China Energy-Efficient Lignite Drying Upgrade Processing Production Line

Energy-efficient production lines for Lignite drying, burning, upgrading and its safe operation
By Lv Yide
Wuhan Aojie Technology Development Ltd. Co
(China-Hubei-Wuhan Post:430223)
Abstract
This paper comes up with a new safe technology for the lignite drying and upgrading. It uses pulverized lignite from bag dust collector, which is combusted in a diffusion-type energy-efficient coal injection burner (stove), to provide energy for efficient rotary dryer. In this way, we can dry lignite. Since the process of drying lignite is easy to explode, so we come up with requirements for every process, to make sure the whole production efficient and safe.
Key words
Energy-efficient production lines for Lignite drying, burning, upgrading; Energy-efficient diffusion-type lignite combustion furnace and its combustion system; Anti-explosive system; Safe operation
1 Introduction
Lignite is the youngest coal in the latest forming solid fossil fuels. It has high explosibility and flammability due to its high volatile; it has low heating capacity and fixed coal content due to its high moisture, which is generally up to 40% and very difficult to remove. Moderate ash content and high tar content make it very necessary to take desulfurization system into considerate during the drying process.
Large coal mines of China that produces lignite have been found mainly in Inner Mongolia. In order to develop lignite utilization, we have carried out comprehensive cooperation with Datang International Group, Datang International Chemical Company, Datang International Chemical Engineering Design Institute, Hebei Electric Power University, Beijing State Power Design Institute (formerly Hebei Electric Power Design and Research Institute) and so on. At the same time, we have signed 500 tons / year lignite drying process line contract.
Pay attention to the following requirements during the lignite drying process:We need explosion protection system no matter in lignite crushing system, conveying systems, warehousing systems or going into the rotary dryer.We should keep the outside air (fresh air) from the lignite rotary dryer and seal the burner (stove), rotary dryer tightly.Lignite drying system process should use fully automatic production line process line and install explosive alarm system in every explosive point. It needs to cut off all power immediately when the explosive gas is excessive, as long as the factory whistle, alarm, evacuation. The entire production line should be far away from the electrical control center, and arrange patrol in DCS. At the same time, we should avoid manually operated system since it have been safety incidents and equipment accidents in lignite drying production line due to not following the design specifications.
The lignite has high tar content and its moisture was composed by internal water and external water. So during lignite drying process, it needs an installation of chiller in the outlet of the lignite at the end of the lignite rotary dryer. In this case, we can achieve security storage, to be packaged and transported.
To sum up, lignite drying is different from drying other materials, which should be paid special attention! We must not build lignite drying process line with conventional means.
2. Block diagram of the new lignite drying process line (downstream process):
Target:Lignite drying capacity: first moisture 30% ~40%, final moisture 8%~15%.Lignite heat (heat value): from 3400 ~ 3700Kcal / kg to 5000 ~ 5500 Kcal / kg or more, making lignite the burning for thermal power plants or other industrial coal furnace.Technical requirements for combustion systems and safe operation in lignite drying:
Lignite is the youngest coal in the latest forming solid fossil fuels. It has low heating capacity, high moisture and low ash fusion point, which is very difficult to burn efficiently.basic principles for lignite combustion:
We have introduced new technology and new experience of lignite drying and combustion, especially using dried pulverized lignite in energy-efficient diffusion-type pulverized coal injection burner (stove). And then it forms closed loop combustion by blowing the lignite to pulverized coal warehouse before the furnace through a high-pressure air tube. Then it doesn't need to introduce other coal.
According to our accurate calculation, the burning off rate can reach 99% when strictly sealed. The slag content is only 1%. And the slag is yellow-white and non-toxic, which can be recycled as building materials used in industry. The flame covers 95% of furnace, which is clear and transparent. Moreover, the chamber temperature can be adjusted from 700 ºC~ 1200 ºC. An unattended work environment can be achieved with an automatic ignition system and automatic slag system. There is advanced control equipments such as PLC control system and DCS control systems aside the furnace.
Among the lignite combustion system, we introduce the most advanced technology in the combustion field to lignite combustion system. Such as: multi-level air distribution technology, multi-staged combustion technology, non-nitride combustion technology, automatic slag discharging technology, CO/CO2backing flowing technology, waste heat circulating technology and anti-coke technologies, and so on.
In particular, most water in the dried pulverized lignite which is collected by the dust bag has been removed, and the heating capacity can reach 5000Kcal / kg or more. However, the ash fusion point is still very low. According to laboratory testing, T1 is still about 1200 ºC. Such as FeS content in lignite is high, coupled with the inaccurate wind, FeS is very easy to generate Fe2O3and SO2 under high temperature, while generate FeO, who has a lower melting point about 900 ºC, in reducing atmosphere,.
Therefore, it is very easy to coke during the lignite combustion process. It affects production if we shut down the process line to clear the coke. So the most important issue in the lignite combustion process is: the flame in the combustion furnace flame must be oxidized, which means, we should strictly control the excess air ratio.
For the burner, it has a very strict requirement for air volume and air pressure of the first air fan and the secondary air the fan. Especially it needs to find the relationship between wind speed and air flow in the coal combustion and coal stove. These issues can be solved when adjusting the furnace.
It mustn't distribute three times cold air (fresh air), or a lot of cold air will going to the rotary dryer and cause explosion and burning of lignite. Therefore, the lignite drying in rotary dryer is anoxybiotic.
Basic principles for lignite combustion:ensure the combustion time of pulverized lignite in the furnace;ensure the temperature of the combustion process;ensure the best mixture of wind and coal in the furnace
only when lignite is combusted according to the above requirements, can we ensure complete combustion of lignite. Our dual swirl pulverized coal burner can reach burning off rate 99%, slag content about 1%. The slag is yellow white. At the same time, we can ensure non-coke and efficient production.safety measures for lignite drying process:
From general survey for the fall behind rotary dryer, we can see the explosion happened just in the rotary dryer, sometimes it dismembered rotary dryer. There was also some explosion occurred in the dust collector, which burned the bag completely and even cause a fire.
Therefore, setting up a reliable safety device and explosion-proof technology is the most important lignite drying conditions. We will set from the following aspects:Finished coal warehouse.
We need bag dust collector and explosion protection valve on the library roof top, level indicator, several liquid CO2pulse gun and thermometer in the warehouse. All these devices must be reasonable designed and allocated. Otherwise, the loss is innumerous if explosion happens.Using energy-efficient diffusion-type coal furnace.
The furnace must have the following functions:Provide qualified air temperature and air volume, meet the heating capacity. It must not have flame and cold air going into the rotary dryer. It needs to connect the furnace and lignite dryer with seal to prevent cold fleeing.The furnace temperature is stable, and the furnace is filled with orange and transparent flame. There is no obvious flame in mixing chamber, just a red light. The flue gas oxygen content is below 4%. And there is a temporary emergency discharging chimney, electricity valves and lignite coal chute on the mixing chamber's roof. Lignite is introduced into the rotary dryer directly. once the follow-up equipment such as dryers have a problem, the emergency discharging chimney will open and stop feeding immediately, so that high-temperature exhaust gas from the stove will emission bypass. Lignite flow cannot enter the dryer to prevent lignite stay in the dryer machine to produce explosion and combustion.The burning off rate of the furnace should reach 98%, with reasonable wind distribution, which will not produce coking phenomenon. Hot air furnace will not use manually slag discharging, while using mechanical automatic bottom slag discharging.It will not have large amount of heating elimination with a good insulation system. The outside temperature will less than 50 ºC.The furnace should equip with automatic diesel ignition system. It can begin production once there is some burning pulverized coal in the pulverized coal warehouse.It should achieve fully automated unattended operation (except meter worker and cleaner).Using specialized lignite feeding chute.
Specialized lignite feeding chute is composed by two-layer heat-resistant steel alloy. It will inflate cooling liquid CO2through two layers of alloy steel directly into the rotary dryer. Then it can prevent high-temperature flue gas oxygen (O2) content is too high, which will result in an explosion and combustion of lignite. The lignite goes to rotary dryer from the first layer of bottom chute.
(4) Using energy-efficient lignite specialized rotary dryer. Its key technology is:Wide-angle screw feeding plate and trap cut lifting plate. Wide-angle feeding plate can make the material flow into the dryer quickly. Trap cut lifting plate can make the lignite flow distributed evenly in the rotary dryer, which makes the hot air flow evenly through the pulverized lignite. As a result, it can achieve efficient heat exchange and improve drying quality and yield as soon as possible. According to the initial moisture, we can adjust the speed of the dryer to ensure the yield and quality of the dryer.
3.we can install a liquid CO2injection device at the end of the dryer to achieve efficient cooling, so that a small amount of molten coal tar will cool immediately to wrap lignite particles. Then it can flow from dryer into the next process.
If the gas temperature is too low, it needs to draw some hot exhaust gas from the furnace into the bag dust collector. At the same time, it needs to add a certain amount of CO2gas into the bag dust collector to prevent it from being burned, and also pay attention to prevent fresh air flow inside.
(5)Explosion-proof measures for the bag dust removal
Because there is a large amount of water vapor in a large amount of lignite smoke a lot of entering the cyclone dusts collector and bag dust collector, so the temperature of the lignite should be kept above the dew point of the flue gas.
If the gas temperature is too low, it needs to draw some hot exhaust gas from the furnace into the bag dust collector. At the same time, it needs to add a certain amount of CO2gas into the bag dust collector to prevent it from being burned, and also pay attention to prevent fresh air flow inside.
Therefore, when using rotary dryer (dryer) in the process of baking or drying lignite, we must apply the following principles:
1. Anaerobicly drying lignite;
2. Strict feeding and discharging systems;
3. Suitable rotary dryer inclination and adjustable rotary dryer speed;
4. Scientific collecting and desulfurization technology;
5. Efficient piping design and insulation installations;
6. Fully automatic control and alarm device.
We should not only achieve the above points but also a strict production management system. only when we manage to do those requirements, can rotary lignite dryer reach safe and efficient operation, environmental protection and fully automatic control.