4.4.1. Technology and measures of exhaust gases treatment is being applied by factory
40 a, Management measures
The factory always focuses on application of mitigation measures from the source. The type of vehicles, machine, equipment are maintained regularly to ensure the best operating. Therefore noise, vibration and emissions are limited. To check the details wear and replace lubricant for machine periodically.
Workers are equipped work wear such as hats, gloves, protective clothes…..
Factory specially concern to workers working in highly noisy department. To arrange alternatively work, organize reasonable timing, divide work time into 3 shifts, 30 minute for break. Time off is calculated in the working time.
Periodic health checks are holded annually 1time / year. Workers in dangerous places will be overhauled 2 times / year to detect early illness due to noise.
Trees were planted in the factory and on transport routes to reduce noise, dust and emissions.
Water was sprayed regularly to reduce dust in the area surrounding the factory.
Using heat exchanger make use of heat into the kiln (drying FO oil from 80- 1200 C) to save energy.
To mitigate the impact from transportation vehicles to the lowest level, the factory has applied the following measures:
+ Before contracting delivery, vehicles as well as vehicle owners have to ensure the technical conditions and other requirements for transport on the road.
+ Cars are maintained regularly in accordance with regulations, ensure proper tonnage and strictly abide the regulations on vehicles circulating.
+ Regulation speed for the vehicles circulating in the factory area.
41 b, Technical measures
Sources of air pollution are mainly of factory exhaust fumes from the kiln stack.
Pollutant agents are temperature, SO2, NOx. Flue gas treatment technology of factory is described as diagrams below:
Figure 4. 5- Diagram of exhaust fumes treatment technology (Source: Can Thep Thai Nguyen Factory)
Notes to the diagrams of exhaust fumes treatment technology:
During operating, the temperature of smoke in kiln is very high from 1100 - 11500C. In order to take this heat back, the indirect heat exchanger will be used. Hot gases from the kiln (1) enters the heat exchanger system (2), clean air from outside is
42 sucked through the sucking end (3) goes to outside of heat exchanger tube. After being heated, the air is sucked by fan (4) then provided to kiln. Exhaust gases coming out of heat exchanger have temperature between 300-4000 0C. Accompany with the heat transferring process is the dust precipitation process. Flue gas passes through a heat exchanger (has been cooled) therefore dust is more likely to precipitate. Under the effect of gravity, suspended dust particles will precipitate to the plate on the surface of heat transfer.
This device imported with steel rolling mill line of DANIELI - ITALIA.
Therefore it gets high level of synchronized. Dust removal efficiency reaches 95% in this device.
4.4.2. Plan proposal
4.4.2.1. Overview of treatment NOx, NH3, dust a, For toxic gases
Adsorption Method
Adsorption is a gas dissociation based on the affinity of some solids for certain gases present in the gas mixture in generally, in the exhaust gas in particularly. In this process, the exhaust gas molecules are retained on the surface of solid materials. Solid materials are used in this process are known as the adsorbent, and the gas trapped in the process known as substance being absorbed.
There are two types of adsorption: physical adsorption and chemical adsorption:
- Physical adsorption: is a multi molecular adsorption (multiple layers adsorption), binding force is an attractive force between molecules (Vander force - Waals) not forming surface compounds.
43 - Chemical adsorption: the single molecule adsorption (a layer adsorption), binding
force is chemical force that create the surface compound.
- Adsorbent materials: the porous material with large internal surface, is formed by synthetic or nature.
- Adsorption materials must meet the following requirements: Adsorption capacity is high, effected scope is broad, having the necessary mechanical strength, ability to revert easily, cheap price.
Adsorbent: conventionally are substances such as activated carbon, silica gel, activated alumina, zeolite …
- Activated carbon: characterized by hydrophobic properties, so it is widely used to treat various forms of moisture. Basic drawback of activated carbon is inferior mechanical strength and flammable.
- Silicagel: the amorphous silica hydrated (SiO2. nH2O), silica is fireproof, low regenerated temperature (110-2000 C) and sufficient mechanical strength. However, it is destroyed by the humidity drops.
- Activated alumina (aluminum oxide is activated Al2O3.nH2O, 0 <n <6) is prepared by heating the different hydrocid aluminum, activated aluminum is durable under the effect of the humidity drops, it can be applied to reclaim the polarized organic compounds and air drying.
- Zeolites are microporous, aluminosilicate minerals, contain alkali metal oxides and earth alkaline. General chemical formula of Zeolite is Me2 / nO.Al2O3.SiO2.7H2O (Me is alkali metal cations, n is its valence). Zeolites have great sorption capacity of
44 steam. Maintaining highly active at relatively high temperatures (150 - 250 ° C) is the outstanding advantage of zeolite.
Adsorption Efficiency
NOx are strongly adsorbed by activated carbon. But during contacting with nitrogen oxides, coal can fire and explode. In addition, coal has low mechanical strength and their recovering can turn NOx into NO.
Absorption capacity of Nox and silica gel is lower than activated carbon despite of their mechanical strength and no blaze. Their recovering can turn NOx into NO as activated carbon
Both adsorption NOx efficiency and mechanical strength of Activated alumina is low
Absorption method
Absorption is the method based on principle of contacting between exhaust emission and solid, whereas these gases either are dissolved in a liquid or change their composition into less toxic. Absorption methods based on interaction of air with solid sorbent, which means dividing into two phases, depend on the interaction between the absorber and absorbed substances in the gas phase.
Absorption method is divided into two types: physical absorption and chemical absorption:
- Absorb physics based on the solubility of the gas phase constituents in the liquid phase.
- Absorb Chemical: absorber involved chemical reaction with absorption solvent.
45 - To absorb nitrogen oxides people use water, alkaline solution, acids and
oxidizers.
Absorption efficiency
Depending on the exposed area between the flue gases and liquids, duration of exposure, environmental concentration and reaction rates between the absorber and emissions.
b, For dust
Based on the concentration of dust, chemical properties, physical properties, demand for air circulation and filter dust away from the air is divided into 3 levels:
- Cleaning: only retain dust particles of size> 100 mm (preliminary filter)
- Intermediate cleaning: not only big particles but also small particles are retained.
Dust concentration after filtering is about 30 - 50 mg / m3.
- Refined cleaning: can filter dust particles smaller than 10 mm with high performance. Dust concentration after filtering is only 1-3 mg / m3.
4.4.2.2. Proposing schemes
Measures to limit and handle pollution from noise and vibration:
Pollution being caused by noise and vibration in the steel mill with capacity 300.000 ton/year will impact on workers directly. Some measures limiting the noise pollution will be implemented:
- To minimize noise at source and allocate anti-noise devices such as earplugs for employees working in the noise departments.
- To isolate reasonably sources of noise into private locations
- The great shocking machine was concerned to deal during laying the foundation
46 - Noise and vibration emitted in production chain are mostly from the actuator of the machine, but the equipments are imported from DANIELI - ITALIA having high quality, running without vibration and less noise.
- To install silencers system in some locations that wind blowing out.
- Planting trees around the factory to create noise separators.
Measures of reducing thermal pollution and against heats:
From the beginning the workshops was designed so that make use of advantage of natural ventilation windows at the maximum. Factory is ventilated, great workshops height (about 14 -16m). Workshops are arranged window system, natural ventilation skylights around the house. To use insulated roofs and roofing walls. At work, ventilators and coolers are equipped for the workers. Also around the workshops are airy so heat dispersion is relatively good
Measures to prevent environmental incidents:
The environmental problems can occur in the factory due to fire. Factory is likely to fire because of fuel oil, flame welding and cutting. So the explosion prevention regulations were strictly carried out.
Machinery and equipment working at high temperatures and high pressures have clear origin and strict implementation of periodic inspection regime.
Factory has sufficient earthling system for the equipment, machinery, buildings ... as well as the firefighting equipment, water system.
Measures of sanitary - occupational safety:
To comply thoroughly the provisions of labor safety for employees, organize training annually for employers and periodic 2 times/year for workers.
47 To inspect and surveillance periodically the health of workers and staffs, timely detect occupational diseases.
To organize annual work environment inspection for the direct producing areas to ensure the working environment of the employees are the best.
Management measures:
Raising the environmental consciousness of workers through media programs billboards, posters....
- To build regulations on environmental protection in the factory as follows:
- Regulations on the functions and responsibilities of each office and workshop in industrial hygiene- safety and environmental protection.
- Regulation on the powers of managers in the implementation of their duties.
- Regulation of reward for the collective and individual having initiatives in the reducing pollution for factory
- Regulations dealing with the collective and individual acts of irresponsibility in occupational safety - explosion prevention - environmental protection.
The factory is being implemented environmental management system ISO 14000 by Thai Nguyen Steel Joint Stock Company:
- To strengthen education on safety, industrial hygiene, environmental protection.
- To inspect and supervise regularly the implementation of procedures and technical rules to ensure industrial safety.
- To establish specialized forces and appropriate means to actively deal and tackle the consequences quickly when incidents occur
48 Proposal selection exhaust gases treatment technology for Can Thep Thai Nguyen Factory:
From the results obtained during the survey in Can Thep Thai Nguyen factory in March 2015 showed that emissions of NOx, NH3, and dust in the kiln kiln stack of the plantis increasing over the years. So we need to have the appropriate remedial measures for kiln stack emissions in order to limit the exceeding permitted standard in the coming years.
Proposal NOx treatment technologies for kiln stack.
Kiln stack need to be installed SNCR system to control NOx emission levels.
SNCR technology is advanced technology. So the kiln stack is not necessary to install other treatment systems. With this technology, NOx emissions can be reduced to 800 mg/Nm3. Factory can install additional SNCR system to minimize the amount of dust dispersed into the atmosphere.
Figure 4. 6- Diagram of SNCR technology
(Source: Report on status of air quality in Hon Chong Xi Mang Company)
49 In steel production line, factory should install NOx control technology in source base on SNCR technology (Selective Non Catalytic Reduction). The principle being applied to reduce NOx is a chemical reaction between NOx being generated during combustion and ammonia (NH3) in the form of gas at a temperature of 800 – 1100 0C, as follows:
4NO + 4NH3 + O2 → 4N2 + 6H2O 6NO2 + 8NH3 → 7N2 + 12H2O
Reducing agent may be ammonia in gaseous or liquid, or urea aqueous solution CO(NH2) 2 25 - 45% by weight. At temperatures over 200 ° C, urea is decomposed into CO2 and NH3. Solution 40% urea is diluted with water being pumped inside and be atomized via two nozzles of level 1 and 2. The purpose of level 1 spraying is to cover maximum surface with the highest retention time, and level 2 spraying is to optimize efficiency. Capacity urea is controlled by measuring the concentration of NOx at the kiln stack. By keeping constant solvent capacity and urea solution, atomized efficiency is maintained during reducing NOx.
Proposal of dust treatment technologies for kiln stack: Depending on the selection filter level, the factory can use the dust filters as follows:
The dry dust collecting equipment based on mechanism: Dry dust collecting equipment include the main following types: Dust-deposition chamber and Cyclon:
single cyclon and combinatorial cyclone
The wet dust collector: in the wet dust collector, dust collecting process is often accompanied by the process of cooling air and gases absorption. Therefore the
50 devices of this type are often used to solve simultaneously the problems: dust revocation, gas absorption and gas cooling for treating exhaust fumes from burning fuel sources, or emissions from metallurgical kiln. The wet dust collectors can reclaim dust sized from 100 - 0,1am with performance from 85-99%. The wet dust collectors include the main types as follows: Empty gas washing equipment and Gas washing with cushioning material equipment
Equipment ESP: ESP equipment operating under the principle of ionized dust and separate them from the gas stream passing through magnetic field. This equipment can filter extreme small dust particles (size from 10 - 0,005am) with significantly high efficiency (85-99%) and has many advantages such as lower energy costs, high temperature, easy automation...
Dust filter using membrane: operating principle of this device is using porous material to retain dust when dust streams pass through that membrane. These devices are classified into several types as follows: Oil refinery equipment and Fabric Filter Equipment
Fabric Filter Equipment is the most reasonable technology to handle dust volume arising from producing process with its advantages as follows:
- Very high collection efficiencies possible (99.9 + per cent) with a wide range of inlet grain loadings and particle size variations. Within certain limits fabric collectors have a constancy of static pressure and efficiency, for a wider range of particle sizes and concentrations than any other type of single dust collector.
- Collection efficiency not affected by sulfur content of the combustion fuel as in ESPs.
51 - Reduced sensitivity to particle size distribution.
- No high voltage requirements.
- Flammable dust may be collected.
- Use of special fibers or filter aids enables sub-micron removal of smoke and fumes.
- Collectors available in a wide range of configurations, sizes, and inlet and outlet locations.
Figure 4. 7- Diagram of Fabric Filter Equipment (Source: Hitachi plant construction, Ltd)
Operating Principle: Firstly air including dust passes through a filter fabric.
Initially dust particles larger than the gap between the fibers will be retained on the surface of cloth base on sieve principle, the smaller particles cling on the fiber surface
52 by collisions, gravity, and electrostatic attraction. Gradually, obtained dust layers are thickning that create filter aid membranes retaining small dust particles.
When air accesses in the direction from outside to inside of bag, the bag has to stretching frame being made by metal to keep bag from out of flat situation during working. With this scheme, mouth bag being connected with sieving surface always turn upward.
Filter surface reverting can conduct after stopping the air passing through the device and clean the dust on the cloth in 2 ways:
- Mechanical vibration by a special structure.
- Backward blowing by compressed air or clean air.
Air load capacity of conventional filter cloth is 150 -200 m/h. Resistance of device is about 120 ~ 150 kg/m2. Dusted cycle is 2-3 h
Filter efficiency reaches 99.8% and filter tiny particles is due to filter aid layer.
After a period, thick extreme dust layer will raise over resistance of the membrane, we have to stop gas passing through and remove the dust on the cloth. This operation is called filtering capability reverting.
53 PART 5: DISCUSSION AND CONCLUSION