As a follow-up to my last post, here is some supporting information on PVC and HCl/Cl2 in fires: Thermal Degradation of PVC and the Fate of HCl Emissions
The thermal degradation of pure PVC and PVC blended with plasticizer and other additives has been extensively studied for a wide range of formulations heated in air, inert gases or under vacuum. Dehydrochlorination at ~ 300 °C is the dominant decomposition reaction for all formulations although the initiation temperature depends on the particular additives and may be as low as 250 °C. Nevertheless, the loss of HCl from PVC is invariably close to the theoretical maximum of 56.7 % of the sample weight for all formulations heated above 400 °C.
- [CHCl – CH 2 ] n - + Heat = - [CH = CH ] n - + n HCl
See for example: J. Polymer Sci. Vol 12, 737, (1974); Vol. 16, 3139, (1978); Vol. 18, 3101, (1980); Polymer Vol 25, 1337 (1984), etc.
HCl is an extremely corrosive and reactive gas. In fact it has been reported that the concentration of HCl in a fire involving PVC “decreases rapidly after its generation due to its consumption in reaction with almost all the surface of the building. Accordingly, the actual concentration of hydrogen chloride in the atmosphere of a fire accident involving PVC is far lower than the calculated value. For instance, some reports say that the hydrogen chloride concentration is about 10% of the calculated value in a fire involving PVC made cable insulator.”
In the case of the fires in WTC 1 & 2, HCl was released not only from the electrical wiring but also from PVC-based plastics in materials such as flooring tile, window blinds, TV monitors, etc, present in the workstations on the fire-affected floors. NIST NCSTAR 1-5C states that plastics accounted for about 20 % of the combustibles in a typical workstation. Given that PVC is one of the most widely used plastics, I would estimate that there was at least 3 tonnes of PVC on every WTC floor in addition to the PVC in the electrical wiring. Thus as much as 2 tonnes of HCl was available to react with exposed surfaces on the fire-affected floors in WTC1 & 2.
HCl reacts with zinc, copper and iron, forming the corresponding chloride and liberating hydrogen gas:
Zn + 2HCl = ZnCl2 + H2
Cu + 2HCl = 2CuCl + H2
Fe + 2HCl = FeCl2 + H2
Secondary reactions also occur:
FeCl2 + HCl = FeCl3 + ½ H2
Fe2O3 + 6HCl = 2FeCl3 + 3H2O
Studies, (See for example Oxidation of Metals 26, 157, (1986)), have shown that the formation of ferric chloride, FeCl3, is favored by the presence of oxygen via the reactions:
2HCl + ½ O2 = Cl2 + H2O
2FeCl2 + Cl2 = 2FeCl3
Or the reaction:
Fe + 3HCl + ¾ O2 = FeCl3 + 1½ H2O
In the present context the most significant feature of the chlorides of zinc, copper and iron is that they have low melting and boiling points, (See Table 1), and are therefore very mobile.
Table 1. The Melting and Boiling Points of Some Zinc, Copper and Iron Chlorides
Species Melting Point (°C) Boiling Point (°C)
Zn 420 908
ZnCl2 275 765
Cu 1083 2582
CuCl 430 1463
Fe 1539 2887
FeCl2 677 1026
FeCl3 304 319
Because the metal chlorides listed in Table 1 have significant vapor pressures above their melting points, exposure of specimens of iron, zinc or copper to HCl at temperatures above 400 °C results in substantial weight loss through vaporization of the metal chloride reaction product. Thus iron loses up to 600g/m2 per hour as volatile FeCl3 through exposure to HCl-air mixtures at 500 °C, (See Corrosion Science 21, 805, (1981)).
The Production of SO2 in the Fire-Affected Regions of the WTC:
While hydrochloric acid appears to have been the main corrosive agent in the fire-affected zones of the Twin Towers, the sulfidation of steel discussed in the FEMA Report and in NIST NCSTAR 1-3C suggest that sulfur-containing species also contributed to the wastage of metals in these buildings. Live load materials that were present in the WTC would be very similar to the materials found in offices or dwellings in any modern urban environment. In fact, researchers investigating airborne particulate over New York City in October 2001 observed that the plume of smoke and dust released during the WTC disaster “resembled in many ways those seen from municipal waste incinerators”, (See T. A. Cahill et al. “Analysis of Aerosols from the World Trade Center Collapse Site, New York, October 2 to October 30, 2001.” Aerosol Science and Technology 38, 165, (2004)).
D. O. Albina et al. in the article “Effects of Feed Composition on Boiler Corrosion in Waste-to-Energy (WTE) Plants”, published in the 12th North American Waste to Energy Conference (NAWTEC 12), 2004, have studied the composition of flue-gases from the combustion of municipal solid waste, (MSW):
“The calculated flue-gas composition upon combustion of 1 kg (dry) NYC MSW was 7.4 % CO2, 11 % H2O and 7.2 % excess O2 and the balance N2 with 334 ppm HCl, 210 ppm, NO and 227 ppm SO2. These results are in relatively good agreement with flue-gas compositions obtained in the combustion chambers of present WTE facilities. Typical HCl concentrations in combustors were in the range 200 – 900 ppm while SO2 concentration were in the range of 10 – 300 ppm….. Concentrations of gaseous SO2 were observed to increase at 600 °C and to peak at 900 °C.”
Thus I would argue that the corrosion of iron, copper and zinc in the fire-affected regions of WTC 1 & 2 was dominated by the presence of HCl and SO2 produced by the decomposition of PVC, gypsum and other materials containing labile chlorine and sulfur species. I have also pointed out the similarity of the gaseous emissions from the WTC fires to the emissions from waste incinerators. This is very significant because high temperature corrosion of incinerators is a well-documented problem for heat transfer surfaces such as the boiler walls and super-heater, evaporator and economiser tubes made from low alloy steels.
P. Rademakers et al. in a report entitled “Review on Corrosion in Waste Incinerators and Possible Effect of Bromine”, issued in October 2002, (http://www.ebfrip.org/statements/TNO-AK ... -Final.pdf
) note that:
“High temperature corrosion in waste incinerators is caused by chlorine, either in the form of HCl, Cl2, or combined with Na, K, Zn, Pb, Sn and other elements. In particular both gaseous HCl, with and without a reducing atmosphere, and molten chlorides within deposits are considered to be major factors. Sulfur compounds, which under certain circumstances are corrosive compounds themselves, can enhance or reduce the corrosion by chlorine…(depending on) the SO2 / HCl ratio.”
Of particular concern in waste incinerator operations is a type of corrosive attack known as “active oxidation”. A well-known example of this occurs in oxygen-starved regions exposed to HCl or Cl2 where volatile iron chlorides form and migrate away from the original reaction site. In regions exhibiting higher oxygen partial pressures, these chlorides are converted to oxides with the liberation of chlorine. However, the newly formed oxide does not form as a perfect layer and offers little or no protection to the underlying metal. Under these circumstances Cl2 can penetrate pores or cracks in the oxide/deposit and react with the metal to form more volatile chlorides. Thus we see that the chlorine is acting as a catalyst leading to enhanced corrosion:
Fe + Cl2 = FeCl2
4FeCl2 + 3O2 = Fe2O3 + 2Cl2
On the role of sulfur compounds during waste burning P. Rademakers et al. state:
“In waste incinerators deposit formation is one of the main reasons for corrosion at relatively low metal temperatures… (in the range of 250 to 400°C). Analyses of deposits have shown that outer scales contain mainly sulfates like CaSO4 , Na2SO4,, K2SO4 , ZnSO4 , PbSO4 . The inner scales near the metal surface show considerable amounts of chlorides like CaCl2 , KCl, ZnCl2 and PbCl2 . These salts are able to convert the protective oxide layers to complex oxy-chlorides .”
The presence of sodium and potassium salts in waste incinerator deposits is noteworthy and stems from the fact that alkali metals, and in particular potassium, are enriched in the organically bound metallic elements that are organically bound or dissolved in salts in wood or wood derived products such as paper and cardboard (See for example, T. R. Miles et al. in “Alkali Deposits Found in Biomass Power Plants”, NREL Report, April 1995). Research presented in a paper entitled “Aerosols in Fixed –Bed Biomass Combustion”by I. Oberberger presented at a Bio-energy Conference in October 2003, shows that the aerosol concentration in the flue gas from burning biomass increases with increasing concentrations of K, Na, Zn and PB in the fuel.
Table 2 provides some data on chloride – sulfate mixtures with particularly low melting points.
Table 2. Low Melting Point Chloride – Sulfate Mixtures
Salt Mixture with wt % of each Component Melting Point (°C)
60 ZnCl2 + 34 FeCl3 200
ZnCl2 + KCl + ZnSO4 + K2SO4 226
68 ZnCl2 + 32 KCl 230
84 ZnCl2 + 27 PbCl2 262
ZnCl2 + ZnSO4 300
35 FeCl2 + 47 KCl 355
The very low melting point of ZnCl2–based salt mixtures accounts for the poor corrosion resistance of galvanized steels to high temperature environments containing HCl or Cl2. Indeed researchers in the U.K. have developed a process for the removal of the zinc coating on galvanized steel scrap metal using air containing 10 % gaseous chlorine at 800° C. (See the article by J.K.S. Tee et al. in the Journal of Metals 51, 24 (Aug 1999)). These authors report over 90 % removal of the zinc on a specimen of electro-galvanized steel after only 10 minutes of exposure to the 10 % Cl2 / air gas mixture. Interestingly, the same authors have patented a process for zinc removal utilizing the combustion of waste PVC as the source of chlorine.
An analysis of the thermodynamics of metal chlorides and sulfates shows that, in addition to zinc, copper and lead may be mobilized under the conditions prevailing in solid waste incinerators. (See: J. of Mat. Cycles Waste Manag. 4, 143 (2002) and Envi. Sci. & Tech. 30, 50, (1996)). In the case of copper it appears that the degree of volatilization is strongly dependent on the CO/CO2 ratio. Under reducing conditions, (CO/CO2 ³ 0.1), 100 % volatilization of copper is predicted at temperatures of 800° C or higher. Similar behavior is expected for lead at temperatures above 500° C with no sulfur species in the system, or 800° C under conditions favoring sulfate formation.