Formusatin Vladimir

Energy-efficient technologies

Improving dust-systems' performance with BTM

Many industries, such as construction material production, power engineering and mining operations, use the following process technology: decomposition of source material, sorting final product into fractions, removing the gas phase, usually air, from the solid phase. Let's here remind that aerosol is an assembly of solid or liquid particles suspended in air. Aerosols, appearing as the result of dispersion of solid powder-like source material and its transformation into the suspended state, are called dispersion aerosols or dusts.

The dust system used at thermoelectric power stations, allows getting coil dust - minute particles suspended in the air, burned in boiler furnaces. The pulverization system, presented below, consists of the following components: ball mill, dust separator, dust extractor (whirler), mill fan (not shown in the Figure) and pulverized fuel pipe.

In the mill the coal is dried and pounded; then coal particles are divided into fractions in the separator by the action of centrifugal and gravitational forces. The biggest particles follow the reversing circuit to return to the mill to be re-crushed. The whirler is the place where dust is separated from air flow by centrifugal action, while the conditioned dust (powder) is stored in the bunker, ready to go into the boiler furnace. Mill fan provides hot air circulation around the whole circuit of the system.

Basic criteria for evaluating efficiency of the system are: maximum performance and low energy consumption (Kwh/ ton) used to pound and transport coal, provided the dispersibility of the coal dust is satisfactory.

It is very important to mention that ball tumbling mill (BTM) has a very high level of energy consumption when talking about its no-load operation, reaching more than 90% of the total energy consumption when the mill is loaded with coal. Hence, the more load is on the mill, the higher is its performance, and, accordingly, the lower is energy consumption. However, when the mill is overloaded (its grinding capability is exceeded), it suffers fuel excess, which can lead to an emergency. Loading the mill with smaller amounts of coal than it can process leads to increase in energy consumption.

Here comes the first conclusion: there is an optimal load for ball tumbling mill. We suppose that the drying performance of the mill is higher than the grinding one, but the values of ball load and ventilation are constant.

Mill and dust system performance are generally depended on the ventilation level of the whole circuit. It is determined by the performance of mill ventilator, aerodynamic resistance of the machines and dust concentration in different sections of the system. The fact that performance of the ventilator and machines resistance are constant values, leads to the second conclusion: in order to reach high performance indices of the system we should eliminate high level of dust concentration in some parts of the system.

It's a fact that the particles of coal dust, transported by the air flow through the pipes, acquire certain electric charge - thus the triboelectric effect of the coal dust particles takes place. Electric charge appears due to friction happening between particles of coal dust, those accumulating on the walls of the pipe and those moving in the gas flow.

Moving particles are charged with a certain level of electricity. The material, which the pipe is made of, doesn't have a considerable influence on the value and sign of the charge. The latter is confirmed by laboratory investigations at test bench, with epoxy covered metal walls. In case the system is provided with the return circuit, certain portion of charged particles can repeatedly get from the separator into the mill and back. Due to this circulation the dust particles acquire 20 kW charge value or even more.

Coal dust, as well as other aerosols, mostly the charged particles of the same polarity, are prone to undergo the effect of electrical dispersion of particles by the action of coulomb forces. Dispersion margins are the walls of system machines - mill, separator, whirler and pipes. Mutual interaction of the charged particles and influence of the air flow determine joint behavior processes of the coal dust, conditioned by the laws of disperse systems electro-gas-dynamics.

Under the direct influence of aerodynamic forces the charged particles of dust are accumulated in front of system sections, having local increase in aerodynamic resistance, which are mill nipple, separator and whirler outer throats. As a result, in these places appear huge electrical charge, carried by coal dust particles. As the system ventilation increases the dust cloud thickens, and charge density grows. Parts of the system, where the charged dust has a higher level of concentration, are marked with blue on the Figure.

The charged cloud of dust in front of the outer throats creates additional aerodynamic resistance of system components and leads to electro-separation of dust particles depending on their charge. The charged cloud in front of the outer throat of the mill prevents conditioned particles equally charged with the cloud from leaving the mill cylinder. Therefore, the part of coal in the mill is grinded; another part is untimely let out. The same way the process of dust electro-separation in the separator goes, which can explain the fact that the separator output product contains 50% of final dust, while the mill cylinder is very hard to be released from the dust for a long period, even when proper ventilation is provided and the coal is not loaded onto the mill.

GENERAL CONCLUSIONS:

1. Electrical charge of coal dust appearing in the dust systems produces a lot of undesirable effects, decreasing dust systems' performance by 25-30%.
2. Some sections of the system, having increased aerodynamic resistance, produce huge dust electrical charge.
3. Appearing electrical charge leads to increase in total resistance of the dust system and dust electroseparation.
4. Dust electroseparation decreases system performance and increases dispersibility, as a result of return of the output dust from the separator to the mill.
5. Dust electrization and the processes it involves cause unsatisfactory work of mill controller in charge of coal load, as far as basic criteria defining load level - the amount of coal, vibration, noise, mill cylinder resistance, etc. don't actually influence the process of dust electrization.
6. In order to make the system efficient some actions should be taken to decrease or eliminate the influence of coal dust electrization, which allows loading the system according to main criterion - output dust consumption.
7. Dust system designers or rebuilders need to take into account the influence of electrical charge of the coal dust so as to eliminate sudden local changes in sections of pipes and system machines.

There has been a lot of research work in the dust systems area, testing the BTM-50 at power units of thermal power stations. The distinctive features of the experiments are: continual registration of dust electrization current, coal dust consumption (using specially designed electronic flowmeter (dust meter) and pressure differential at the outer throat of the mill. Besides, the indications of all regular devices of the system have been recorded.

We have obtained and processed large experimental material, including the one concerning the efficiency of electronic dust meter.

We offer consulting and recommendations, provide technical support while the dust systems are initially tested; basing on test results offer different technical solutions so as to eliminate negative effect of coal dust electrization; provide necessary information concerning electronic dust meters. We welcome your suggestions concerning everything mentioned above.

E-mail: vformus@mai.ru     Tel. (+373 22) 727817, (+373 22) 253320
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