592
593 594
4.1 Operation guidelines for controlling NO
xemission in blasting operations
595596
The Australian State of Queensland’s Guidance Note 20 (DEEDI, 2011) provides useful
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precautionary measures for preventing, controlling and managing the formation of NOx 598
fumes in open cast mining to minimise the contamination of atmospheric environment. Note
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20 comprises standards for manufacturing, storage time-limit, selection of proper initiating
600
devices, better design of mine shots and confinements, as well as appropriate planning and
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personnel training. At present, all major manufacturers of AN explosives offer formulations
602
that can tolerate wet conditions, usually, with the resistance to these conditions related to the
603
loading of emulsion in prill/emulsion (heavy ANFO) mixes. In practice, selection of
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appropriate explosive formulation, dewatering of holes prior to loading and minimising the
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sleep time prior to the detonation display the most significant impact on preventing the fume
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formation (DEEDI, 2011). Moreover, from a managerial perspective, Note 20 recommends
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considerations for establishing fume management zones (FMZ) and determination of blast
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exclusion zones (BEZ) that account for effects of meteorological (e.g., wind speed and
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direction, temperature, humidity, etc.) and geological conditions. FMZ represents an area
610
likely to contain fumes after blast, requiring workers to remain outside the zone. For
611
industry, managing exclusion zones adds to the cost of mine operations.
612 613 614
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304.2 Abatement techniques
615 616The NOx mitigation technologies employed in post combustion plants (Baukal et al., 2001; 617
Dvořák et al., 2010; Gómez-García et al., 2005; Hill and Smoot, 2000; Skalska et al., 2010)
618
do not apply to blasting activities. This is because: (a) the post-explosion atmospheric
619
mixture does not yield itself to capturing, scrubbing, treatment and/or reprocessing; (b) AN
620
exhibits incompatibilities with materials (Table 3) (Bretherick et al., 1999) that could serve
621
for scavenging NOx. However, strategic research into NOx abatement led to the development 622
of the following mitigation technologies:
623 624
Table 3.
625 626
(i) Alkalimetric neutralisation: One of the earliest studies on tackling the formation of
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NOx in blasting operation involved the use of neutralising additives. Azarkovich 628
(1995) proposed the addition of inexpensive substances (to AN explosive charges)
629
that possess the capacity of binding oxides of nitrogen. The author recommended
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additives that are able to react as alkalis, viz., bases and salts of weak acids.
631
Azarkovich (1995) reported the neutralisation reaction of NO2 occurring in a Dolgov
632
bomb and an underground chamber, especially for additives such as slaked lime
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Ca(OH)2, chalk CaCO3 and soda Na2CO3, uniformly spread into the charge in small
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aliquots of 0.1 – 0.2 mass %, or added into sand stemming. (Stemming denotes the
635
practice of placing soil, sand or rocks on the explosives in the blast hole.) Equations
636
30 – 32 suggest the operation of the additives that reduce NO2 emissions by 40 – 80 637
%, as monitored by an electromagnetically relayed air sampling vessel (Azarkovich,
638
1995; Ishibe et al., 1995).
639 640
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31 Ca OH 2+2NO2+ 1 2O2 ⇌ Ca NO3 2+H2O (30) CaCO3+ 2NO2+ 1 2O2 ⇌ Ca NO3 2+CO2 (31) Na2CO3+2NO2+ 1 2O2⇌ 2NaNO3+CO2 (32) 641(ii) Application of stabilising and scavenging additives: In the search of enhancing the
642
stability of AN, Oxley et al. (Oxley et al., 2002) screened a large number of
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formulations comprising grounded salt of sodium, potassium, ammonium and calcium
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sulfates, phosphates, carbonates, oxalates, formates, urates and guanidinates at about
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10 mol %. The authors revealed that, the more stabilising the additive, the lower the
646
N2O/N2 ratio, i.e., the more selective N2 formation. Although Oxley et al. (2002) did
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not report measurements of NO and NO2 concentration, the insight into the selectivity
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towards N2 suggests NOx reduction. Opoku and Dlugogorski (2012) and Opoku et al. 649
(2014) inserted additive (non-ammonium) nitrates into the nanocrystalline structure of
650
AN, and studied their effect on formation of NOx in deflagration of AN. The authors 651
showed that additives, especially potassium nitrate that forms 5 mol % K in the co-
652
recrystallised AN salt, achieve up to 40 % reduction in NO emission.
653 654
(iii) Reburning-like technique: In boilers, reburning abates NOx by using a supplementary 655
fuel to reduce NOx (Wendt et al., 1973), to achieve an overall decrease in NOx 656
emission of about 50 – 85 %. Reburning constitutes a three-stage process. In the first
657
stage (primary zone), the main fuel burns under slightly fuel-lean condition. The
658
produced NOx then reacts with supplementary hydrocarbon radicals in the “reburning 659
zone” to form intermediate nitrogenous species, and then molecular nitrogen (N2).
660
Finally, addition of air in the last stage (burnout zone) completes the combustion
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32process by oxidising all unreacted fuels and N-species (Hill and Smoot, 2000; Smoot
662
et al., 1998; Oluwoye et al., 2017a). These interactions involve several elementary
663
steps summarised in the following overall equation (Smoot et al., 1998).
664 665
CiHj + NO → HCN +... (33)
666
In presence of NOx, the HCN decays through a series of intermediates, and ultimately 667
reaches N2 via the reverse Zeldovich reaction (Equation 37):
668 669 HCN+O→NCO+H (34) NCO+H→NH+CO (35) NH+H →N+H2 (36) N+NO→N2+O (37) 670
Many practitioners applied reburning-like technology to reduce NOx emission in 671
blasting of AN explosives adding supplementary fuel (usually solid) of higher
672
oxidation stability compared to fuel oil used in AN explosives. As seen in Figure 6,
673
the fuel with lower oxidation stability (fuel oil) participates solely in primary
674
detonation reaction, with the resulting NOx species subsequently reduced to nitrogen. 675
However, the excess fuel conditions, if they arise in the reburning of detonation-
676
generated NOx, may lead to formation of CO, as shown in Equation 10. 677
678
Figure 6.
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33Sapko et al. (2002) demonstrated that, the use of pulverised coal dust (mean particle
681
size of 74 µm), as an additive, mitigates the total NOx emission by 10 – 50 %. 682
Likewise, biomass briquette and advanced reburning materials such as urea (Smoot et
683
al., 1998) offer better NOx-mitigation performance (Oxley et al., 2002; Sapko et al., 684
2002). Other fuel additives include non-hazardous waste polymers, such as
685
polyethylene (Oluwoye et al., 2015, 2016, 2017b).
686 687
(iv) Chemical trapping: Spin traps can provide an effective reduction of the overall
688
release of nitric oxide in sensitisation (chemical gassing) of emulsion explosives.
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Spin trapping reaction removes radical species, such as NO, by forming stable
690
adducts. Venpin et al. exploited the spin-trapping technique to develop NO
691
scavengers to control NOx emission from industrial processes, including sensitisation 692
of emulsion explosives (Venpin et al., 2012, 2013a, b). In their experiments, they
693
applied aromatic ortho substituted nitroso compounds, such as nitrosobenzene
694
sulfonate (NBS), 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS), 3,5-dimethyl-4-
695
nitrosobenzene sulfonate (DMNBS) and 3,5-dichloro-4-nitrosobenzene sulfonate,
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obtaining up to 70 % removal efficiency of NO during sensitisation of AN emulsion
697
blends. Equations 38 – 40 depict the reaction pathways leading to the formation of N2
698 (Venpin et al., 2012): 699 700 X X NO + NO X X N O NO X X N O N O
Aromatic nitroso sulf onate
X = H, Br, CH3and Cl Nitroso-NO adduct
O3S O3S O3S
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34 (39) (40) 701(v) Use of alternative oxidising agents: Even though the AN technologies have been
702
established for applications in civilian explosives, there exist at least one non-nitrogen
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based formulation. Araos and Onederra (2015) replaced AN with hydrogen peroxide
704
(H2O2), and used micro balloons, rather than chemical gassing, to avoid NOx 705
formation both in detonation and in emulsion sensitisation. Such H2O2/fuel blends
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detonate with heat release and velocity of detonation similar to AN explosives (Araos
707
and Onederra, 2015; Armstrong et al., 2013; Sheffield et al., 2010), but without NOx 708
hazards. The technology requires further testing to verify its large-scale performance.
709 710
Some noteworthy patents include formulating the blasting agent to contain from about 1 % to
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about 20 % silicon powder (Granholm, 2003), or appreciable amount of urea in the
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discontinuous oxidiser salt phase (Forshey and Mason, 1973; Granholm and Lawrence,
713
1997), improved composition of hydrogen peroxide based explosives (Araos, 2013),
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application of other nitrogen-free oxidisers (Day, 1999), and appropriate gassing method
715
(Dlugogorski et al., 2009).
716 717 718