«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Captured air emissions from thermal processes carried out after finishing treatments The previous section dealt with calculated emissions. As for measured emissions at the stack, typical emission levels are reported in Table 3.45 for a sample of finishing mills (each with one or more different processes). When emission abatement systems are installed, the values shown in the table will correspond to the resulting clean gas. In the case of direct heated stenters, the portion of Organic-C emission attributable to the fuel (methane, propane, butane) is mentioned separately and is not included in the data for the Organic-C emission values (concentrations, emission factors and mass flows).
The following general comments apply to the reported data:
· thermal treatment of textiles on stenters can be influenced by upstream finishing processes (and by the efficiency of the previous washing treatment, if applied), as can be seen for dyeing carriers in process 4.4 in Table 3.39 and for perchlorethylene in process 12.1 and
12.2 in Table 3.45 (this aspect is analysed in more detail later in this section) · an emission factor of 0.8 g Organic-C per kg textile can be achieved in nearly all processes in textile finishing (however, it has to be kept in mind that for sites with installed emission abatement systems, the reported emission figures correspond to the clean gas).
· Organic-C emissions caused by uncombusted fuel are in a range of 0.1 g/kg textile up to 5 g/kg textile in the case of poorly maintained burners · formaldehyde emissions may originate not only from the auxiliaries applied, but also from the directly heated stenters themselves due to partial burning-out of the gas (methane, propane, butane). Concentration levels from stenters range from 0.1 to 60 mg/Nm3.
Carry-over of pollutants from upstream processes to drying and fixation Textile auxiliaries and chemicals (and their by-products/ impurities) with a certain affinity with the fibres can be fixed temporarily on the textile, especially if washing/rinsing is inadequate. In down-stream thermal treatments these substances can be released from the textiles and end up in
the exhaust air. Typical substance classes to be regarded from this point of view are:
· carriers · levelling agents · aftertreatment agents · wetting agents · hydrocarbons from printing pastes (this aspect has already been analysed in Section 18.104.22.168.5) · acetic acid · perchlorethylene (if dry-cleaning is carried out) Data on emission potential of carrier-dyed fabrics and dry-cleaned fabrics are given below.
Emission potential of carrier- dyed fabrics Carriers are mainly used for dyeing of PES and PES blends (see also Sections 22.214.171.124 and 4.6.2). Part of the carrier (in some cases up to 50 % or more) is absorbed on the fabric and
released during heat treatment. The degree of carrier exhaustion/absorption mainly depends on:
· liquor ratio · quantity used · dyeing process · textile substrate · processing conditions during rinsing.
Emission potential of carrier-dyed fabrics is summarised in Table 3.46. Data are based on fabric that has been dyed with carriers (on industrial scale), but not dried. Drying of the fabric and airemission measurement was carried out on a laboratory stenter.
Table 3.46: Air emission factors from drying carrier-dyed textiles Table 3.
47 shows a representative selection of air-emission values from four textile mills during drying/fixation of carrier-dyed wool fabrics. It is clear from reported data that, especially if carriers based on aromatic solvents are used, the active compounds of carrier formulations can lead to a considerable off-gas load during thermal treatment. The efficiency of the emission abatement systems (calculated by comparing the Organic-C concentration in raw and in clean gas) can be insufficient (only 10 - 40 %,) for these classes of compounds.
For non treated off-gases, concentrations ranging between 30 and 4600 mg C/m3 were observed with emission flows of 0.2 - 28 kg C/h and emission factors (WFc) of 0.8 - 24 gC/kg textile.
The most critical substance found in the exhaust gas was biphenyl with concentration levels of 60 - 110 mg/m3 at emission flows of 350 - 600 g/h (WFs: 0.9 - 1.5 g/kg textile).
Table 3.47: Air emission levels during drying/fixation of carrier-dyed fabrics Emission potential of dry-cleaned fabrics
Dry-cleaning is used in the textile industry for the following purposes:
· cleaning of grey textiles and especially elastane blends (conventional washing processes are insufficient to remove silicone preparations widely used for elastane fibres) · aftertreatment for wool/elastane or wool/PES fabrics to achieve improved colourfastness especially for dark shades · quality corrections (removal of spots).
Besides the intentional use of perchlorethylene, a considerable amount of dry-cleaned fabric is finished in Germany because imported goods are often dry-cleaned. The retention of perchloroethylene (main solvent used in dry-cleaning) on textiles is high. As a result perchloroethylene can be released during thermal processes (especially drying).
Due to a potential risk of PCDD/PCDF formation by drying/fixation of perchloroethylenecleaned fabrics, treating perchloroethylene-cleaned fabrics on directly heated stenters is banned in some countries (e.g. Germany).
The emission potential of non-dried fabrics treated with perchloroethylene from five textile finishing plants has been investigated on a laboratory stenter (process temperature 150 °C).
Table 3.48 summarises the resulting measured emissions.
Ranges of air-emission values for perchloroethylene-cleaned fabrics during drying/fixation are:
· 0.1 – 0.8 g perchloroethylene/kg textile (dry-cleaning on-site at finishing plant) · 0.3 – 1.7 g perchloroethylene/kg textile (external dry-cleaning).
* External dry-cleaning ** On-site dry-cleaning at the finishing plant Table 3.48: Emission values from perchloroethylene-cleaned fabrics 126.96.36.199.7 Coating and laminating The main environmental concerns from coating operations are volatile organic compounds from solvents, softeners, etc. as well as ammonia and formaldehyde from stabilisers and cross-linking agents. These aspects are discussed in more detail in Section 2.10. Process-specific emission values for coating processes (including one example for carpet back-coating) derived from measurements carried out in five installations are presented in Table 3.49. However, it has to be noted that since in-house recipes are normally used, the emission levels can vary greatly.
Therefore the examples in Table 3.49 are intended to give only a first insight on the topic.
Additional data can be found in Table 3.45 (see processes 6.1, 6.2, 7.1, 8.3).
Note that in the case of direct heated stenters, the portion of Organic- C emission attributable to the fuel (methane, propane, butane) is mentioned separately (see column "machine based emission") and is not included in the data for the Organic-C emission values ("concentration", "emission factor", "mass flow").
3.4 Carpet industry 3.4.1 Wool and wool-blend carpet yarn dyehouses The processes dealt with in this section are described in detail in 188.8.131.52. Literature data on consumption and emission levels for the wool carpet yarn sector is very poor. Information presented below comes from the report that ENco has submitted to the EIPPC Bureau [32, ENco, 2001]. Quantitative data have been gathered from a group of UK companies representative of this sector. The survey covered a range of enterprises, varying in size from a yarn dyehouse processing approximately 1000 tonnes/year of fibre to an integrated loose fibre and yarn dyeing/finishing plant, processing over 7000 tonnes of fibre per year.
Three categories of companies were involved:
· loose fibre dyehouses which dye and dry only loose fibre · yarn dyehouses which scour, dye and dry only yarn. In one or two cases the scouring of previously dyed yarn is also included · integrated loose fibre and yarn dyehouses.
In this document, data on consumption and emission levels are presented for the first two categories of mills in Table 3.50 and Table 3.53. Integrated dyehouses can be seen as mixtures of these processes. Figure 3.14 attempts to give ranges of inputs to and output from the wool carpet yarn activities (outputs after municipal waste water treatment are not included). The reported figures should be used with caution, as the diversity of the sector makes these generalisations subject to considerable error.
Consumption and emission figures are based on information for a twelve month period within a 1999 - 2000 time frame.
Emission data are given only in respect of water pollution, from which the main environmental issues associated with the activities carried out in this sector arise.
Emissions factors have been calculated from waste water volume, measured waste water concentration at the outfall to public sewer (after flow balancing only) and textile product throughput over a corresponding time frame. There are typically wide variations in the composition of waste water due to the predominantly batch nature of the dyeing process used and natural variation in the fibre. Data available from longer term measurements have been preferred, as individual measurements made on a few batches of material are unlikely to reflect long-term trends.
The emissions have been quantified in terms of the following environmentally significant
· Chemical Oxygen Demand (COD) · suspended solids (SS) · metals (copper, chromium, cobalt, nickel) · organochlorine pesticides (HCH, dieldrin, DDT) · organophosphorus pesticides (diazinon, propetamphos, chlorfenvinphos) · synthetic pyrethroid insecticides (permethrin and cyfluthrin from mothproofing agents) and cypermethrin (from sheep-dip ectoparasiticides).
Table 3.50: Overview of emission and consumption levels for three typical loose fibre dyehouses Water & energy consumption As a general consideration, it has to be noted that the water consumption figures reported in the table above are inevitably higher than the theorethical values obtained considering the liquor ratio of the machines (which is typically 1:10 for loose fibre dyeing) and the subsequent water additions for rinsing or other aftertreatments.
These values include water used to raise the steam for heating the liquor, spillage on loading and reloading, cooling additions made for shade matching, etc.
The three loose fibre dyehouses referenced in Table 3.50 indicate a wide range of specific water consumption figures, attributable to different working practices and water recycling measures.
Plant A operates a standard dyeing regime consisting of the dyeing cycle followed by rinsing in a separate bath, with all waste liquors discharged directly to drain. Site C operates similar machinery, but rinses and cools dyeings using the overflow methods in which clean water is allowed to overflow from the dyeing machine to drain. Site B recycles a proportion of both the dye liquor and rinse liquor and has the lowest overall water consumption factor.
Reported data on energy consumption account for the operations of raising the temperature of the dye liquor from ambient to boiling point and evaporating water from the textile during drying.
Therefore, the theoretical requirements for loose fibre dyeing and drying would be:
· for dyeing (heating 10kg of water per kg of textile): 4.2GJ/tonne · for drying (water content when the fibre enters the dryer: 0.5kg/kg textile): 1.3GJ/tonne.
The total theoretical energy requirement is, therefore, 5.5 GJ/tonne of textile. In practice the energy requirements of individual plants are significantly higher than the above figures would suggest, due to losses in steam generation and transmission and the use of process water at modest temperatures in rinsing and the application of finishes.
Data presented in Table 3.50 fall within a narrow range despite their widely differing water consumption figures. This is understandable on the basis that all three enterprises consume energy in essentially the same way (to raise the temperature of the dye bath and to dry the wet fibre) and that the additional water usage arises from cooler rinsing operations.
Chemical Oxygen Demand
The organic substances discharged in the waste water and the corresponding COD emission factors reflect the pattern of usage of dyestuff and dyeing auxiliaries. In loose fibre dyehouses the use of metal-complex dyes is predominant. Dyeing may require the use of both levelling agents and polyamide reserving agents.
COD emission factors for the three selected mills range from 20 to 30 kg/tonne of processed fibres. A portion of this COD load, however, is attributable to contaminants already present on the incoming raw material. Scoured wool may contain variable amounts of residual wool grease and detergent, depending on the efficiency of the wool scouring process. Synthetic fibres on the other hand contain residual spin finish whose content in oxygen-demanding material varies depending on the lubricant employed.
The figures in the table below come from laboratory analyses performed by submitting samples of raw material to aqueous extraction procedures to simulate the removal of the contaminants in the first wet process.
Table 3.51: Concentrations of compounds present on raw loose fibres, which contribute to the waste water COD load Synthetic pyrethroids from mothproofer The indicated emission factors of mothproofing active agent permethrin show a wide range of values, which reflects very different procedures on each site.
Site A operates a conventional mothproofing process in which each of the dyeings that will ultimately make up a bulk blend is treated at a level consistent with the desired overall application rate (typically 100 mg permethrin/kg fibre). Waste water from each dyeing is discharged.
Site B operates with an overtreatment and dye liquor re-circulation system designed to minimise mothproofer emissions. In this process only some of the fibre (perhaps as little as 10 % of the total blend weight) will be mothproofed, this portion receiving a correspondingly high application rate. Spent dye baths from this process contain correspondingly high levels of permethrin, but they are retained in a holding tank and re-used for the next sequence of dyeings or are used to dye fibre which requires no treatment. Overall the waste water residues from this sequence of operations are significantly lower than from the conventional process. The remainder of the blend receives no treatment and spent process liquor from these dyeings contains no residual mothproofer. Finally the overtreated and untreated fibre is intimately mixed during mechanical processing and yarn formation, resulting in an insect-resistant yarn with the correct average treatment level.
Site C does not carry out mothproofing. The low levels of permethrin detectable in the effluent arise from the dyeing of fibre which is inadvertently contaminated with low levels of permethrin. The source of this contamination is difficult to identify but may arise, for example, through the reprocessing of previously treated waste fibre, contamination during the raw wool scouring process when this process is used to mothproof scoured wool, etc.