PVC: To Burn or Not to Burn?

Polyvinyl chloride, or 'PVC', is one of the most widely used polymers in the world. Due to its highly versatile nature, PVC is used extensively in many industries including construction, automotive, electronics, packaging, fashion and design amongst others. However once it enters the waste stream, it has traditionally be seen as a cause of harmful emissions from incineration. But should that view change? By Carlo Ciotti and Arjen Sevenster Micro-pollutants as well as acidic gases and solid wastes are produced during combustion processes in general, and therefore also during waste incineration. Until a few years ago it was alleged that PVC was the main cause for these hazardous emissions. This focus on PVC was due to the presence of chlorine, which leads to the production of dioxins, the emission of HCl and the production of solid hazardous waste because of the presence of heavy metals from the additives used in various formulations. Because of this, many will be critical if one talks about waste incineration being 'sustainable' for PVC and its products, or if one states that the environmental impact of PVC is at most equivalent, if not lower, compared to alternative materials. However, such critics would not take into account the improved knowledge acquired over the last decades on thermal combustion systems. The development of technologies for the reduction/elimination of emissions, the reduction and recycling of solid waste and the use of new additives substantiate this view, which may surprise those who did not follow these developments, but will be perfectly normal for those who have in-depth knowledge and/or worked on these issues. To clarify the reasons why the thermal valorisation of PVC can be considered a real alternative to mechanical recycling in solving and optimising its end of life management, it's important to explore the following aspects that relate to the environmental impact of thermal valorisation: 1) the formation of dioxins;2) the emission, recovery and recycling of gases;3) the reduction of waste. The Formation of Dioxins First one must keep in mind that there are many production processes or activities that emit dioxins, not only the production of chlorine and the PVC lifecycle. Despite this fact PVC has for a long time been accused of being the cause of the dioxin production, although PVC is not the only substance in Municipal Solid Waste (MSW) which contains chlorine - plants, food and paper are all among the waste streams which contribute to the chlorine present in MSW. In fact, the production of dioxins depends more on the quality of incineration than on the type of materials burned. The PVC industry has made significant inroads into collecting end-of-life PVC products, such as window and door frames Credit: Recovinyl History Before the mid-eighties it was believed that the presence of dioxins in the gaseous emissions from incinerators could result from their formation during the combustion of chlorinated organic compounds or from the pyrolysis of lignin and cellulosic substances present in waste. These assumptions, formulated almost thirty years ago, were for some time considered plausible and should not be underestimated. To prevent the formation of these micro-pollutants it was thought advisable to: Reduce the content of chlorinated organic substances in the waste in order to reduce the content in hydrochloric acid in the flue gas of combustion Neutralise the hydrochloric acid resulting from their combustion as soon as it is formed in the combustion chamber, by addition of basic reagents (carbonate or calcium hydroxide). Subsequently it was found that the formation of dioxins takes place primarily on heat exchange surfaces of the boiler, at a temperature of 300°C to 400°C - and then only if ash and products of incomplete combustion of carbonaceous nature (for example soot) are deposited on the tubes, and in the presence of copper chloride, which is inevitably present in ash along with many other metal compounds. To limit the presence of these micro-pollutants in gaseous emissions, technologies have been developed over the years that have enabled: The complete combustion by suitable dosage and distribution of combustion air Maintaining sufficient temperature and ensuring an optimum degree of turbulence Keeping heat exchange surfaces as clean as possible, especially in the area of the boiler characterized by gas operating temperatures between 300 and 400°C. Use of soot blowers to do this is more efficient as it also improves the thermal efficiency of the boiler Using activated carbon in conjunction with neutralisation for the removal of acid gases. Given this knowledge of the mechanisms of dioxin formation, the scientific community now unanimously considers that PVC cannot be given the full responsibility for the presence of dioxins in the flue gas emitted from incinerators. This is confirmed by a trial done a few years ago at an MSW incinerator in Hamburg, in order to verify the impact of an increase of PVC in the waste on emissions. At the time of testing the composition of waste fed to the combustion chamber normally contained about 6% of plastic material and 0.7% of PVC. As a comparative test, about 170 kg/h of pure PVC were added to the normal flow of household waste incinerated, thus achieving a concentration of PVC in municipal waste fed to the combustion of about 5%. The analysis of dioxins during these co-incineration tests did not show significant deviations in quantity and distribution of dioxin/furan isomers compared to those observed with the normal composition of municipal waste. No influence on the quality of gas emissions was observed. In conclusion, since the second half of the eighties the problem of reducing emissions of dioxins and organic micro-pollutants has been considered solved at the scientific and technology level. The combination of the following features ensures the reduction of dioxins down to values below the acceptable limits: Reaching temperatures of 1100°C Having a post combustor ensuring a contact time of 2 seconds Maintaining an oxygen concentration of 6% in the output gases Ensuring a fast cooling down to 250°C to avoid the 'de novo' synthesis of dioxins Having also a catalyst for destruction of dioxins or a system with adsorbent activated carbon. The Emissions, Recovery and Recycling of Gas In addition to hydrochloric acid (which is produced whether PVC is present or not), incinerator emissions also include sulphur and nitrogen compounds. During the waste combustion process the rupture of the polymer chain, resulting in the release of chlorine in the form of HCl gas. Even in the absence of PVC, due to other sources such as household chlorine in the waste, combustion will always produce gaseous HCl that must be destroyed prior to the release into the atmosphere. It must be emphasised that the removal of HCl gas is facilitated by its chemical characteristics, whereas it is more difficult to remove SOx, which should be blamed as the largest contributor to acid rain. Chlorine also has a positive contribution in flue gas, as it allows better capture of the heavy metals present in MSW waste, thereby reducing emissions into the environment. Abatement of Gaseous Emissions The acid gases, above all SOx and HCl, are mostly neutralised by adding alkaline substances, thus producing the corresponding salts. The amount of neutralisation residues depends on the type of technology used (dry, semi dry, wet, semi wet). In the case of municipal solid waste incineration, assuming for example that PVC is responsible for approximately half the production of HCl while the wood, paper and other materials present are responsible for the rest, it was estimated that the traditional neutralisation process based on the use of hydrated lime requires that the residues are disposed of in a landfill for hazardous waste. Today there are other processes that enable recycling a significant part of the neutralisation residues, among which the NEUTREC process uses sodium bicarbonate injected dry in the acid fumes, after they have passed an electrostatic filter to mostly eliminate them from the fly ash. Sodium bicarbonate neutralises the acids and transforms them into sodium salts that are captured by a filtration section and collected, while the purified flue gas can be emitted to the atmosphere. Note that the sodium bicarbonate also contributes to absorb a large part of the heavy metals and dioxins, when injected together with activated charcoal. The sodium salts generated by the neutralisation of acid gases, once collected in the final stage of filtration, may be recovered in a dedicated section, where they are dissolved in water with additives to promote the precipitation of metals, and subjected to a filtration. The insoluble phase is sent to disposal while the soluble phase (brine), after being further purified, is recycled for the industrial production of sodium carbonate. A further development of the recovery process (Revasol®), also leads to the recycling of the insoluble phase which, with the SOLVAL®/Resolest® process, is sent for disposal. In this case however, the it is recovered for use as a material for the construction or road foundations, The HALOSEP® process is another option to recover neutralisation residues. The primary function is to recover chlorine and obtain a reduced amount of Flue Gas treatment Waste (FGW) to be disposed of. The HALOSEP process can also be used to obtain a treated FGW that has improved leaching properties and complies with the leaching limit criteria for heavy metals. The salt product typically has the form of salt brine with a salt content of about 20% (w/w). It is free from dioxins and furans and the concentration of heavy metals is below detection limits. It is possible to produce a Sodium chloride rich product, a Calcium chloride rich product. Cadmium, zinc and lead can be extracted in various amounts in the HALOSEP process. The Recovery of Chlorine as HCl The incinerator in Hamburg previously mentioned can also be taken as a reference for the recovery of hydrochloric acid and its reuse as muriatic acid (aqueous solution of HCl) as a raw material for other industrial activities. The incinerator has in fact, besides the treatment sections for NOx and SOx, also a facility of purification of chlorinated gas. The gaseous stream, still containing the chlorinated combustion products, is sent to a HCl 'rectification column' where a 30% aqueous solution of hydrochloric acid is produced, which can be sold to the chemical and construction industries, or used in energy production. Conclusions An MSW incinerator produces three types of waste - bottom ash, fly ash and waste neutralisation residues. The bottom ashes are the heavier solid residue of combustion. Solid particles are also formed during combustion, and they contain metals entrained by the gases. In order to avoid atmospheric release, the solid particles must be absorbed or removed from the gas phase sent to the stack. So the first step is to 'capture' particles through mechanical or electrostatic filters. They constitute the so-called 'fly ash'. The recovery of this fraction, together with the recovery of the neutralisation residues is possible thanks to processes such as Resolest and Revasol, or Halosep. The contribution of PVC to the production of both bottom ash and fly ash is very limited and is estimated around 0.5% of total ash produced. Regarding the influence of the presence of heavy metals in PVC, one should also note that: 1. The contribution due to the heavy metals in PVC is not significant except for cadmium, which has been phased out2. Stabiliser formulations containing heavy metals are used less and less, and are substituted by other chemical substances, which are not hazardous as shown by the Reach Regulations. Carlo Ciotti is environmental affairs manager at the PVC Forum Italia and Arjen Sevenster is manager of technical and environmental affairs at the European Council of Vinyl Manufacturers (ECVM).