Page 242 - New Trends in Eco efficient and Recycled Concrete

210 New Trends in Eco-efficient and Recycled Concrete

combustion and the processing of raw materials in the kilns. Around 70% 95% of
the fraction not attributable to energy production is absorbed due to the high alka-
linity of clinker (Houghton et al., 1996). Thus, the majority of the SO 2 leaves the
kiln with the clinker (Schuhmacher et al., 2004). The NO x values refer to both NO 2
and NO emitted to the air. These are mainly an output from fuel usage during clin-
ker production and energy consumption throughout the process chain.
Local scale: Cement kiln dust (CKD) emissions are the main contributors to the
local impact. The size of CKD (0.05 5 μm) is within the size range of respirable
particles (Schuhmacher et al., 2004). Since the diameter is smaller than 10 μm,
10
CKD is classified as PM . According to the Environment Protection Agency
(EPA) (EPA, 1994, 1999), these fine particulates of unburned and partially burned
raw materials present in the combustion gases of the cement kiln, are considered as
a potential hazardous waste due to their caustic and irritative nature. As listed in
Table 9.1, the amount of CKD generated per kg of clinker produced equals about
15% 20% (by mass) (Van Oss and Padovani, 2003). However, it must be said that
the latter estimation was based on very limited, informal data and conversations
with plant personnel. The discontinuous nature of CKD measurements is another
reason that could explain the large variability in the reported emission values. It is
also not always clear which are the CKD emissions that are included in the reported
values. This could be the fraction that is initially captured by the electrostatic preci-
pitators or the baghouse filters, the fraction that is returned to the kiln or only the
fraction that is landfilled. Nowadays, both the environmental and health risks asso-
ciated with CKD can be reduced significantly by means of mineral carbonation. As
observed in the carbonation of other industrial wastes, sequestering carbon may
yield additional benefits by stabilising the waste (reducing the pH) which reduces
health risks and the generation of harmful leachate (Huntzinger and Eatmon, 2009).
In addition, the utilisation of CKD for carbon sequestration by means of mineral
carbonation appears to have its advantages on the global scale, since about 7% of
the carbon emissions from the cement kiln can be captured this way (Huntzinger
and Eatmon, 2009).
On the local scale, attention should also be paid to the emission of metals and
polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). After emission
into the atmosphere, these chemicals can be transmitted to humans through direct
(air) and indirect (groundwater, soil, vegetation) pathways. Health risks turned out
to be quite low (Schuhmacher et al., 2004). When located in an urban area, noncar-
cinogenic and cancer risks derived from exposure to metal and PCDD/Fs coming
from a cement plant were within the ranges acceptable according to national and
international regulations, with the exception of only a few elements (e.g., As and
Cr) (Rovira et al., 2010). In addition, no significant increases in the environmental
levels of metals and PCDD/Fs were detected when comparing a conventional fossil
kiln fuel with an alternative fuel (15% on average partial substitution of fossil fuel
by refuse-derived fuel from municipal solid waste). Also, no changes in airborne
particulate matter were noted, while significant reductions were found for a number
of pollutants (PCDD/Fs, Co, Cr, Mn and Ni) in vegetation, as well as in soil (Ni)
and air (Sn) (Rovira et al., 2010).