The author has participated in the design of many cement plant projects in different countries and regions of the world. This article is a summary of the differences between various raw material grinding and waste gas treatment schemes.
Dust emission treatment
Figure 1 is the typical process flow of water-cooled low concentration bag (electrical) filter (Scheme 1).
The raw material mill (which is used throughout) is fitted with a dedicated typical three-fan system. The high temperature exit gas from the preheater enters the high temperature fan through the conditioning tower. When the mill is in the operating mode, all the waste gas from this fan enters the raw mill as a drying heat source. When the mill stops and the kiln opens, its temperature is reduced and conditioned by the conditioning tower, after which the dust is removed by the kiln’s bag filter. Finally, the gas is discharged into the atmosphere through a stack. The waste gas from the raw mill is also filtered by the bag filter. The fine raw meal is conveyed into the homogenisation bin or feed elevator to the kiln.
This scheme is usually used in countries where there are abundant water resources, such as in Kazakhstan, Azerbaijan, and China. The design inlet gas temperature of the bag filter for this particular system is normally between 150 and 180 °C, so the cost of filter bag material is reduced and investment is reasonably low.
Further savings can be made by spraying cooling water onto the vertical pipeline between the preheater and the inlet of the fan instead of using a conditioning tower.
The advantage of this process is that, if the combined moisture of the raw materials is not high, the use of a conditioning tower (or by spraying the pipeline) to lower the temperature of waste gas will enable the fan to operate at a low temperature and reduced flow. This, in turn, will improve the service life and reduce power consumption. However, if the air lock at the discharge end of the conditioning tower is not efficient, air leakage from the conditioning tower will adversely affect the volume of air for the firing system.
Figure 2 illustrates the typical process flow of a water-cooled low concentration bag filter.
The difference between Schemes 1 and 2 is the position of the ID fan. In Scheme 2, which is usually employed when a tube mill is used and high temperature waste gas is needed, the fan is placed in front of the conditioning tower. However, the waste gas induction point of the raw mill needs to be relocated to the rear of the fan. If a tube mill was used in Scheme 1, it would not be able to use up all the kiln exit gas. As a result, the mill would require hot waste gas, and this would be in conflict with the requirements of a gas precipitator for low temperature inlet gas. The conditioning tower would not be able to meet both requirements at the same time.
The advantages of this process is that in the event of a conditioning tower failure, the resulting air leakage would not affect the air volume of the firing system. The disadvantage is that, as such ID fans operate constantly at high temperatures and with high flow conditions, their service lives are inevitably short and power consumption is high.
Figure 3 represents the typical process flow of a gas-cooled low concentration bag filter.
Here the raw material grinding arrangement employs a dedicated circulation pump - typically a three-fan system. Gas from the preheater passes through the ID fan and all the waste gas leaving it is sent to the raw mill where it dries the raw material. When the mill stops and the kiln opens, a cooling fan reduces the temperature in the pipeline before conveying the waste gas into the bag filter. Once through the bag filter, the gas is released into the atmosphere via the stack, while the exit gas from the raw mill is also treated by the bag filter.
This scheme is usually used in countries with low water supply, such as in Saudi Arabia, Libya, and Yemen. The design inlet gas temperature of the bag filter is normally 200 °C. If the design temperature is too low, the resulting high volume of cool gas will be too much for the bag filter to handle. In addition, this scheme will increase the power consumption of the cooling and waste gas fans in the future. It is not possible to use an ESP in this scheme because the processing waste gas temperature it requires needs to be 130 – 150°C, and the waste gas needs to be conditioned to possess a specific resistance.
This Scheme is suitable for 4000 tpd cement plants.
Figure 4 shows the typical process flow of gas-cooled low concentration bag filter.
This is a variation of Scheme 3, but with the addition of an air mixer and the high temperature and cooling gas flows entering the mixer simultaneously from where they flow into the filter. This scheme is suitable for cement plants with a production rate of more than 4000 tpd.
Figure 5 illustrates the typical process flow of a water-cooled, high concentration electrical precipitator (ESP).
This Scheme does not employ a typical two-fan system. The hot exit gas from the preheater enters the high temperature fan via a conditioning tower. All the waste gas from this fan is fed into the raw mill where it acts as the drying heat source. When the mill stops and the kiln opens, the gas, after its temperature is lowered and treated by the conditioning tower, enters the ESP. Finally it is discharged into the atmosphere through the stack. The exit gas from the raw mill is also treated by the kiln’s ESP. The fine raw meal collected is conveyed to the homogenisation bin. The grinding system in this Scheme normally does not require a cyclone; the finished products are collected by the ESP with an inlet concentration requirement of 550 g/m3. In general the ESP does not use a bag filter because it has higher resistance. Since the system fan static pressure needs to be higher than 13 000 Pa, the choices are rather limited.
This features a low concentration bag filter (ESP) that combines kiln inlet and outlet gas (Figure 6).
This scheme is also a typical three-fan system. The hot exit gas from the preheater flows into the hot fan via the conditioning tower. When the mill is in operational mode, all the exit gas from the fan is fed into the raw mill as a drying heat source. When the mill stops and the kiln opens, the bag filter absorbs the gas after its temperature has been reduced and conditioned. Finally, it is discharged into the atmosphere via the stack. The exit gas from the raw mill is also treated by the bag filter. The waste gas from the grate cooler, after passing through a cyclone dust collector, is sent to the cooler or raw material grinding system by the end fan on the kiln via a horizontal pipeline. The biggest advantage of this Scheme is that the combination of kiln inlet and outlet gases can reduce the combined moisture content of the raw materials entering the mill. It is therefore suitable for countries that handle raw materials with a high moisture content or operate in low ambient temperature, such as exists in Togo and Russia. The design wind speed of the waste gas pipeline from kiln inlet to outlet usually is 20 – 25 m/s. The pipeline can be built overhead or buried.
Schemes 1-6 can lower the temperature of the waste gas by incorporating a low-temperature waste heat power generation system.
When the waste heat boiler is running, the hot exit gas from the preheater flows into the high temperature fan via a waste heat boiler. When this is shut down, it flows into the high temperature fan via the conditioning tower. When the mill is operating, all the exit gas from the high temperature fan is fed into the raw mill as a drying heat source. When the mill stops and the kiln opens, its temperature is reduced and conditioned by the conditioning tower before entering the ESP. Finally, the gas is discharged into the atmosphere via a stack. The exit gas from the raw mill is also treated by the ESP. The low temperature waste heat power generation system is currently used in China; however, plant owners rarely mention the use of waste heat low-temperature power generation systems in international projects. During the author’s project designs, he sometimes incorporates a low temperature waste heat power generation option in order to save energy. Nevertheless, plant owners usually ignore such advice.
Parameters of key equipment
Table 1 shows the parameters of the key equipment, for reference. Design conditions are as follows:
- Clinker yield 5000 tpd
- Altitude 500 m
- Ambient temperature 40 °c
- Fuelled by heavy oil
- Calorific value 10 000 kcal/kg
- Coal-fired rate at kiln inlet: 40%
- Heat consumption of clinker kcal/740 kg-cl
- 5 Stage preheater.
Treatment of emissions: SO2
SO2 mainly comes from the fuel or sulfur-containing materials used in cement production. During the clinker cindering process, a large quantity of SO2 is produced. However, within the temperature range of 800 ~ 1000 °C, most SO2 can be absorbed by oxides (for example, alkaline-oxide) in the materials to form intermediates such as sulfuric acid and calcium sulfite that are retained in the clinker. Since the material has adequate contact with the gas, the retention rate can be higher than 98%. It is for this reason that the desulfurisation design scheme has not been considered for cement plants so far.
Treatment of emissions: NOx
NOx emissions in clinker generation are associated with the high temperature burning process inside the kiln, emission volume and combustion temperature, excess air and reaction time. The higher the combustion temperature, the larger the amount of excess air, the longer the reaction time and the more NOx will be generated. At present, de-NOx design is mainly based on the following concept:
Low NOx combustion technology.
Selective non-catalytic reduction method (SNCR).
As NOx is generated primarily in rotary kilns and associated with N content in fuel, the formula for calculating NOx emissions is as follows:
GNOx – NOx emission load kg
B – coal consumption, t (based on batching, coal consumption/t of clinker = 0.1093 t)
N – content in coal %
ß – conversion rate of nitrogen in coal % (conversion of N 25%)
Heavy oil estimate
GNOx – NOx emission load, kg
B – heavy oil consumption, t
N – content in coal %
ß – conversion rate of nitrogen in oil % (conversion of N 35%)The above calculated fuel consumption is for kiln inlets that account for 40% of system burning fuel consumption, the calculation results can be used as a theoretical reference for reducing agent system design.
Read the article online at: https://www.worldcement.com/asia-pacific-rim/09022012/treating_waste_gases/