«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Applicability This measure is applicable in all new and most existing wool scouring plants.
For plants scouring hair, or wools giving low yields of poor quality grease, the measure might not be an economically attractive proposition.
The COD concentration of the effluent resulting from the dirt removal and grease recovery loop may be too high for on-site aerobic treatment plants. The installation of a coagulation/flocculation or anaerobic biological treatment before the aerobic biological plant would overcome this problem.
Economics A calculation of net economic benefit per tonne greasy wool can be done, based on the assumptions reported in Table 4.10. The unit costs refer to the UK situation at the time of the research, therefore they can only be indicative for considerations at European situation.
Table 4.10: Estimate of the economic benefits achievable with the installation of integrated dirt removal/ grease recovery loops It is estimated that the installation of dirt removal/grease recovery loops at a mill processing 15000 to 25000 tonnes/year of greasy wool would cost between 400000 and 800000 euros, depending on the nature, the quality and the capacity of the particular system chosen.
The payback time on the installation, ignoring the benefits of reduced effluent disposal costs, would be between 2.04 and 4.08 years [187, INTERLAINE, 1999].
Driving force for implementation
The driving forces are economic benefit for medium and large mills, especially those processing fine (high grease content) wools. Economic benefit derives from savings in water, energy, sewage treatment and chemical costs and the proceeds from sales of wool grease. Disincentives are the high capital cost, high maintenance costs and complexity.
Reference plants Many plants throughout Europe (see also survey referred to in Chapter 3).
4.4.2 Use of integrated dirt removal/grease recovery loops combined with evaporation of the effluent and incineration of the sludge Description The technique involves the closed-loop treatment of wool scouring effluents by evaporation/incineration with recovery of water and energy. Thereby the whole system of effluent and waste management is closely integrated with a dirt removal/grease recovery loop applied to the scouring process (described in Section 4.4.1).
As far as can be determined, there is only one mill in the world presently using this technique.
That is Mill N, mentioned in the survey presented in Chapter 3 [187, INTERLAINE, 1999] (see schematic diagram of the effluent and waste managemtn system in the figure below). The features of the system, as it is applied in the referenced mill, are therefore used as the basis for the description of this technique.
Figure 4.8: Schematic diagram of the effluent and waste management system at Mill N [187, INTERLAINE, 1999] Mill N has eight scour lines.
Flowdown from the rinse bowls is treated biologically, using aeration in a series of 5000 m3 circular tanks. Biological sludge is removed in a settling tank and partly returned to the first aeration tank. Excess sludge is treated in a sludge thickener, then dewatered in a decanter centrifuge and finally disposed of to agricultural land.
Heavy scour effluent is sent to a settling tank. The dirt- or sand-rich sludge from the bottom of the tank is dewatered in decanter centrifuges and then partly used in brick-making and partly landfilled. The grease-rich phase from the top of the tank is taken to the wool wax separators (grease centrifuges). Here the grease is separated. The middle phase from the centrifuges is returned to the scours and the bottom (dirty) phase sent to the evaporation plant.
A seven-stage steam-heated falling film evaporator is installed. The steam used for heating is produced in a boiler which uses waste heat from the incinerator. The boiler also powers a steam
turbine, producing electricity. The integrated evaporator/incinerator/boiler system is selfsufficient in energy, all the energy used being derived from the sludge.
The condensate from the evaporator is treated in a steam stripper to remove ammonia, then passed through a fixed bed aerobic bioreactor, which removes residual odorous compounds and 90 % of steam-volatile ectoparasiticides, before the water is recycled to the rinse section of the scour. The ammonia is used in a catalytic reactor to reduce the NOx content of the incinerator flue gases.
The evaporator concentrate entering the incinerator has a calorific value of 9.5 MJ/kg and its combustion is self-supporting (no fuel added from external sources). The operating temperature of the incinerator is 1200°C in order to destroy the polychloro-dioxins and -benzofurans. The exhaust gases are used to heat the boiler, as already stated, and fly ash is removed from the boiler flue gases in a bag filter system. The ash is extracted with water to recover sodium and potassium carbonate in solution, which is used as a builder in the scour. The extracted ash and the solidified liquid ash from the incinerator are landfilled.
Main achieved environmental benefits
In addition to the environmental benefits achievable with the application of a dirt removal/grease recovery loop of the type described in the previous section, the proposed
technique allows further reductions of:
· the organic load discharged to the environment (see Table 4.11) · water consumption, thanks to the additional amount of water recovered from the evaporator.
Assuming a consumption of 4 - 6 l/kg greasy wool as best performance achievable by a company using high capacity dirt removal/grease recovery loops, a further 70 – 75 % reduction in water consumption may be obtained (Mill N declared a net water consumption of 1.31 l/kg greasy wool) · the amount of sludge to be disposed of. The evaporation/incineration process produces 20 g of ash per kg of greasy wool, but no sludge. Sludges (75 g/kg greasy wool, dry weight) arise from the dirt removal/grease recovery loop and from the biological treatment of rinse effluent. In other companies where the sludge is not incinerated the amount produced is in the region of 185 g/kg of greasy wool (dry weight), as for example in Mill L (see Chapter 3, Section 3.2.1).
The environmental performance of the plant at Mill N has been described in detail8. The plant was installed in stages over a period of 13 years from 1982 – 1995. In 1982, the settling tank and wool wax recovery plant were installed. By 1987, the aerobic biological effluent treatment plant had come on line, and at that time was used to treat heavy scouring effluent. In 1988, the evaporation/-incineration plant was built and since then used for heavy scouring effluent and only rinse water treated in the aerobic biological plant. Later refinements included the fixed bed bioreactor, for removing odours and volatile ectoparasiticides from the recycled evaporator condensate, and then the ammonia stripper used to prevent recycling of ammonia in the condensate thereby reducing ammonia and nitrate levels in the treated effluent from the aerobic biological plant. At a similar time, the mill started to use ammonia to reduce NOx in the exhaust gases from the boiler, and bag filtration of the gases (replacing a water scrubber) to remove fly ash, so reducing air emissions and emissions of sodium and potassium salts to water.
The performance of these various stages of plant improvements and additions in reducing emissions to water, air and land are given in the next tables. Figures are related to greasy wool consumption.
R Hoffmann, G Timmer and K Becker, The Environmentally Friendly Production of Wool Tops – Waste Water Treatment at BWK, Proc. 9th Int. Wool Text. Res. Conf., Biella, 1995; R Hoffmann, G Timmer and K Becker, Wool and the Environment – Effluent Treatment and Recycling, Recirculation of Useful Substances in Wool Scouring and Combing Plants, Bremer Woll-Kämmerei AG, 1995.
Source: [187, INTERLAINE, 1999]
(1) figures are related to greasy wool consumption Table 4.13: Solid wastes at Mill N, 1982-1995: production-specific values Operational data A plant such as this requires extensive monitoring. All the parameters listed in the abovementioned tables should be monitored regularly [187, INTERLAINE, 1999].
Cross-media effects Significant cross-media effects should only occur if the plant is operated incorrectly. Although the plant is self-sufficient in energy, it uses a large amount of self-generated thermal and electrical energy and produces CO2. However, conversion of the carbon content of the evaporator concentrate to CO2 is preferable to its conversion to methane in landfill [187, INTERLAINE, 1999].
The availability of this “complete solution” to the problems of wool scour effluent and waste management is restricted, for existing installations, by a number of considerations [187,
· the economics – the very high capital cost and high running cost – probably make the system unaffordable for any but the largest scourer (Mill N’s throughput – 65000 t/yr – is almost double that of any other European scourer).
· the technology is very complex and the required expertise is beyond the scope of many scourers. It would be necessary to find and employ an engineer with the requisite skills and experience.
· the space occupied by the plant is large and many scourers would not have room for the plant on their present site.
Mill N has offered to make its expertise available to other scourers who are contemplating installing similar plant. This may increase availability because other mills would not need to go through the learning curve, which began in 1982 at Mill N and is still ongoing, though all the developments described above were in place before the end of 1995 [187, INTERLAINE, 1999].
Economics In addition to savings achievable with the dirt removal/ grease recovery loop (see Section 4.4.1), recycling of condensate from the evaporator saves the cost of water and the cost of effluent disposal.
Mill N reported in 1995 that its capital expenditure on environmental improvements, since 1982, had been DM64 million (33 million euros) and that the annual running cost of the plant is DM10 million (5 million euros). Despite the mill’s large size and economies of scale its effluent and waste management costs, per tonne of wool processed, are higher than all but the smallest scourers in the survey [187, INTERLAINE, 1999].
Driving force for implementation It is believed that the driving force at Mill N has been stringent local and national regulations regarding emissions to air and water [187, INTERLAINE, 1999].
Reference plants Mill N.
As already stated above, as far as can be determined, this is the only mill in the world presently using this technique.
Reference literature [187, INTERLAINE, 1999]
4.4.3 Minimising energy consumption in wool scouring installations Description Wool scouring is an energy-intensive process. In addition to the generally applicable good housekeeping techniques already mentioned, the biggest energy savings in a wool scouring process come from reducing effluent flowdown (and consequent heat losses) to drain or to onsite effluent treatment plant, by the installation of a dirt/grease recovery loop. Technique include fitting a heat exchanger to recover heat from the dirt/grease loop flowdown.
Further savings arise from each of the following measures [187, INTERLAINE, 1999]:
· fitting of covers on scour bowls to prevent heat loss by convection or evaporation.
Retrofitting, however, is sometimes difficult on existing installations · optimising the performance of the final squeeze press in order to improve mechanical removal of water from the wool before it enters the evaporative dryer. The presses used for squeezing wool usually have steel bottom rollers and a porous top roller. Traditionally, the top roller was a steel roller wound with crossbred (coarse) wool top (a sliver of parallel fibres). More recently, this has been replaced with a blended top of wool and nylon (polyamide), a nylon top, or a square section rope, usually of wool and nylon blend. The last option combines durability with good performance. Porous composition rollers are offered commercially, but no information is available on their performance in this application · running the last bowl at relatively high temperature in order to improve squeezing efficiency. Many scours are set to run with bowl temperatures decreasing from first or second to last bowl. Last bowl temperatures in the survey ranged from ambient (say 20°C) to 65°C, with an average of 48 °C. Since heat losses from the last bowl will increase as its temperature increases and heat consumption in the dryer will correspondingly decrease as the squeezing efficiency improves, it follows that there is an optimum temperature for the last bowl. It has been shown that this temperature is 60 – 65 °C for wool throughput rates above about 500 kg/h · retrofitting heat recovery units to dryers. However, this is expensive and the heat saving available is only about 0.2 MJ/kg. Scourers’ practical experience with heat recovery units on wool dryers is also negative; the units quickly become blocked with fibre and may even cause increases in energy consumption · direct gas firing of scour bowls and driers in order to avoid the losses which occur in the generation and distribution of steam for use in direct or indirect steam heating. Retrofitting is not always possible in existing plant and the cost is relatively high. Energy saving is
Main achieved environmental benefits Reduction in energy consumption will have the effect of reducing emissions of CO2, SOx and NOx, either from the scouring plant itself or off-site.
Energy savings from a dirt/grease recovery loop can be estimated as about 2 MJ/kg of greasy wool if a scour with loop and heat exchangers is used. It is assumed that a conventional scourer discharging 10 litres of water per kg greasy wool needs 2.09 MJ to heat 10 litres of fresh water from 10 °C to 60 °C (209 kJ/l). A scouring installation with loop and heat exchangers discharges only 2 l/kg (see Section 4.4.1) and recovers 80 % of the heat contained in the effluent (the energy input needed becomes 0.084 MJ/kg greasy wool).
It is also interesting to show the energy savings achievable in the dryer by operating the last bowl at optimum temperature (65°C) as discussed earlier.
Source: [187, INTERLAINE, 1999] with reference to L A Halliday, WRONZ Report No R112, 1983
(1) Calculations were made considering an indirect steam dryer Table 4.14: Energy savings from operating the last bowl at optimum temperature (65°C) Energy savings for the other measures described have already been reported under the heading “Description”.
In conclusion, medium-to-large wool scouring plants can be operated with energy consumptions of 4 –4.5 MJ/kg greasy wool processed, comprising approximately 3.5 MJ/kg thermal energy and less than 1 MJ/kg electrical energy. Smaller plants may have higher limits for specific energy consumptions, but no information is available to confirm this [187, INTERLAINE, 1999].
Many mills do not have separate metering for monitoring the energy consumption of individual machines or processes – or even whole departments. In these circumstances, it is difficult for mill personnel to identify potential energy savings or to become quickly aware of problems, such as loss of efficiency of a dryer. Installation of energy monitoring equipment would probably have an early payback, but little evidence is available to support this claim. In the absence of individual metering, whole-plant monitoring should be done on a frequent basis and any departures from the norm investigated [187, INTERLAINE, 1999].
Cross-media effects Energy conservation has positive effects on emissions to air and land. No negative effects are to be expected.