Distribution Of Heavy Metals Between Gas, Liquid and Solid Wastes from Sewage Sludge Combustion

Transcription

1 Cracow University of Technology, Poland From the SelectedWorks of Witold Zukowski 2006 Distribution Of Heavy Metals Between Gas, Liquid and Solid Wastes from Sewage Sludge Combustion Jerzy Baron Witold Zukowski Stanislaw Kandefer Available at:

2 Jerzy BARON * Witold ŻUKOWSKI * Stanisław KANDEFER ** thermal utilisation bubbling fluidised bed sewage sludge * Institute of Inorganic Chemistry and Technology, Cracow University of Technology, Cracow, Poland baron@pk.edu.pl; pczukows@pk.edu.pl ** Institute of Thermal Engineering and Air Protection, Cracow University of Technology, Cracow, Poland kandefer@pk.edu.pl DISTRIBUTION OF HEAVY METALS BETWEEN GAS, LIQUID AND SOLID WASTES FROM SEWAGE SLUDGE COMBUSTION Results are presented on the partitioning of metals between the different waste streams produced from the co-combustion of sewage sludge with wood chips in an installation with a bubbling fluidised be combustor, equipped with a flue gas cleaning system. The system consisted of a dust separator and two wet scrubbers in series. For selected metals, their concentrations were determined in the fuel and in the ashes as well as carried with the flue gas stream. Heavy metals were mostly found in the dust carried with the flue gases, either as vapours condensing on solid surfaces or present in the fines and reaching the scrubbers, The alkaline metals are carried to the scrubbers, aiding in the capture of SO 2. INTRODUCTION Sewage sludge is now one of the most important wastes associated with civilisation, produced when industrial or residential waste waters are cleaned at waste water treatment plants (WWTP) before being discharged to the environment. As waste waters are collected from increasing numbers of communities and are treated, the quantity of sludge produced gradually grows [1]. Sewage sludge is very wet and contains up to over 80% water when it comes off a filter press and about 50% if it is first deposited in drying beds. The dry mass consists of mineral and organic fractions. The mineral fraction contains most of the metals previously present in the waste waters, including heavy metals. This may be the decisive factor for the utilisation of the sludge. According to the present regulations sewage sludge can be used on soils as a fertiliser or for the recultivation of derelict land, provided certain conditions are satisfied [2], including that the concentrations of specified metals in the dry mass lie below the limits defined by the law. The content of heavy metals in the sewage sludge from individual WWTPs depends on the character of the waste waters processed. In certain areas the load of heavy metals in the sewage sludge is such as to preclude its use in agriculture or for recultivation. The concentrations of heavy metals found in the sewage sludge from a number of WWTPs at different localities are given in Table 1. In the first two areas specific industries are present, responsible for higher concentrations of most of the listed metals and particularly Cr at Myślenice (many small tanneries) and Ni, Cd at Lublin (automotive industry).

3 Table 1. The concentrations of heavy metals in the sewage sludge from different localities and the limiting upper levels for use on land, in agriculture and for land recultivation for non-agricultural purposes. All values are in mg/kg (dry mass). Waste water treatment plant at Upper legal limit Myślenice Lublin Niepołomice agriculture recultivation Mn not specified Cu Pb Ni As not specified Hg Cd Cr Zn no data When the sewage sludge cannot be used on land, it has to be stored or subjected to thermal treatment. Because of its very high water content co-combustion with coal in conventional boiler installations does not appear to be feasible. However, other technical solutions for burning low calorific value fuels are possible. The combustion process can take place in a fluidised bed, which assures excellent mixing with combustion air and the sewage sludge can be either only partly dried or mixed with a more calorific fuel, preferably a bio-fuel. Moreover, some of the heat carried by the flue gases can be used to preheat the fluidising air. Such a fluidised bed combustor installation, burning sewage sludge with wood chips as the supplementary fuel, has been used to obtain the results presented here on the distribution of heavy metals and selected other elements in the combustion products. The measurements were made when the operation of the installation was tested, still under sub-optimum conditions from the point of view of flue gas treatment. It was, however, possible to detect certain regularities and arrive at some practical conclusions. EXPERIMENTAL INSTALLATION The installation used to obtain the results presented burned sewage sludge, with wood chips supplementary fuel, and employed the following technical solutions [3]: 1. A dual feed system, to make it possible to burn either sewage sludge alone or together with the supporting fuel. 2. A thermally insulated combustion chamber, with a bubbling fluidised bed of sand at the bottom and a freeboard space above. This ensured uniform distribution of the hot combustion air, rapid drying of the wet sludge and combustion at about 900 o C both in the bed and above it.

4 3. A heat recuperator, to preheat the fluidising air to about 500 o C. A technological scheme illustrating the proposed bubbling fluidised bed technology for the thermal treatment of wet sewage sludge is given in Fig. 1. The installation has been designed to treat about 400 kg/h of raw sewage sludge, with about 18% of solids, or up to 700 kg/h of sludge with 50% of solids. WATER STACK COOL AIR 3214 kg/h RECUPERATOR WATER HEAT EXCHANGER o C S C R U B B E R S C R U B B E R FLUE GASES 3898 kg/h WASTE WATER 900 o C FREEBOARD AFBC 1.5 MW tbed= 900 o C FEEDER HOPPER FEEDER HOPPER CONVEYOR CONVEYOR SEWAGE SLUDGE (82% MOISTURE) 400 kg/h WOOD CHIPS 322 kg/h ASH 38 kg/h HOT AIR 400 o C Fig. 1. A schematic of the proposed technology for the incineration of wet sewage sludge Before the flue gases leave the installation they are passed through a scrubber system. Its chief functions are to bring down the SO 2 concentration (there is no added SO 2 sorbent in the bed) and to capture any particulates escaping the dust removal system. The scrubber water is also meant to capture alkali and alkali metal compounds. This should raise the ph of the water and increase the efficiency of SO 2 removal (lime water could be used to increase the efficiency further). The scrubbers also help to reduce the stream of heavy metals carried with the flue gases, through capturing much of the fine flue dust and possibly metals that might be carried as vapours or volatile compounds. The fact that the temperature of the gases is eventually brought down to under 100 o C also plays a part. The schematic in Fig. 2 shows the whole installation.

5 fluidized air flue gases Fig. 2. A schematic of the installation for burning sewage sludge built at the Waste Water Treatment Plant at Niepołomice The most important element of the installation is the 1.5 MW bubbling fluidised bed combustor. The combustor unit comprises a wind box (1), with a distributor attached at the top (2), a combustion chamber (3) and freeboard space (6), where the combustion process should go to completion. The sewage sludge and auxiliary fuel (wood chips), are supplied by a screw feeder and a conveyor belt respectively, to two bunkers (5). From the bunkers, using pipe feeders and via sloping ducts (4) the fuels are directed to the combustion chamber (3). Hot flue gases leaving the freeboard are directed to a counter-current plate heat exchanger (8). Its function is to preheat the fluidising air to a suitable temperature. Hot air from the recuperator (8), via a wind channel (7), reaches the wind box (1) and passing through the distributor (2) enters the combustion chamber (3). In addition, the system contains tubular heat exchangers (9), to provide hot water. The construction of the wind channel (7) is such as to facilitate installing tubular heat exchangers, depending on need, either between the combustor and the plate heat exchanger or beyond the recuperator (8). The air flow rate is measured in the duct (10) between the air pressure fan (11) and the recuperator (8) and the duct is equipped with a flap seal which can be used for cutting off the air supply to the combustor instantaneously. After leaving the heat exchangers the flue gases travel along a flue duct (12) to a scrubber system (13), from where an exhaust fan (14) draws them into the chimney (15). In the flue and air ducts, freeboard and recuperator there are sockets where sensors can be placed to measure the temperature and through which probes can be inserted to withdraw gases for chemical analysis.

6 TEST PROCEDURE, RESULTS AND DISCUSSION The tests were made both when sewage sludge was burned alone and during its co-combustion with wood chips as the supplementary fuel, with combustion under steady conditions. Steady conditions were obtained and maintained with the aid of the automatic control system. The bed temperature was the most important parameter, and the system was set for 900 o C. In the course of the process the operational parameters were recorded and the flue dust load in the exit gases was determined. At the point of entry to the stack, i.e. after the scrubbers, samples of the flue dust were collected on a heated filter as well as of the condensate obtained from the flue gases in a watercooled condenser cooler. The condensate, together with the pollutants carried, was collected by passing 10 m 3 of the flue gases through the condenser. Samples of the water circulating in each of the two scrubbers were also collected, soon after the test was started and then again at the end, with no water renewal in the scrubbers, apart from topping-up. Samples of solids were taken from the burned-out bed material, the elutriated particulates collected at o C in the separator ash trap, situated between the heat recuperator and the first scrubber. The tubular heat exchanger was not yet in operation. The various materials were collected chiefly when sewage sludge was burned alone, since the concentrations of metals in the sludge were considerably higher than in the wood chips. They were also much higher in the solid wastes from burning the sludge, collected at different points in the installation, although the sludge contained very considerably more mineral matter than the wood chips, so that the average concentrations in the ash, with respect to those in the original fuel, should not increase so strongly. The metals determined in the samples were the alkali and alkaline earth metals, as well as all those for which emissions to the atmosphere or concentrations in waste waters are limited by law. The analysis also included sulphur, phosphorus, arsenic and silicon. Table 2 gives the concentrations of the analysed elements in the sewage sludge burned, the bed ash, dry ash captured in the ash separator and particles collected from the flue gases on the heated filter, at the bottom of the stack. The heavy metals are divided into three classes, since the limit concentrations are specified not for each of the 13 metals but for the sums of the concentrations of Cd and Tl and of the remaining 10 metals. Only for Hg there is an individual limit. To check how the individual measured concentrations affect complying with the emission standards the appropriate sum is compared with the limit. It is also interesting to examine the concentrations of the various elements in the bed material, separator trap and the filter dust and to assess how the various metals are distributed between the bed ash and the particulates carried by the flue gases and captured in the separator and at the exit from the installation, on the filter. This can best be done by referring to the theoretical concentrations, obtained by multiplying the experimental concentrations in the dry fuel by the theoretical enrichment factor, where F = 100/[fuel ash, %]). This rests on the

7 assumption that all the metals from the fuel pass into the ashes. The relative enrichment factor for each element in the ash is then given by the ratio of the concentration found in the sample to the theoretical value. The results are given in the last three columns of Table 2. Table 2. The concentrations of selected elements in the dry solids, in mg/kg Concentration [mg/kg] Element fuel bed separator filter calculated conc. after combustion experimental conc./calculated conc. bed separator filter Heavy metals I - limited Cd Tl TOTAL II - limited Hg III - limited Sb As Pb Cr Co Cu Mn Ni V Sn TOTAL TOTAL I+II+III Other metals Na K Ca Mg Al Zn Fe Other elements P S Si

8 If all the compounds in which the metals are present in the original sewage sludge or which are formed during the combustion process were thermally stable, nonvolatile and were uniformly distributed in the three solid materials analysed, the relative enrichment factor should in all cases be close to unity. The relative enrichment factors obtained from the results can both be much less and much more than 1. Clearly, the behaviour of the elements examined can be very different. All of them except silicon tend to leave the combustor with the finer ash fractions and are concentrated in the filter dust, with the separator material showing intermediate behaviour, but closer to that of the bed ash. The silicon (probably mainly as SiO 2 from sand and soil) is concentrated in the bed and the ash separator material, with appreciably less in the filter dust. The degree to which the other elements are depleted in the bed ash and the extent to which they become concentrated in the filter dust shows considerable variation. The metal most strongly depleted both in the bed and in the trap is Hg, followed by Cd and Sb and Cu. There is only slightly more Hg in the trap, and no strong enrichment in the filter dust, but with Cd enrichment starts in the trap and for the filter dust is the highest observed, over 50 - fold. Very high filter dust enrichment factors are also found for As, Zn, and Pb, respectively over 24, 18 and nearly 15. This partitioning of the elements (including the non-metals) can be ascribed to at least three factors: A. The volatility of the metal. This is certainly very important for Hg. With As, Zn and Pb it could play a part, but not for e.g. Cu [5,6]. Tab. 3. The inorganic components of the solids, in oxide form, % compound fuel bed separator filter other metals as oxides Na 2 O K 2 O CaO MgO Al 2 O ZnO Fe 2 O TOTAL other components, as oxides P 2 O SO SiO TOTAL minerals, calc additional parameters Moisture [%] Organics [%] B. The detailed high temperature chemistry of the metals. Many of them can react with combustion gases to form quite stable gas phase species, such as hydroxides or chlorides (if enough chlorine is present) [5,6]. These can be transported with the hot gases, to condense on solid particles at lower temperatures and possibly entering into secondary reactions. C. The form in which the given metal appears in the fuel. Both the chemical compounds of the metal present in the sludge and the degree of dispersion may play a part. If the compound is thermally stable at combustor temperatures it will remain

9 unchanged. On the other hand, if it is strongly dispersed, it may tend to pass into small particles elutriated from the bed and captured in the separator or in the scrubber, either as sediment, or in the scrubber solution. The remainder carried in the flue gases should be detected in the dust captured on the heated filter or in the condensate. This could be the case with e.g. Ca and Mg, Al and Fe. The balance of the main mineral components of the ash, expressed in oxidised form, is shown in Table 3. As expected, the total is increasingly deficient for the finer materials, in which the trace elements tend to become concentrated. On the basis of the analysis of the fuel and of the bed material, the quantities of the metals leaving the combustor with the flue gases, in the gas phase or with the particulates elutriated from the bed can be calculated. Knowing the volume of the flue gases, the average concentrations carried can be obtained, to be compared with the legal limits [4]. The minimum efficiency of the gas cleaning system necessary to comply with the regulations can be then assessed. This is illustrated by Table 4, with Tab. 4. Material balance for the elements analysed, for 1 tonne of fuel and the minimum cleaning efficiency required by the standard amount [g] amount held in bed [g] amount in flue gas [g] conc. in flue gas [mg/m 3 ] standard [mg/m 3 ] Minimum cleaning efficiency, Element Heavy metals I - limited Cd Tl TOTAL % II - limited Hg % III - limited Sb As Pb Cr Co Cu Mn Ni V Sn TOTAL % other metals Na K Ca Mg Al Zn Fe the figures referred to 1 tonne of the sewage sludge (dry mass), containing 65% of mineral matter. On combustion, this fuel gives 3788 Nm 3 of flue gases (at 11% of O 2 ). In the present installation considered, the gas cleaning process takes place in three stages. The gases first pass through the dry separator and then trough two scrubbers, working in series. At the start of the tests, the scrubbers were filled with water. In the scrubbers particulates are captured and insoluble components collect in the sediment, while the soluble components should pass into

10 solution. The presence of alkaline and acidic pollutants will affect the ph and pollutants may interact with one another, so detailed interpretation of the results at this stage could not be attempted. Tab. 5. The concentrations of heavy metals (ng/dm 3 ) and other selected components (mg/dm 3 ) in the scrubber water and in the condensate from the flue gases at the entry to the stack Component Start of test scrubber I End of test change changei / total Start of test scrubber II End of test change changeii / total total change condensate condensate / scrubber II heavy metals [ng/m 3 ] I limited Cd % % Tl % % Total % % II limited Hg % % III limited Sb % % As % % Pb % % Cr % % Co % % Cu % % Mn % % Ni % % V % % Sn % % Total % % Total I+II+III % % other metals [mg/dm 3 ] Na % % K % % Ca % % Mg % % Al % % Zn % % Fe % % other compounds [mg/dm 3 ] Cl % % P % % SO % % additional parameters Conductivity [ms/cm] % % ph

11 The gases reached scrubber I while still hot (the tubular heat exchanger was not in use) and much the heat was given up to the scrubber water and gradual water loss by evaporation led to the need to add more water, to maintain the level of circulation, but there was no continuous water exchange - i.e. the pollutant concentrations in the water changed with time. The process could be monitored, but here samples of the scrubber water were obtained only twice, early in the test and at the end of it. The results are shown in Table 5, separately for each scrubber, together with those for the condensate. The proportion of each metal captured by each scrubber, with respect to the effect of both scrubbers together is also given, as well its concentration in the condensate, with respect to that in the second scrubber at the end of the test. On examining them it is found that most of the soluble inorganic pollutants were captured in the second scrubber and only Hg and Sn appeared mainly in the first scrubber, as well as the alkali metals. Partial desulphurisation took place mostly in the second scrubber, with rising concentration of sulphur compounds. This could be on account of higher temperatures in the first scrubber. The first scrubber removed most of the Na and K, probably as hydroxides (the water remained slightly alkaline, in spite of the presence of acid gases) and chlorides and sulphites. The water remained slightly alkaline, in spite of the gases passing through carrying acid gases, such as SO 2. In contrast, the water in the second scrubber gradually turned from practically neutral to acid. This indicates that some solubilities would change with time and shows that to make effective use of the scrubbers, lime water should be continuously added, so as to maintain permanent alkaline conditions in both scrubbers. The metal concentrations found in the condensate are strongly differentiated. In many cases the concentrations are lower than the final ones in scrubber II, but there are exceptions. The outstanding ones are Sn and Pb. There is hardly any Pb in scrubber I, much more in scrubber II, but the concentration in the condensate is higher still. Sn appears to be lost from the scrubbers, while in the condensate its concentration is higher. There is no ready explanation for such effects and the problem of the heavy metals in sewage sludge deserves further study. CONCLUSIONS 1. Sewage sludge can be burned, but it is necessary to use a sufficiently sophisticated technical solution in order to be able to burn a very wet material and a system to keep the pollutant emissions within the legal emission standards. 2. It is necessary to bring down the emissions of heavy metals and their partial elimination is possible in a multistage gas cleaning system, employing both dry and wet stages. 3. The staged system with scrubbers makes it possible to eliminate most particulate emissions and to keep down the emissions of acid gases.

12 4. The alkaline components carried in the flue dust and captured in the scrubbers turn the circulating water alkaline, aiding the removal of acid gases from the gases discharged to the atmosphere. ACKNOWLEDGEMENTS This work has been carried out within the project NNE5/2001/468 Sludge for Heat financed by the European Union and the Polish Ministry of Science and Information Society Technologies. The authors would also like to thank Prof. Elżbieta M. Bulewicz for useful suggestions and discussions during the preparation of this paper. LITERATURE [1] Ochrona Środowiska 2005, red., M. Grzesiak, W. Domańska, Zakład Wydawnictw Statystycznych, Warszawa [2] Rozporządzenie Ministra Środowiska z 1 sierpnia 2002, Dziennik Ustaw Nr. 134, poz [3] J. Baron, S. Chrupek, S. Kandefer, M. Olek, M. Pilawska, J. Porzuczek, J. Wrona, W. Żukowski, Small Scale Incinerator for Biomass, with a Bubbling Fluidized Bed. I. Combustion of Sewage Sludge Development in Production and Use of New Agrochemicals, Chemistry for Agriculture Volume 6, CZECH-POL TRADE, Jeseník, Czech Republic, pp , 2005 [4] Rozporządzenie Ministra Środowiska z dnia 4 sierpnia 2003 r. w sprawie standardów emisyjnych z instalacji, Dz. U. 2003, 163, [5] J. W. Hastie, High Temperature Vapors, Academic Press Inc., N.Y., 1975 [6] C. Th. J. Alkemade, Tj. Hollander, W. Snelleman, P. J. Th. Zeegers, Metal Vapours in Flames, Pergamon Press, Oxford, 1982