Rducing cements CO2 footprint Hendrik G van Oss

12
Cement,
confronting
ecological
responsibility
and economic
imperatives
Reducing
cement’s CO2 footprint
The manufacturing process for Portland cement causes high levels of greenhouse
gas emissions. However, environmental impacts can be reduced by using more
energy-efficient kilns and replacing fossil energy with alternative fuels. Although
carbon capture and new cements with less CO2 emission are still in the experimental
phase, all these innovations can help develop a cleaner cement industry.
Hendrik G. van Oss
U.S. Geological Survey
H
ydraulic cement, chiefly Portland
cement or similar cement having
Portland cement as a base, is the
binding agent in concrete and most mortars,
and is thus a key component of construction
activity worldwide (van Oss and Padovani,
2002). Hydraulic cements derive their
strength through the hydration (chemical
combination with water) of their component
cement compounds or minerals. World output of cement in 2009 of about 3 Gt was sufficient for about 24 Gt of concrete, or about
3.5 metric tons (t) of concrete annually per
person on the planet. Worldwide, concrete
is thus the most abundantly manufactured
material. Most of the environmental issues
surrounding cement production concern
the manufacture of clinker, the dominant
issue of global concern being emissions of
carbon dioxide (CO2), an important greenhouse gas (GHG).
Manufacturing Portland cement involves
converting limestone and a variety of other
raw materials into clinker, and then grinding this with about 5% of calcium sulphate
and other additives into a fine powder. The
composition of clinker does not vary much
worldwide, and most production involves
rotary kilns that share a similar technology. Thus, the manufacturing process for
HENDRIK G. VAN OSS
Hendrik van Oss is an economic geologist who since 1996 has
worked for the U.S. Geological Survey’s National Minerals Information
Center as a commodity specialist covering cement, ferrous slags, and
coal combustion byproducts. Between 1988-1995, he was a country
specialist with the U.S. Bureau of Mines, and prior to that, he spent
a decade in the mineral (chiefly gold) exploration industry in the
western United States.
www.proparco.fr
cement, and the associated environmental
issues, are common worldwide.
Calcination and heating requirements,
highly CO2-emissive
Clinker is composed mainly of four oxides:
about 65% calcium oxide or lime (CaO), 22%
silicon dioxide (SiO2), 6% aluminum oxide
(Al2O3), and 3% ferric (iron) oxide (Fe2O3). The
remaining 4% is made up of minor amounts of
oxides of magnesium (MgO), usually less than 2%, and various alka- “Most of the
lis. In ‘straight’ Portland cement, environmental issues
the major oxides are combined surrounding cement
within four hydraulically reactive production concern
cement minerals in the clinker the manufacture
component – tricalcium silicate of clinker.”
or ‘alite’ (C3S, typically 50-55%),
dicalcium silicate or ‘belite’ (C2S, 19-24%), tricalcium aluminate (C3A, 6-10%), and tetracalcium aluminoferrite (C4AF, about 7-11%), to
which about 5% gypsum is added.
Because of the predominance of calcium oxide
(C), raw materials must include an abundant
and inexpensive supply of it for clinker manufacture to be practicable. Traditionally, calcium oxide has been supplied by limestone or
similar rocks. Limestone is mainly composed
of calcite, which is calcium carbonate (CaCO3).
This reliance on limestone is the root of most
of the environmental problems associated
with cement manufacture.
Calcium carbonate in the raw material mix is
thermally decomposed in the kiln to make its
calcium oxide available, via a reaction called
calcination. Because calcium carbonate is composed of 56% CaO and 44% CO2, calcination
releases a great deal of this GHG. If calcium
carbonate is the only source of CaO available
to the kiln, the plant will need to calcine 1.16
metric tons (t) of CaCO3 to yield 1 t of clinker
of 65% CaO content, and this calcination will
release 0.51 t of CO2.
13
The energy required for calcination is enormous. Because most limestones are not pure
calcite, the mass ratio of limestone to clinker
will actually be more like 1.5 instead of 1.16,
and along with some other materials such as
clay and silica sand, it takes about 1.7 t of total
raw materials to make 1 t of clinker. Calcination of the raw material takes place at 7501,000°C, with the heat being supplied by the
combustion of fossil fuels, chiefly coal and
petroleum coke, which releases more CO2.
Once calcination is complete, the subsequent
formation of C3S, C2S, C3A, and C4AF requires
only a little extra heat, even though the reactions occur at higher temperatures (1,0001,450°C). This is in part because some of their
formational reactions, especially
“Developing that to form C3S (via the reaction
countries have more C2S + C à C3S), actually release
modern cement heat (are exothermic).
plants than many Overall, to make 1 t of clinker,
developed countries.” about 3.9 billion joules (GJ) of
heat are required. This is for a dry
kiln, which takes its raw materials in a dry
state. Some older plants, however, operate
wet kilns, which take their raw material mix in
the form of a slurry containing about 35-40%
water. For wet kilns, evaporation of this water
ahead of the preheating step will require an
additional 1.6-1.8 GJ/t of clinker.
Kiln technology determines fuel needs
These heat requirements are notional; in reality, it is usually higher because of varying
amounts of heat loss from equipment (particularly the enormous kiln tubes). But there
are opportunities for saving heat, particularly relating to the combustion air/exhaust
and the air used to cool the clinker. On the
latter, clinker emerges from the kiln red hot
and must be cooled in a dedicated apparatus to 100-200°C before it can be ground into
cement. This superheated air can be rerouted
to the kiln burner or be used for preheating
raw material, saving fuel.
Most kilns built in recent years are modern preheater-precalciner kilns. The majority of new cement plants in recent years have
been built in developing countries, leading to
developing countries typically having more
modern cement plants than many developed
countries. However, because of ongoing plant
FOCUS
The U.S. Geological Survey (USGS), established in 1879, is the
major scientific agency within the U.S. Department of the Interior,
and conducts studies of the overall geological and biological
framework of the United States (and overseas), with the emphasis
on mineral resources; geological, biological, and topographical
mapping; geological hazards; and water resources.
upgrades, installed kiln technology varies
worldwide.
Abandoning wet kilns in favour of dry kilns,
and upgrading or replacing older dry kilns
with more modern dry technology, improve
fuel efficiency. Data from the U.S. illustrates
this: in 2007 wet kilns required an average of
6.5 GJ/t of clinker; long, dry kilns averaged
5.3 GJ/t; preheater kilns averaged 4.1 GJ/t,
and preheater-precalciner kilns averaged
3.6 GJ/t. Preheater kilns use a separate apparatus for preheating rather than the less efficient kiln tube, and the heating is from hot
‘waste’ air. Preheater-precalciner kilns also use
an independently fueled calciner apparatus
that is far more efficient than a kiln tube for
the calcination task; the kiln tube only has to
perform the final stage of clinker mineral formation. Efficiencies of scale were also evident:
larger plants tended to be more fuel efficient.
Additionally, most plants offer opportunities
to achieve smaller improvements in efficiency
through upgrades or ‘tweakings’ of existing
systems, especially for electricity savings. The
results can be cumulatively significant.
Reducing fuel consumption or using alternative fuels
CO2 emissions from fuel combustion are typically around 0.40-0.45 t CO2 per 1 t of clinker,
and with calcination emissions added, the total
becomes about 0.91-0.96 t of CO2, unless lowercarbon fuels and/or non-carbonate sources of
calcium oxide are used. Some hydraulic cements
have a lower clinker content than Portland
cement; still, it is estimated that at current
manufacturing rates, the world cement industry releases about 2.2-2.6 Gt of CO2 annually.
To save fuel , the cement industry has long been
on a trend of lowering its per-unit (per ton of
product) energy consumption, by installing
modern technology. Ongoing modernisation is
part of the industry’s strategy to further reduce
CO2 emissions (U.S. Environmental Protection
Agency, 2010).
Cement plants can also burn a wide variety of
alternative fuels (AF), including a variety of
industrial wastes, some hazardous. Many of
these have a lower carbon content than conventional fuels. In carbon-emissions-reporting protocols, deductions may be allowed for AF use.
Likewise, deductions may be allowed for biofuels (including the natural rubber content of
used tyres). Biofuels are generally considered
to be carbon-neutral in climate change modelling. Constraints on the use of AFs include environmental permission (especially for hazardous
waste fuels); availability in sufficient quantity;
cost of procurement, storage, and blending; and
quality, as they are commonly more variable
Private Sector & Development
14 Reducing cement’s CO2 footprint
Cement,
confronting
ecological
responsibility
and economic
imperatives
in their heat and moisture contents than
conventional fuels.
Three other current practices are also part of
the emission reduction strategy; two of these
reduce overall plant emissions, and all three
reduce emissions on a per-ton of cement basis.
Towards a greener cement mix
Cement plants can make use of a wide variety of alternative raw materials (ARM), in
addition to traditionally used materials,
such as limestone. Among the ARMs in common use are the industrial ‘wastes’, such
as coal ashes from power plants, iron and
steel slags, and industrial residues. Of particular interest are the slags and coal ashes,
many of which have a similar composition
to clinker. Most importantly, certain ARMs
(but especially ferrous slags) can be significant non-carbonate sources of CaO, reducing limestone consumption and attendant
calcination emissions of CO2 during clinker
production. These ARMs require less heat
for combustion, reducing fuel consumption
and fuel emissions of CO2.
Limits on ARM use revolve around availability and cost (especially for transport),
environmental permission for their use,
and their oxide balances. Within these limits, by consuming ARMs, the U.S. industry
in recent years has reduced its calcination
emissions of CO2 by about 0.7-1.3 million tons per year (or about 2.4-3%); reductions at the ARM-using plants themselves
have been in the range of 2-10% or so. It is
harder to gauge the reduction of fuel-related
emissions, but ARM-consuming plants typically have energy consumption levels 3-30%
lower than U.S. industry averages for the
kiln technologies concerned.
The clinker content of finished cement can
be reduced by incorporating supplementary
cementitious materials (SCM), such as fly
ash, ground granulated blast furnace slag,
silica fume, metakaolin, and pozzolanic volcanic ash, to make blended cements. These
have many of the same uses in concrete
manufacture as Portland cement. The use of
SCM reduces the carbon ‘footprint’ attributable to the cement industry, but most SCMs
derive from industries that also emit CO2.
SCMs develop their cementitious properties
by reacting with the CaO released during
the hydration of Portland cement. Concrete
producers can also directly introduce SCMs
into the concrete mix to reduce the Portland
cement (hence clinker) content. In either
case, the use of SCMs commonly improves
the quality of the concrete. Typical SCM
contents in blended cements worldwide,
www.proparco.fr
and substitution ratios for Portland cement
in concrete, are in the range of 5-50%, but
can be higher for some applications. Limitations on the use of SCMs mainly revolve
around availability and whether local building codes allow their use.
The clinker proportion of cement can be
further reduced, where allowed, by incorporating relatively inert bulking agents or
extenders, the most common of which is
(uncalcined) ground limestone. Incorporation can be as high as 20% or more in some
Portland-limestone cements, but is typically less than 10%. Between inert extenders and SCMs, the world average clinker
content of hydraulic cement is currently
around 75-80%, compared with about 95%
for traditional ‘straight’ Portland cements.
Importantly, although using SCM or other
extenders in cement and concrete does not
reduce the cement industry’s emissions of
CO2, overall, it reduces unit emissions and
thus allows more cement (and concrete) to
be made from the same amount of clinker.
Carbon sequestration
at experimental stage
Because they are large stationary emitters of
CO2, cement plants are considered good candidates for the future incorporation of carbon
sequestration technology, especially if the CO2
content of the exhaust stream can be concentrated through using oxygen, rather than air,
for combustion. A concentrated CO2 stream
reduces the overall volume of gas to be processed, and may reduce the size of the sequestration facility needed as well as the consumption of any absorptive reagents. Proposed
sequestration methods include producing
a CO2 gas or liquid stream for use elsewhere
or for permanent underground injection,
absorption by some reagent, which would
need to be disposed of, and the formation of
a marketable product such as sodium bicarbonate. Overall, carbon sequestration technologies for cement “Cement plants can
plants are presently perceived burn a wide variety
as being largely experimental, of alternative fuels.”
costly, and, for some proposed
systems, requiring a facility of similar size to
the cement plant itself. Very few plants have
as yet installed carbon sequestration technology and, indeed, it may be unaffordable for
many smaller or older plants. Also, with few
exceptions, cement plants are situated next to
limestone quarries, and these locations may
not be conducive to future CO2 transport piping infrastructure.
Cement plants have been cited as potential
consumers of the calcium carbonate formed
15
by some new CO2 sequestration technologies or by so-called calcium-looping circuits
proposed for thermal power plants. The use
of such calcium carbonate by cement plants
would, of course, return the CO2 to the atmosphere, but would at least reduce the need for
the plant to burn its own limestone.
New cements to be considered
in the medium to long term
Although made today in tiny quantities, a
number of new cements have been developed
in recent years that could be suitable for at least
some forms of construction.
“The few new Among these are geopolymer
cements are several cements and several MgO-based
hundred dollars binders. Advantages claimed for
per ton more these cements include a lower
expensive than energy (heat) required for manuPortland cement.” facture and hence lower CO2 emissions, and for MgO binders, that
they actually absorb CO2 from the air and may
thus be CO2-neutral or even net-negative. MgO
binders develop strength through ‘carbonation’.
Apart from issues in getting any new cement
accepted into local and national building codes,
there are constraints on the widespread use of
binders (CaO or MgO) that work via carbonation. Carbonation (hence strength-development) requires sustained exposure to the atmosphere, and although suitable for some high
surface area applications (such as stuccos, thin
slabs, and small blocks), this may not occur sufficiently rapidly in bulk concrete applications
where CO2 permeability could be problematic,
and raw materials of sufficient purity for MgO
binder manufacture may be more limited in this
regard than for Portland cement.
Even where shown to have suitable strength,
durability, and applicability, to significantly
reduce CO2 emissions, billions of tons of these
new cements will have to be manufactured
annually. The new cements will have to compete
against an established output from thousands
of Portland cement plants worldwide, representing billions of dollars in investment. And the few
new cements are several hundred dollars per ton
more expensive than Portland cement. Yet the
cost of Portland cement has been increasing over
the years, largely because of fuel cost increases,
and is likely to continue to increase over the long
term. If some of the new cements could be manufactured in large quantities, economies of scale
would be realised in their production costs, and
within the next 30-50 years, some may become
cost-competitive with Portland cement. During that time, many existing Portland cement
plants may have exhausted their local limestone
reserves, or their equipment may be in need of
replacement, and the original cost of the plants
will have been fully amortised. At that time, the
world may enter a post-Portland cement age.
References
/ U.S. Environmental Protection Agency, 2010. Available and emerging technologies for reducing GHG emissions from the Portland cement industry, rapport, October. // U.S.
Environmental Protection Agency, 2011. Inventory of U.S. greenhouse gas emissions and sinks-1990-2009, report April 15. // van Oss, H.G, 2011. Cement: chapter in the U.S. Geological Survey Minerals
Yearbook. // van Oss, H.G., and Padovani, A.C., 2002. Cement and the environment-Part 1, Chemistry and technology, Journal of Industrial Ecology, volume 6, n°1, January, 89-105. // van Oss, H.G., and
Padovani, A.C., 2003. Cement and the environment-Part 2, Environmental challenges and opportunities, Journal of Industrial Ecology, volume 7, n°1, January, 93-126.
Private Sector & Development