient. However, the issue of CO2 emissions from calcination of the raw materials needs innovative solutions. Examples of processes that enable easier capture of CO2 (ITM-, CA-based), processes which convert the CO2 either into fuels or mineral carbonates, and processes which replace cement-based concrete in innovative ways were discussed. Although the Regional Greenhouse Gas Initiative (RGGI) does not regulate CO2 emissions from cement plants currently, the Western Climate Initiative will likely include cement plants in its regional cap-and-trade umbrella. The MidWestern Greenhouse Gas Reduction Accord might also regulate CO2 emissions from cement plants. Given that some of the largest cement plants in the US are in states which will be participating in the WCI, the cement industry should be prepared for this and future federal legislations. Intra-US carbon leakage will probably be negligible for the cement industry because the cement plants are located either close to the markets or close to the limestone quarries (due to high transportation costs). On the other hand, robust policies need to be enacted to ensure that the US cement industry remains competitive with imported cement to prevent carbon leakage out of the US (which does not reduce global GHG emissions).
In the short-term, processes which utilize CO2 would provide low-cost CO2 offsets to cement producers, whereas the long-term approach likely involves developing the infrastructure for low-cost carbon capture and storage (CCS).

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