Pneumatic conveying systems enable the transport of fine bulk materials in closed pipelines and thus often have lower investment costs compared to mechanical conveying systems, although many different feeding systems makes it difficult to choose the best and most economical conveying solution.

There are system suppliers who have different feeding systems in their portfolio. However, each supplier always has a preference for "their own" system. When working out the best and most economical pneumatic conveying solution, considering all the general conditions, independent advice can often be helpful.

Introduction

In the cement industry, pneumatic conveying systems compete with mechanical conveying systems. Pneumatic conveying has the advantage of transporting fine bulk materials in closed pipelines. This means that conveying routes can be adapted to existing plants and constructions. The piping can be laid in bends and contain horizontal and vertical sections. Such a simple and flexible arrangement is not possible with mechanical conveying systems, which run in straighter lines. Another crucial point is that with pneumatic conveying we are talking about a closed system. There are no transfer points that have to be dedusted. Dust-free conveying is a big advantage here compared to mechanical systems such as belt systems. Mechanical systems also have more moving parts that are subject to wear and usually much higher initial investment costs for equipment than pneumatic systems.

The advantage of mechanical systems, however, is their lower energy consumption. But it is on this point that pneumatic conveying systems differ significantly and can even be more energy efficient than mechanical systems.

Overview of pneumatic conveying applications in the cement industry

In the cement industry, not only cement is pneumatically conveyed, but also raw meal, filter dust, bypass dust, gypsum, fly ash, ground granulated blast furnace slag (GGBFS), pulverised coal and other fuels. The materials can vary greatly in terms of density, particle size distribution, moisture, temperature, and wear behaviour - all parameters that have a significant impact on pneumatic conveying and the equipment selected. However, the optimal choice of pneumatic conveying system depends not only on the material, but also on conveying capacity and conveying distance. All this cannot be adequately covered by a single pneumatic conveying principle.

Structure of pneumatic conveying systems  

Only the so-called overpressure conveying systems in connection with conveying pipelines that can transport several hundred tonnes per hour over several hundred metres are considered. In these systems, compressed air is generated by compressors or blowers to move the bulk material through the conveying pipeline. This principle creates a pressure difference which, in addition to the air flow, is responsible for moving the bulk material from the feed point to the receiving point. The energy requirements of the various systems depend on the material being conveyed, the conveying capacity and the conveying distance. Every bend or diverter valve or other irregularities increase the pressure loss in the pipeline and thus the energy requirement. Therefore, if pneumatic systems are to be optimised in terms of energy, the system design or conveying line design is of great importance.

Pneumatic conveying systems are basically divided into lean-phase conveying and dense-phase conveying, with various conveying phase types in between (Fig. 1). In lean-phase conveying, the bulk material load in the air stream is low and a certain air velocity is required to capture the material at the pipe inlet. This is also the most critical part of the pipe, as the air velocity is lowest here and as it expands towards the end of the pipe, the velocity increases and the material is always entrained in the air. In dense-phase conveying, a much higher pressure difference is used to move the material through the pipe. In such systems, however, it is important that the material plugs that form in the conveying pipe do not exceed a critical length, otherwise conveying will be blocked.

 

 

Pressure vessel systems are typical dense-phase conveying systems. With all other systems, such as the conventional rotary valve, the high-pressure (ceramic) rotary valve and the screw pump, only transport in the lean-phase is possible. Pressure vessel systems can be designed for back pressures of 7.5 bar and more and enable very flexible dense-phase transport over longer distances of more than 2 km. This feed system results in discontinuous transport. Even when several pressure vessels are interconnected, only "quasi" continuous conveying is ever achieved. Compared to other pneumatic conveying systems, a high process and automation effort is required to achieve optimal conveying. Due to the time-delayed filling and emptying of the pressure vessels, some flap and/or pinch valve technology is required. Wear and spare parts and the corresponding costs must be taken into account. In terms of energy consumption, the pressure vessel system has clear advantages over the other feed systems and can adapt better to changed conveying pipe runs. This is the case, for example, when such systems are installed on self-discharging cement ships and call at different ports.

To overcome critical conditions in dense-phase conveying, there is a combination of air slide system with overpressure systems. These so-called "FLUIDCON" or "Fpipe" systems offer all the advantages of pneumatic conveying systems, but with significantly lower energy requirements. The use of the transport principle of an air slide in the conveying pipe enables dense-phase conveying with an increased bulk material load.  Such a system can be combined with different feeding systems and clogging of the material in the pipe is avoided by transporting the material in the pipe via an air-permeable fabric and injecting aeration air along the entire length of the pipe or only at the most critical sections of the pipe. The aeration air is not used to transport the solids but keeps the bulk material in the pipe in a fluidised state with minimal internal friction. This type of conveying enables the transport of fine bulk solids at low differential pressures and low air velocities.

Rotary valves and screw pumps are the most commonly used feeding systems in the cement industry. With conventional rotary valves, unfortunately, higher back pressures are not possible due to the high leakage air volumes via the rotary valves and the associated wear problems. The counter pressure should remain below 1 bar. With ceramic rotary valves and up to 1 bar counter pressure, the leakage air volume becomes much larger, but the wear can be kept at a lower and still acceptable level due to the ceramic lining. The drive power of the rotary valve is significantly lower than that of the screw pump. However, attention should be paid to the wear of the airlock. There are ways to limit this disadvantage, e.g. by rotating the housing, as the wear mainly occurs in the area of the air leakage connection.

Screw pumps allow counter pressures of up to 2 bar. The disadvantage of screw pumps is, on the one hand, the higher motor power required to drive the screw. On the other hand, the material plug effect at the end of the feed screw, i.e. in the transition area from the pressure less to the pressurised area, must be ensured by a constant material feed. This plug serves as a seal and prevents the backflow of conveying air.  Only a pulsation-free, steady, and constant feed of the material into the feed line ensures low wear of the end of the feed screw. The screw pump as a feeding device also has disadvantages in the case of changing conveying line courses, as the pressure conditions change as a result. These framework conditions are given, for example, in the case of self-discharging cement ships that call at different ports of discharge. However, screw pump systems, if designed correctly, are very durable and low-maintenance and requires a low installation height.

Summary

Pneumatic conveying systems are very widely used in the cement industry. Often there is no alternative to these systems, even though they are considered energy intensive. However, significant energy savings are possible with the different pneumatic conveying systems. Of course, this depends on the requirements that are placed on the transport. It is therefore advisable not to rely on just one supplier, but to consider and compare the task as a whole. Even if priorities differ from customer to customer, a comparison should be made that takes all aspects into account. In addition to the investment costs, spare parts and maintenance costs, the energy costs of all consumers used for the task should be compared. Only a comparison of the operating costs over several years provides certainty as to which system is really the best or most economical.