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).


adolfo said...

Most of the cement companies involved operate facilities in multiple markets, including many developing economies. As the Initiative moves from research and planning to implementation, it is expected that smaller cement companies will be recruited to join and participate, both for training and technology transfer purposes. Participants would be engaged in guideline formulation, establishing monitoring and reporting programs, and communicating results.

Anonymous said...

I think you ignore the concentration of metals in coal fly ash like Mercury that make disposal in cement kilns - which have no Mercury pollution controls - unattractive.

Also, what about bio-fuels like Sawgrass, etc?

Pradeep said...

"I think you ignore the concentration of metals in coal fly ash like Mercury that make disposal in cement kilns - which have no Mercury pollution controls - unattractive."
Fly ash is milled with cement clinker and gypsum in a separate step downstream of the cement kiln. Therefore, there is no danger of the mercury volatilizing at high temperatures. Most of this mercury (Hg) ends up in the cement concrete. I have not looked for studies claiming Hg releases from concrete products specifically.

I think Class C fly ash is considered safe enough to make bricks (which replace the coal-fired clay bricks), The Green Brick Company already does this.

BTW, most of the Hg in the coal ends up in the vapor/gas phase, at least for coal combustion.

About cement kilns not having Hg controls, the conversations I have had with people in the industry indicate that the EPA is still looking into this and will probably come up with a ruling.

Using saw grass etc: This is a good option, if the saw grass is cheaper than coal or other fuels. However, fuel contributes to ~4% of the CO2 emissions of cement kilns, most of the CO2 emissions are a result of the raw material calcination. Cement production and concrete manufacture essentially is as follows:
1. Mining limestone, clay, alumina etc.
2. Milling the raw materials in desired proportions.
3. Calcining the limestone to form lime (CaO), and further reactions of the lime with silica and alumina forming calcium {alumino) silicates (clinker).
4. Milling of the clinker with additives (fly ash, gypsum, limestone) to produce various grades of cement.
5. Mixing this cement with sand, and other aggregates to produce concrete.
6. As the concrete ages in buildings, it reacts with atmospheric CO2 and water and undergoes hydration. The CaO in cement reacts slowly with CO2 forming calcium carbonate again.

Essentially, in the overall cement life cycle, man takes CO2 out of CaCO3, forms calcium (alumino) silicates and Nature puts CO2 (and water) back into the mix slowly.

Gerald said...

Because the calcination process in cement production makes up about 60% of the CO2 released (40% from combustion)it would be very useful to have separate streams,(calcination and combustion). Basically not allowing the two gases to mix in the process. The calcination component would be relatively pure and could be geologically stored. Under this concept cement manufacturers could make much headway toward the future cap and trade goals (europe, and soon to be in the USA). The capture component of CCS is very expensive, around $40/ mt. One might also use some of the calcined component as a scrubber in the combustion stream with regeneration on the calcination side. There has been a lot of work done on preheaters and precalcination, but this all needs to go to the next step.

What do you think?

Gerald the geologist

Pradeep said...

Interesting idea. The cement process technologies I know of involve direct-contact heat exchange.

IMO, essentially, the function of the preheaters and precalciners is to increase the effective length of the kiln, so that drying and calcination can happen much earlier in the process..this increases the plant output for a given energy input.

What you propose might be feasible with oxy-fuel combustion. Here, instead of burning the fuel in an atmosphere of air, the combustion takes place in a O2:CO2 atmosphere.

Apart from the challenges of figuring out the clinker:CO2:O2 chemistry to preserve the clinker quality, cryogenic air distillation plants have a big energy/land foot print.

In this context, novel air separation processes become significant. For example: Air Products' ion transport membrane (ITM).

I should also point out that capture of the CO2 does not lead to carbon credits, it is the effective storage or utilization component that closes the deal.

Anonymous said...

Anyone out there know, or able to guesstimate, what the impact is on the emissions of dust from five mills and separators of blending in all sorts of "toxic" fillers like fly ash, and bypass dust from the co-incinerating cement industry? If you look at you can see a 2 million tonne a year Cemex plant that was built and operated in 2000 without a valid planning or operating permit. Year on year production increases as do the HGVs and the use of waste fuels and industrial waste substitutes for raw materials such as slag, mill scale, aluminium dross, foundry sand etc, and of course emissions also increase, but most are unmonitored and only "sampled" twice a year.

You can see that it is very close to the town and has many vents and stacks as well as the main 115 metre stack. The stack emits about one million cubic metres gas each hour - or more - and the low level sources emit about another 600,000 cubic metres an hour of particulate laden gas/air in "what they call pure air".

The EA refuse to give us any information about the emissions from the mills and LLPS and they say continuous particulate monitors were fitted about 2 years ago. Cemex however say that RMC fitted these monitors in 2004. Which is true - anyone's guess! But they both refuse to allow us to see any data from these emission sources, but we have found out several occasions when the twice yearly contractor does the tests as in the permit and finds sometime the mills running at SEVEN times the permitted 30 mg/m3, i.e. at 20,000 micrograms/m3. The local people are frequently covered in thick sticky dust and they refuse to tell us what is in it and refuse access to information which heightens the very real suspicion that they are dumping even more pollution on us than we can find out about.

They also break the ELVs for the main stack for dioxin, particulate, etc and we are very concerned about the dust that is coming out of the stack and also particularly from the LLPS - with all the waste blended into the clinker. There is also mercury and arsenic etc from main stack and the UK's EA says it has no capability of testing for these pollutants - even chromium 6 etc though the USA EPA does test and recently found the Davenport Cemex plant to be emitting high levels of this cancer causing agent from the burning of mill scale and slag as a replacement for the iron oxide.

Has anyone got any idea, or any peer reviewed reports, about the impact on the emissions from all these substitutions in the mills. They also refuse to give us the mandatory EU waste catalogue number and just keep on saying misleading things - like the bypass dust is not hazardous, even though they know it is.

Any help anyone please?
Thank you.


Pradeep said...

As far as I know, foundry sands should not contribute to heavy metal emissions. I do not have direct experience in air quality issues, but does the EA check the air quality around the town regularly?
I think the main issue is to know what is being emitted and establish if the emissions are toxic. Various heavy metals end up in different places during the cement production process depending on the volatility.

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