NO X , combined double filtration and wet scrubbing
2 APPLIED TECHNIQUES .1 Overview and introduction
2.3 The thermal treatment stage
2.3.3 Fluidised beds
Fluidised bed incinerators are widely applied to the incineration of finely divided wastes e.g.
RDF and sewage sludge. It has been used for decades, mainly for the combustion of homogeneous fuels. Among these are coal, raw lignite, sewage sludge, and biomass (e.g. wood).
The fluidised bed incinerator is a lined combustion chamber in the form of a vertical cylinder. In the lower section, a bed of inert material, (e.g., sand or ash) on a grate or distribution plate is fluidised with air. The waste for incineration is continuously fed into the fluidised sand bed from the top or side [66, UllmansEncyclopaedia, 2001].
Preheated air is introduced into the combustion chamber via openings in the bed-plate, forming a fluidised bed with the sand contained in the combustion chamber. The waste is fed to the reactor via a pump, a star feeder or a screw-tube conveyor.
In the fluidised bed, drying, volatilisation, ignition, and combustion take place. The temperature in the free space above the bed (the freeboard) is generally between 850 and 950 °C. Above the fluidised bed material, the free board is designed to allow retention of the gases in a combustion zone. In the bed itself the temperature of is lower, and may be around 650 °C or higher.
Because of the well-mixed nature of the reactor, fluidised bed incineration systems generally have a uniform distribution of temperatures and oxygen, which results in stable operation. For heterogeneous wastes, fluidised bed combustion requires a preparatory process step for the waste so that it conforms with size specifications. For some waste this may be achieved by a combination of selective collection of wastes and/or pretreatment e.g. shredding. Some types of fluidised beds (e.g. the rotating fluidised bed) can receive larger particle size wastes than others.
Where this is the case the waste may only require only a rough size reduction.
[64, TWGComments, 2003] [74, TWGComments, 2004]
Pretreatment usually consists of sorting out and crushing larger inert particles, and shredding.
Removal of ferrous and non-ferrous materials may also be required. The particle size of the waste must be small, often with a maximum diameter of 50 mm. However, it is reported that average acceptable diameters for rotating fluidised beds are 200 - 300 mm. [74, TWGComments, 2004]
The schematic diagram below shows an installation that pretreats mixed MSW for incineration in a fluidised bed incineration plant. Several pretreatment stages are shown including mechanical pulverisation and pneumatic separation, along with the final stages of incineration, FGT and residue storage:
Figure 2.12: Schematic diagram showing pretreatment of MSW prior to fluidised bed combustion
During incineration the fluidised bed contains the unburned waste and the ash produced. The ash surplus is usually removed at the bottom of the furnace. [1, UBA, 2001, 33, Finland, 2002]
The heat produced by the combustion can be recovered by devices either integrated inside the fluidised bed or at the exit of the combustion gases or a mixture of layouts.
The relatively high cost of pretreatment processes required for some wastes has restricted the economic use of these systems to larger scale projects. This has been overcome in some cases by the selective collection of some wastes, and the development of quality standards for waste derived fuels (WDF). Such quality systems have provided a means of producing a more suitable feedstock for this technology. The combination of a prepared quality controlled waste (instead of mixed untreated waste) and fluidised bed combustion can allow improvements in the control of the combustion process, and the potential for a simplified, and therefore reduced cost, flue-gas cleaning stage.
The following table shows the properties of various waste fractions that are treated in fluidised beds [33, Finland, 2002]:
Commercial waste
Pretreated construction waste
Sorted and pretreated household waste Lower heating
value as received
MJ/kg MWh/t
16 – 20 4.4 – 5.6
14 – 15 3.8 – 4.2
13 – 16 3.6 – 4.4
Moisture Wt % 10 – 20 15 – 25 25 – 35
Ash Wt % 5 – 7 1 – 5 5 – 10
Sulphur Wt % <0.1 <0.1 0.1 – 0.2
Chlorine Wt % <0.1 – 0.2 <0.1 0.3 – 1.0
Storage properties Wt % Good Good Good as pellets
Table 2.6: Properties of various RDF (Refuse Derived Fuel) fractions treated in fluidised beds.
[33, Finland, 2002]
The following fluidised bed furnace technologies can be differentiated according to the gas speeds and design of the nozzle plate:
stationary (or bubbling) fluidised bed (atmospheric and pressurised): The inert material is mixed, but the resulting upwards movement of solids is not significant (see Figure 2.13)
a version of bubbling fluidised bed is the rotating fluidised bed: Here, the fluidised bed is rotated in the incineration chamber. This results in longer residence time in the incineration chamber. Rotating fluidised bed incinerators have been used for mixed municipal waste for about ten years
circulating fluidised bed: The higher gas speeds in the combustion chamber are responsible for partial removal of the fuel and bed material, which is fed back into the incineration chamber by a re-circulation duct (see diagram Figure 2.14).
In order to start-up the incineration process, the fluidised bed must be heated to at least the minimum ignition temperature of the added waste (or higher where required by legislation).
This may be accomplished by preheating the air with oil or gas burners, which remain operative until incineration can occur independently. The waste falls into the fluidised bed, where it is crushed through abrasion and incineration. Usually, the major part of the ash is transported with the flue-gas flow and requires separation in FGT equipment, although the actual proportion of bottom ash (removed from the base of the bed) and the fly ash depends on the fluidised bed technology and waste itself. [1, UBA, 2001].
Fouling problems, common in waste incineration boilers can be managed by controlling waste quality (mostly keeping Cl, K, Na and Al low) and by boiler and furnace design. Some boiler and furnace designs can be used in fluidised beds (but not in mixed waste grate boilers) because of the more stable temperatures and the presence of the bed material.
2.3.3.1 Stationary (or bubbling) fluidised bed incineration
This type of fluidised bed is commonly used for sewage sludge, as well as for other industrial sludges e.g. petrochemical and chemical industry sludges.
The stationary, or bubbling fluidised bed (see Figure 2.13), consists of a cylindrical or rectangular lined incineration chamber, a nozzle bed, and a start-up burner located below.
1
2
3 4
5 6
7
8 9 1 Sludge feed with disintegration/spraying
2 Additional fuel 3 Atmospheric oxygen 4 Waste gas
5 Fluidized bed 6 After-burner chamber 7 Start-up incineration chamber 8 Inspection glass
9 Air preheater
Figure 2.13: Main components of a stationary/bubbling fluidised bed Source [1, UBA, 2001]
Preheated airflows up through a distribution plate and fluidises the bed material. According to the application, various bed materials (silica sand, basalt, mullite, etc.) and bed material particle sizes (approx 0.5 – 3 mm) can be used.
[2, infomil, 2002], [64, TWGComments, 2003]
The waste can be loaded via the head, on the sides with belt-charging machines, or directly injected into the fluidised bed. In the bed, the waste is crushed and mixed with hot bed material, dried and partially incinerated. The remaining fractions (volatile and fine particles) are incinerated above the fluidised bed in the freeboard. The remaining ash is removed with the flue-gas at the head of the furnace.
Drainage and drying pretreatment stages can be used so that the waste burns without the need for additional fuels. Recovered heat from the incineration process may be used to provide the energy for waste drying.
At start-up, or when sludge quality is low, (e.g. with old sludge or a high share of secondary sludge) additional fuel (oil, gas, and/or waste fuel) can be used to reach the prescribed furnace temperature (typically 850 °C). Water can be injected into the furnace to control the temperature.
The furnace is usually preheated to its operating temperature before waste feeding starts. For this purpose a start-up incineration chamber (see Figure 2.13) may be located below the nozzle bed. This has an advantage over an overhead burner, as the heat is introduced directly into the fluidised bed. Additional preheating may be provided by fuel lances that protrude over the nozzle bed into the sand bed. The sewage sludge is supplied when the furnace temperature reaches the operating temperature, e.g. 850 °C.
The size of the furnace is largely determined by the required evaporation (furnace cross-section), the heat turnover in the furnace (furnace volume) and the required amount of air.
Example operational parameters for a fluidised bed sewage sludge incinerator are shown in Table 2.7:
Parameter Units Value
Steam load kg/m2h 300 – 600
Feed air amount Nm³/m2h 1000 – 1600
Heat turnover GJ/m³h 3 – 5
Final incineration temperature °C 850 – 950
Residence time, open space and afterburner zone sec. min. 2
Preheating of atmospheric oxygen °C 400 – 600
Table 2.7: Main operational criteria for stationary fluidised beds Source [1, UBA, 2001]
The preheating of air can be eliminated completely with higher caloric fuels (e.g. dried sewage sludge, wood, animal by-products). The heat can be removed via membrane walls and/or immersed heat exchange systems.
Some processes incorporate drying as a first step. Steam for the drying may be produced by a boiler and then used as the heating medium with no direct contact between the steam and the sludge. Sludge vapours can be extracted from the dryer and condensed. The condensed water typically has a high COD (approx. 2000 mg/l) and N-content (approx. 600 - 2000 mg/l) and may contain other pollutants (e.g. heavy metals) from the sewage sludge, and therefore will often require treatment before final discharge. The remaining non-condensates may be incinerated.
After incineration, the flue-gases can be cooled in a heat exchanger in order to preheat the incineration air to temperatures of approximately 300°C and in some cases over 500°C. The remaining heat in the steam boiler can be recovered and used for the production of saturated
2.3.3.2 Circulating fluidised bed (CFB) for sewage sludge
The circulating fluidised bed (CFB see Figure 2.14 below) is especially appropriate for the incineration of dried sewage sludge with a high heat value. It works with fine bed material and at high gas speeds that remove the greater part of the solid material particles from the fluidised bed chamber with the flue-gas. The particles are then separated in a downstream cyclone and returned to the incineration chamber.
Primary air
Coarse ash
Air
Fluidized bed condenser Fluidized bed
Incineration chamber
Recycling cyclone
Flue gas to the boiler Lime bunker
Sewage sludge
Secondary air
Figure 2.14: Main components of a circulating fluidised bed Source [1, UBA, 2001]
The advantage of this process is that high heat turnovers and more uniform temperature along the height can be reached with low reaction volume. Plant size is generally larger than BFB and a wider range of waste inputs can be treated. The waste is injected at the side into the incineration chamber and is incinerated at 850 - 950 °C. The surplus heat is removed through membrane walls and via heat exchangers. The fluid bed condenser is placed between recycling cyclones and the CFB, and cools the returned ash. Using this method, the heat removal can be controlled.
2.3.3.3 Spreader-stoker furnace [64, TWGComments, 2003]
This system may be considered as an intermediate system between grate and fluidised bed incineration.
The waste (e.g. RDF, sludge etc) is blown into the furnace pneumatically at a height of several metres. Fine particles participate directly in the incineration process, while the larger particles fall on the travelling grate, which is moving in the opposite direction to the waste injection. As the largest particles are spread over the greatest distance, they spend the longest time on the grate in order to complete the incineration process. Secondary air is injected to ensure that the flue-gases are adequately mixed in the incineration zone.
Compared to grate incineration the grate is of less complicated construction due to the relatively smaller thermal and mechanical load. When compared to fluidised bed systems the uniformity of particle size is less important and that there is a lower risk of clogging.
2.3.3.4 Rotating fluidised bed [74, TWGComments, 2004]
This system is a development of bubbling bed for waste incineration. Inclined nozzle plates, wide bed ash extraction chutes and upsized feeding and extraction screws are specific features to ensure reliable handling of solid waste. Temperature control within the refractory lined combustion chamber (bed and freeboard) is by flue-gas recirculation. This allows a wide range of calorific value of fuels, e.g. co-combustion of sludges and pretreated wastes.
2.3.4 Pyrolysis and gasification systems