Sorbent injection systems can be divided into four types. These are:
- furnace sorbent injection;
- economiser sorbent injection;
- duct sorbent injection; and
- hybrid sorbent injection.
The simplest technology is furnace sorbent injection where a dry sorbent is injected into the upper part of the furnace to react with the SO2 in the flue gas. The finely grained sorbent is distributed quickly and evenly over the entire cross section in the upper part of the furnace in a location where the temperature is in the range of 750-1,250°C. Commercially available limestone (CaCO3) or hydrated lime (Ca(OH)2) is used as sorbent. Whilst the flue gas flows through the convective pass, where the temperature remains above 750°C, the sorbent reacts with SO2 and O2 to form CaSO4. This is later captured in a fabric filter or ESP together with unused sorbent and fly ash. Temperatures over 1,250°C, result in sintering of the surface of the sorbent, destroying the structure of the pores and reducing the active surface area. To encourage fast sulphation, it is important to have an even distribution of the sorbent in the flue gas. If the temperature can be kept within 750-1,250°C, the major part of SO2 transformation occurs within 1-2 second. At temperatures below 750°C, reactions practically cease. Sorbent injection can be carried out at several levels to deal with part load conditions. The process is optimised by using adjustable injection nozzles.
Removal efficiency of up to 50% can be obtained with a Ca/S ratio of 2 with Ca(OH)2 used as sorbent. If CaCO3 is used as sorbent the removal efficiency will be considerably lower, or the Ca/S ratio will have to be much higher. Bjerle and others (1993) discussed the results of laboratory scale experiments, which showed that an SO2 removal efficiency of 95% could be achieved with furnace sorbent injection. The following conditions were applied when these results were achieved: residence time was about 2 seconds at a Ca/S molar ratio of 2 and the sorbent particle size was <3 µm. In the experiments it was found that the particle size of the sorbent and the mixing/dispersing conditions are two parameters, which need much more attention. Fine sorbent particle size (<5 µm) and an even distribution of the sorbent over the cross-section of a boiler could significantly improve the process performance.
In an economiser sorbent injection process, hydrated lime is injected into the flue gas stream near the economiser zone where the temperature is in the range of 300-650°C. Wang and others (1993) reported the results of an experimental study of the process. They found that in contrast to the furnace sorbent injection process, where the reaction temperature is around 1100°C, Ca(OH)2 reacts directly with SO2 since the temperature is too low to dehydrate Ca(OH)2 completely. In this temperature range, the main product is CaSO3 instead of CaSO4 and the reaction rate is comparable to or higher than that at 1100°C. The production of carbonate in the process is undesirable, since it not only consumes the sorbent but also blocks the access of SO2 to active sorbent surfaces. The tests showed that carbonation significantly increased with reaction temperature.
In duct sorbent injection, the aim is to distribute the sorbent evenly in the flue gas duct after the preheater where the temperature is about 150°C. At the same time, the flue gas is humidified with water if necessary. Reaction with the SO2 in the flue gas occurs in the ductwork and the by-product is captured in a downstream filter. Removal efficiency is greater than with furnace sorbent injection systems. An 80% SO2 removal efficiency has been reported in actual commercial installations. There is a wide range of possibile process variations including the use of the sorbent (Ca- or Na-based, dry or suspended) and recirculation/reactivation of the by-product. A pre-filter, if installed, has the advantage that fly ash and desulfurization products are separated. This makes recirculation of unreacted sorbent easier, resulting in improved sorbent utilisation.
An efficient reaction between SO2 and particles/droplets with the active Ca(OH)2 is achieved if the particle/droplet surface is large and active. Again, this condition is met by small particles with an open pore structure. A dry sorbent has to be finely grained and a sorbent in suspension must be atomised into small droplets. Wet grinding achieves smaller particles than dry grinding which means that a sorbent in suspension results in more efficient particles. On the other hand, this kind of sorbent is much more difficult to handle compared with a dry sorbent. The sorbent may be kept in its most active form Ca(OH)2 with relative humidity. However, the flue gas must be kept above the dew point in order to minimise the risk of undesired deposits in the flue gas duct and process equipment after sorbent injection.
There are many factors, which influence the performance of a duct sorbent injection process. These include sorbent reactivity, quantity of injected sorbent, relative humidity of the flue gas, gas and solids residence time in the duct, and quantity of recycled, unreacted sorbent from the particulate control device. Finally, when designing a duct sorbent injection system, a good working knowledge of the flow conditions in the flue gas ductwork is important. The easiest and most efficient way of achieving this is to ensure well-controlled flow conditions. As the process needs reaction time, the most obvious solution would be to establish a dedicated reaction chamber. However, even a simple reaction chamber can contribute prohibitively to the capital costs of such a process.
The hybrid sorbent injection process is usually a combination of the furnace and duct sorbent injection systems aiming to achieve higher sorbent utilisation and greater SO2 removal. Various types of post furnace treatments are practised in hybrid systems, such as:
- injection of second sorbents such as sodium compounds into the duct; and
- humidification in a specially designed vessel.
Humidification reactivates the unreacted CaO and can boost SO2 removal efficiency up to 90% depending on the process. The hybrid process offers the following advantages:
- relatively high SO2 removal;
- low capital and operating costs;
- easy to retrofit;
- easy operation and maintenance with no slurry handling;
- reduced installation area requirements due to compact equipment; and
no waste water treatment.
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