Nov 24, 2008

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).


Typical yields from algal biofuel technologies

This is an examination of yields of "primary product" (algal oil/ethanol/biodiesel), expressed as barrels of oil equivalent/hectare of land area/year, from various algal (algae) biofuel technologies. I used data from company websites and press releases and converted the algal oil/ethanol production to a BOE/ha/year basis.
From the above figure, typical "yields" range from 100-1000 BOE/ha/yr. Compare this to Dr. Benemann's recent statement that the maximum algal yield without using genetically modified algae would be ~2000 gal algal oil/acre/year (101 BOE/ha/year).

Disclaimer: This is not meant to be a comparison of various processes or an endorsement/critique of a specific process. Utilization of and the value for the algal biomass and the biofuel determines the overall process economics. My assumptions and data are given below:

  1. Algenol: 6000 gal EtOH/acre/year
  2. Solix data from here.
  3. GreenFuel, from a previous post
  4. PetroAlgae: Assumed 200x current soybean oil yields (200x50 gal oil/acre/year).
  5. GSPI: Link here
  6. Theoretical maximum: from CircleBio's website, assuming 20,000 gal biodiesel worthy plant oil/acre/year.
  7. I further assumed that 1 T of algae oil gives 1 T of biodiesel, unless mentioned otherwise on the company's website (the ratio is ~0.96 for soy oil).
  8. Calorific value of 33MJ/L for biodiesel and 20 MJ/L for ethanol.

Related posts:
Analysis: Algae for CO2 capture - II
Analysis: Algae for carbon dioxide (CO2) capture


Nov 9, 2008

Analysis: Algae for CO2 capture - II

I evaluate the process economics of algal CO2 capture from cement plant, using the GreenFuel Holcim facility mentioned in a previous post. Internal rates of return (IRR) and payback periods for various scenarios are presented. As shown in the above figure, both increased yields as well as higher oil prices significantly influence the economics of algal CO2 capture.

Base case
Capital expenditure: 92 million $, CO2 fixed: 50,000 T/year (2011).
Algal oil production: 1.3 million gal/year.
Cost algal oil: 4 $/gal.
Price of CO2 offsets: 20 $/T CO2.
Timeline considered for IRR calculations: 10 years.

The rest of the scenarios are explained in the figure. Doubling the yields (and CO2 captured) does increase the IRR and lower the payback periods more than doubling the oil prices (mentioned in my last post). Moreover, CO2 trading plays only a minor role by itself, but results in higher IRRs and lower payback periods when considered along with other possibilities. The highest IRR and lowest payback occur when both yields as well as the oil prices are significantly higher than in the base case scenario.


Nov 7, 2008

Comments: Marine cement production process

Recently, Calera proposed a process suitable for power plants and other major CO2 emitters which converts CO2 to calcium carbonate (CaCO3). From the Scientific American:

"...The Calera process essentially mimics marine cement, which is produced by coral when making their shells and reefs, taking the calcium and magnesium in seawater and using it to form carbonates at normal temperatures and pressures. "We are turning CO2 into carbonic acid and then making carbonate," Constantz says. "All we need is water and pollution...
....Calera hopes to get over that hurdle quickly by first offering a blend of its carbon-storing cement and Portland cement, which would not initially store any extra greenhouse gases but would at least balance out the emissions from making the traditional mortar. "It's just a little better than carbon neutral," notes Constantz, who will make his case to the industry at large at the World of Concrete trade fair in February. "That alone is a huge step forward."..

In the following, I discuss some of the critical challenges to make this a feasible operation. The roles of the abundance of calcium and magnesium in sea water and its pH are discussed in detail. The formation of calcium/magnesium carbonates from sea water requires energy to proceed. This energy could be supplied either in the form of a pH shift (by adding strong base) or by coupling the carbonate formation to other processes (algal photosynthesis). Why does blending calcium carbonate in cement make economic sense? The unit price for medium ground (4-9 um) CaCO3 is around 95-100 $/T whereas portland cement retails at ~180 $/T . Each ton of CaCO3 blended will save 80 $.

Sea water has 0.01 gram-mol Ca2+/kg-sea water and 0.05 gram-mol Mg2+/kg-sea water . The precipitation chemistry of calcium and magnesium carbonates is complicated, involving many minerals. If we precipitate all of the Ca2+ and Mg2+ as carbonates (CaCO3 and MgCO3), sequestering 1 T of CO2 would require 360 T of sea water. Additionally, carbonate precipitation does not occur below a pH of ~10, whereas sea water has a pH of ~7.5-8.4 , which could be decreased by increasing the partial pressure of CO2. Increasing the pH of the solution to favor carbonate precipitation likely requires the use of a base, such as sodium hydroxide, NaOH.

Apart from the large volumes of sea water to be handled to make carbonates, the manufacture of NaOH involves the electrolysis of brine and is fairly energy-intensive, producing 4.6 T CO2/T NaOH. Depending on the extent of NaOH requirements, this could represent a significant source of CO2 emissions. (Therefore, using NaOH to sequester CO2 does not make much sense).

Switching gears to learning from nature, certain coralline algae form CaCO3 from sea water. This CaCO3 acts as a binder, stabilizing the coral reef. The mechanisms involved are fairly complex for my understanding, but an essential process is the equilibrium between the bicarbonate and carbonate ions, depending on the pH.

pH = 10.33 - log([HCO3-]/[CO32-])

Under sunlight, algae photosynthesize dissolved CO2 or HCO3- to form organic compounds (CH2O). This leads to an increase in the local pH. An example of how this happens is given in a paper by John Dodson (subscription required). This increased pH combined with the presence of ion-selective membranes, which admit only certain ions might be how algae use photosynthesis to drive an endothermic CaCO3 precipitation reaction.

  1. Large volumes of sea water are required to sequester CO2 as calcium/magnesium carbonates. Membrane separations will likely play a critical role in reducing this requirement.
  2. On the other hand, CO2 sequestration in brines (by product of oil and gas production) also has been investigated. The advantage here is that these brines have high concentrations of calcium, iron and magnesium ions. Therefore, potentially less volumes of brine are needed compared to sea water to sequester the same amount of CO2. The obvious problem here is to increase the pH of the solution to precipitate dissolved CO2 as carbonate. Instead of using a strong base such as sodium hydroxide, folks at NETL used residues from bauxite (an ore of aluminium) processing to buffer the solution pH.
  3. One needs a driver to precipitate calcium/magnesium carbonates from sea water. In the chemical process described above, it is the addition of sodium hydroxide.
  4. The smarter way nature has found around that requirement is the formation of carbonates via the bicarbonate<->carbonate equilibrium, shifted towards the right by algal photosynthesis (a reaction converting bicarbonate/CO2 to organic carbon using sunlight, thereby increasing the pH).


Nov 3, 2008

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