Variable Waste Qualities and their Impact on the Operation of Waste Incineration Plants

Transcription

1 ISWA World Congress 2010 on in Hamburg Variable Waste Qualities and their Impact on the Operation of Waste Incineration Plants Dr. Ing. Frank Ehlers, Interargem GmbH 1. Introduction Waste incineration plants with grate firing are long-term tested plants that can exploit energy from waste fractions in a wide quality spectrum and in various compositions without encountering any problems. While doing so, the plant operation runs approval-conform and the emission limits approved for each plant are securely maintained. However, the use of different waste qualities has impacts on the operation of the refuse incineration plants. For instance, the various flowability of the waste fractions have consequences for the stacking behaviour in the bunker and the waste delivery system to the boiler. Furthermore, the wear behaviour and the slagging of the boiler plants, the volume of the operating supplies deployed and the accruing residuals due to the composition of the delivered waste are influenced. That directly affects the cost effectiveness of the plant operation. In this regard, the intensity of the consequences of using varying waste qualities depends on the process technology being used, the structural plant design and the operative management of the plants. Two examples initially describe the current relevance of the topic: 1. The attainable proceeds for the recyclable components in the waste such as iron and nonferrous metals or plastic fractions have been falling strongly since the beginning of Correspondingly, the commercial incentive for pretreating and sorting (screening) the waste has declined and the waste volumes delivered frequently contain, e.g., metal components. Fig 1: Plastic balls and cable remnants 2. In recent years, waste treatment plants with a high portion of commercial waste and screenings have increasingly started operation (substitute fuel power stations, German File: Ehlers.doc Page: 1/10

2 abbreviation "EBS-Kraftwerke"). Alternating waste qualities lead to fluctuating pollutant contents and a temporary increased pollutant level in the crude gas at the boiler outlet. Dependent on the structural and process-engineering design of the flue gas cleaning plant, these pollutant fluctuations and the amount of pollutant quantities in the crude gas have impacts on the amount of the resources consumed and on the accruing residuals. For the consideration of the impacts of the varying waste qualities the following cases can be differentiated in correspondence with the waste composition: Waste treatment plants with a proportion of % domestic waste ca. 50% domestic waste, ca. 50% commercial waste, > 80% commercial waste and treated waste ("EBS-Kraftwerke") Whereas the quality of the domestic waste is relatively constant, commercial wastes or treated wastes can exhibit qualitative fluctuations, for example through monofractions, impurity proportions or the content of harmful substances. With increasing proportions of commercial waste, these fluctuations have corresponding impacts on the plant operations. During this talk, the impacts of varying waste qualities along the process chain of a waste treatment plant will be depicted. Starting with the fuel reception, storage and the fuel feed, the consideration of the impacts on the combustion process and the boiler plant will follow. Subsequently, the impacts on the flue gas cleaning plant will be portrayed and the speech will conclude with a summary of the findings. 2. Waste reception, storage and waste feeding system The impacts of varying waste qualities on the areas of waste reception, waste storage and waste feeding will be described in the following. 2.1 Waste reception An increasing portion of commercial and treated waste in association with a large number of waste suppliers requires intensive monitoring of the delivered wastes already during waste reception. Sufficient operating staff need to be planned in for this and an area for pre-tilting and appraising the delivered waste needs to be available. During delivery of special fractions, the spotter and the crane driver need to be correspondingly informed by the weighing staff. File: Ehlers.doc Page: 2/10

3 2.2 Waste storage With the increasing proportion of treated wastes and commercial wastes on the waste fraction delivered in total, the requirements placed on sufficient mixing of the wastes before the fuel feed increases. As far as possible, monofractions or residual waste for example need to be mixed with the rest of the waste to equalise the varying calorific values and to homogenize the supplied pollutant quantities. By applying careful bunker management, one has to ensure that the bunker filling level does not become too high and that enough space is available to mix the waste. A sufficiently dimensioned waste bunker is essential. For example, for a plant with a yearly throughput of 400,000 t (waste reception 1,500-1,800 t/day) with a waste composition of ca. 50% domestic and 50% commercial waste, a bunker volume of 20,000 m³ has proven to be suitable. When storing and stacking the wastes in the bunker, the fluidity of the delivered wastes needs to be taken into consideration. With the increasing proportion of treated and dry commercial wastes, the flowability increases and along with that the difficultly of structuring a stable wall of waste. Normally, the wastes are dumped into a tipping pit, picked up by a crane and immediately stacked behind the tipping pit (Fig. 2). In the waste bunker, such a waste wall can reach a height of m and can contain several thousand tons of waste. If this wall collapses, the compression wave and the flow of waste can destroy the bunker gates and endanger the employees in the delivery area. A retaining wall recessed into the bunker behind the tipping pit can provide a remedy. It reduces the clear height of the stored wastes by, for example, 15 m, preventing uncontrolled slipping of the wastes in the tipping pit (Fig. 2). Fig 2: Staked wastes, retaining wall in the waste bunker File: Ehlers.doc Page: 3/10

4 2.3 Waste feeding The varying fluidity of the wastes in dependence on the proportion of the dry and treated commercial wastes is also to be taken into consideration in the structural design of the fuel feed. The feeding of very fluid material in association with the shaft construction according to Fig. 3a can lead to the wastes flowing freely onto the grate over the feed table. Controlled fuel supply via a lifter is then no longer feasible. For these waste fractions, the design of the fuel feed system according to Fig. 3b is more suitable. The wastes create a column in the vertical feed chute and are fed to the combustion chamber with a double lifter via a channel. However, in this design one must consider that the feeding of viscous, wet, domestic waste can lead to blockages in the feeding system. Fig 3a/3b: Structural design of waste feeding systems 3. Combustion process and boiler plants Impacts of the varying waste qualities on the combustion are primarily due to the respective calorific values. Furthermore, through large, non-combustible components, blockages can occur in the area of slag removal. 3.1 Calorific value of the wastes The calorific values of the fed wastes have an important influence on the combustion process. Table 1 states the calorific values for individual waste fractions as examples. The calorific value on the combustion grate results correspondent to the waste composition. File: Ehlers.doc Page: 4/10

5 Slurry, e.g. mechanically dehydrated sewage sludge 3,000 kj/kg Domestic wastes 7,000-9,000 kj/kg Pretreated domestic and commercial wastes (MBA output) 12,000-14,000 kj/kg Residual waste 15,000-30,000 kj/kg Table 1: Calorific value of various waste fractions Low calorific values Calorific value fluctuations in the lower range (H u < 6,000 kj/kg) caused by very moist domestic wastes or sludge can cause avalanching on the incineration grate: The boiler demands increased waste feed to maintain the set nominal steam flow. Increased waste feed and sudden flashover of the waste after an extended drying phase leads to an "overshooting" of the steam generation and highly turbulent boiler operation. This process can be counteracted and compensated by firing rate controls within a limited frame. Increased calorific values The incineration of wastes with a rather higher average calorific value level (H u >12,000 kj/kg) results above all in an intensification of the combustion process. The drying phase is shorter or even non-existent and the combustion process is concentrated on the corresponding grate zones. The combustion temperature increases and along with that the thermal load of the combustion chamber, especially the grate materials, the refractory lining of the combustion chamber and the boiler pressure parts located in the combustion chamber. Furthermore, the higher calorific values increases the tendency of the waste to already ignite on the feed table so a correspondingly distinct separation of the combustion chamber from the feed zone through a drop edge from the feed table to the grate is advantageous. During the combustion process, the grate bars are protected by a layer of slag. The tendency towards lower slag portions in the treated wastes and commercial wastes leads to a thinner waste bed on the grate and thus to increased grate bar wear. Water-cooled grate bars provide a clear advantage here by separating the grate cooling from the feed of the necessary combustion air volume. Fig 5: High-calorific commercial wastes File: Ehlers.doc Page: 5/10

6 Deployment of waste fractions with alternating calorific values Along with the chronologically averaged, high or low level of the calorific values of the waste, the acceptance of alternating waste fractions with strongly differentiating calorific values influences the combustion process. During this, limited compensation by mixing the wastes in the bunker and correcting the calorific value fluctuations with the firing rate control is possible. However, uneven boiler operation remains, for instance with temperature fluctuations in the flue gas and implications on the slagging and wear behaviour of the boiler plant. The higher the "calorific value stable" proportion of domestic waste on the fed waste, the lower the calorific value fluctuations during combustion. This results in smooth combustion operation with a clearly lower load on the grate and boiler plant. 3.2 Iron and non-ferrous metals With alternating proportion, metals are always contained in wastes and in normal cases can also be processed by the boiler plants. It can currently be observed that, due to the declining revenues for iron and non-ferrous metals, pre-sorting the wastes is also diminishing and the share of metal in the wastes is increasing. Large proportions of metals such as in steel beams and iron plates are contained in the deliveries despite being excluded from acceptance. They can cause limited mechanical damages in the combustion chamber and above all lead to clogging in the slag chute or in the downstream slag removal plants. Experience has shown this happens especially in small plants with a furnace net heat output of, e.g., 30 MW and a correspondingly narrow chute. The clogs need to be removed by the employees at great expense. Too high a proportion of low-melting-point alloys can lead to problems especially in roller grates. The alloys flow through the gaps between the grate bars in the rollers, solidify and plug up the rollers. The supply of cooling and combustion air is disturbed, resulting in increased wear of the grate bars in a short operating period. 3.3 Boiler dust The composition of the supplied waste fractions in association with high or strongly alternating calorific values has impacts on the composition and the fusibility of the boiler dusts and thus on the fouling behaviour of the boiler. If the boiler operation is uneven, "softened" boiler dusts get into the radiation passes of the boiler and settle there on the membrane walls. The heat transmission to the water-steam system decreases and the flue gases set into the heating convection surfaces with increased temperature. There, the corrosion tendency increases significantly, especially on the superheater surfaces. There are possible countermeasures such as facilities to clean the boiler passes "online" during operation, for example by using water blower or a "shower cleaning system" or File: Ehlers.doc Page: 6/10

7 through explosive cleaning. Otherwise, the plant needs to be shutdown periodically for a cleaning stop. 3.4 Operational experiences with different boiler designs Operational experiences show that small boilers with roller grates (furnace net heat output 30 MW) tend to react more sensitively to alternating waste qualities than larger boiler plants with furnace net heat outputs of, for example, MW and water-cooled moving grates. In a narrow combustion chamber of m width, high combustion temperatures can lead to slag deposits on the side walls which coalesce right across the combustion chamber after a short time. The flow of waste is then blocked and the plant has to be shutdown. When using water-cooled grates, the functions of grate cooling and the combustion air supply are separated from each other, which especially in alternating waste qualities has significant advantages on the wear of the grate bars and the control of the combustion process. Here, roller grates, if necessary, bring along the additional problem that the rollers clog due to low-melting-point alloys. Finally, in the deslagging area in smaller plants, blockages caused for example by large metal components, tree roots and coiled waste balls occur more frequently. Fig 6: Combustion chambers with roller grate and moving grate File: Ehlers.doc Page: 7/10

8 4. Flue gas treatment The use of differing waste fractions has the following impacts on the composition of the flue gases on the crossing point between the boiler and the flue gas treatment plant. Level of the flue gas temperature Flue gas volume Amount and composition of the dust portion in the flue gas Amount and fluctuations of the pollutant content in the flue gas 4.1 Level of the flue gas temperature, flue gas volume Flue gas temperature Whereas in new boiler plants the flue gas temperatures after the boiler amount to C, in older plants, especially with increased boiler fouling, the flue gas temperatures at the boiler outlet can rise to C. The flue gases can be cooled down only with sufficiently dimensioned spray dryers or absorber towers to the point where dust separation in normally downstream baghouse filter is feasible. Flue gas volume The water content of commercial and pretreated waste for example lies at 10-15%, for domestic wastes in the range of 20-30% and with very moist domestic wastes or mechanically dried sludge can rise to over 50%. Through that, for example in a plant with a furnace net heat output of 60 MW, the flue gas amounts can lie between 120,000 m³/h (normal state, moist) when using commercial wastes and 140,000 m³/h (normal state, moist) when using domestic wastes. The flue gas fans, flue gas ducts and cleaning section of the flue-gas cleaning plant must be correspondingly dimensioned if the varying flue gas volumes are to be processed with a wide range of waste. 4.2 Amount and fluctuations of the pollutant content in the crude gas When using waste fractions from domestic wastes with up to 50% commercial waste proportions, the dimensioning of a flue gas cleaning plant for a crude gas content of 700mg/Nm³ SO 2 and 1,500mg/Nm³ HCl in the daily average value can be implemented. But if the proportion of commercial and screenings increases, pollution contents of up to 1,000mg/Nm³ SO 2 and 2,000mg/Nm³ HCl can occur permanently as a daily average. Not only do the crude gas contents need to be considered when dimensioning the flue gas treatment levels, but also when dimensioning the attendant facilities such as drag link conveyors, rotary gate valves and conveyor worms. Furthermore, with an increased proportion of commercial wastes and screenings, large dynamic fluctuations in the crude gas pollution contents can occur, which need to be collected File: Ehlers.doc Page: 8/10

9 by controlling the flue gas treatment. A multi-stage flue-gas treatment plant with graduated cleaning of individual pollutant components is advantageous here. The impermissible delivery of large quantities of mercury is a special case. If this should be the case, the coke feed can be correspondingly increased when using separate storage and feeding of slaked lime. Another possible measure is stocking sulphuric-ester doped activated carbon which with increased amounts of mercury can be pneumatically fed to the flue gas at various points in the flue gas treatment plant. 4.3 Implementation example for a flue gas treatment plant One example for a flue gas treatment plant is shown in figure 7. This plant has been put into operation at the Enertec Hameln site in Fig 7: Flue gas treatment plant 4. Incineration line, Enertec Hameln The precipitation of the acidic pollutant components, essentially HCl, HF, SO 2 and SO 3 is operated in the absorber tower and by adding slaked lime before baghouse filters 1 and 2. To adsorb the heavy metals and the dioxins/furans, heath furnace coke is added before baghouse filter 2, which is added again after separation in the backflow before baghouse filter 1. A 2-ply catalyst exists for the denitration of the flue gases to ca. 60mg/m³ (normal state, dry) at an NH 3 leakage < 1mg/m³. The catalyst is not switched as "tail end" to be able to separate the mercury in case it is deposited in the catalyst in the baghouse filter 2. With this plant, during normal operation the limits in 17 BImSchV (German Federal Ordinance on Exposure Control) are clearly undercut. But also during cases of intermittently in- File: Ehlers.doc Page: 9/10

10 creased pollutant concentrations and strongly fluctuating pollutant contents in the flue gas, the approved limits in 17 BImSchV are securely complied with. 5. Summary Starting out from the statement that incineration plants with grate firing can reliably exploit energy even with alternating waste compositions and do so in conformity with the approval, the impacts of using varying waste qualities on the operating behaviour of the plants was presented. In particular, varying waste fractions can impact the fouling and wear behaviour of the boiler and flue gas treatment plants and the use of resources along with the accruing residual volumes. Through additional cleaning stops, increased maintenance expenditures or increased resource and disposal costs, the cost effectiveness of the plant operation can thus be significantly influenced. The effects in the above talk can be mitigated or even completely prevented. If plant operation with an increased proportion of commercial and pretreated waste is assumed, that should be taken into consideration already when selecting the process technology for the boiler and the flue gas treatment plant. Furthermore, the possibly deployed varying waste qualities should be taken into account through structural measures and sufficient dimensioning of the plant components. Due to the current macroeconomic situation, it is to be expected at least in the short-term that presorting and pretreatment of the wastes will continue to decline. As a consequence, there is a wide spectrum of waste available for energy exploitation especially for commercial waste and screenings. Incineration plants with grate firing and a sufficiently dimensioned flue gas treatment plant provide an ecologically sustainable and reliable technology for this, which has proven itself in practice for decades. File: Ehlers.doc Page: 10/10