Using coal for energy production is one of the causes of global climate change. At the same time, it also creates a large amount of ash and slag and is a source of environmental pollution of soil, water and air. The recycling and use of ash when burning coal in Vietnam has achieved certain results. To sustainably develop the energy industry in Vietnam in the coming time, it is necessary to have measures to continue to innovate and apply scientific and technological advances as well as to invest in the construction of recycling facilities and use of ash waste. slag industries.
KS. Kieu Cao Thang - TS. Nguyen Duc Quy
Vietnam Mineral Recruitment Association
Coal is one of the non-renewable energy mineral resources. The process of mining and processing and using fuel coal is shown in Figure 1.
Ash and slag is one of the solid wastes generated in the coal burning process of thermal power plants, production of cement, chemicals - fertilizers, metallurgy, paper industry and agricultural-food drying ...
Depending on the source of fuel and combustion technology, the volume and composition of ash and slag varies. In addition to the need of thousands of hectares of land to fill such as in India, America, China ... coal ash is also a source of environmental pollution of land, water and air.
Since the middle of the 20th century, the recycling and use of coal ash as construction materials (building materials), roads and the manufacture of new materials… are increasingly developed. Below is an introduction to the situation, recycling and ash use directions of thermal power plants in Vietnam.
1. Regarding the resources and use of coal in Vietnam
Vietnam is a poor country with coal resources in terms of both reserves and types. According to geological documents, Vietnam's coal resource is only 34 790 058,103 tons, of which the verified reserve is 6 140 683,103 tons.
The situation and forecast of coal mining and consumption by economic sectors are shown in Table 1.
2. Sources of thermal ash and slag
2.1 Source of ash, slag, thermal power
Currently, most coal-fired thermal power plants are mainly located in the North, due to their proximity to coal sources. Total capacity of thermal power plants in operation as of 2010 is 4 250 MW.
In the period 2010-2020, it will continue to build large thermal power plants such as Mong Duong (2 000 MW), Nghi Son (3 000 MW), Vung Ang (2 000 MW), Tra Vinh (3 800 MW), Soc. Moon (4 400 MW), Kien Giang (1 200 MW) ... According to the national electricity development plan for the period 2011-2020 with consideration to 2030, in the Decision No. 1028 / QD -TTg dated July 21, 2011 , the total capacity of thermal power plants according to the base load plan, in 2020 will be 36,000 MW and by 2030 will be about 75,000 MW.
The domestic source of fuel coal for power plants is usually low-quality coal, with ash greater than 31 ÷ 32%, even 43 ÷ 45%. Therefore, thermal power plants emit a large amount of ash and slag, possibly from 20-30% of coal used. With an average coal consumption rate of about 500 g / kWh, the total amount of coal used for thermal power and the ash and slag formed is shown in Table 2.
Thermal power plants must thoroughly collect all the original ash and slag and store them in storage yards, to avoid spreading and polluting the environment.
2.2 Raw ash and slag quality
Before 2010, power plants often used traditional coal-fired furnace technology. Raw material coal is finely ground -200 mesh over 90%, Coal powder mixture with hot wind is sprayed into the combustion chamber of the boiler, the combustion efficiency is usually <70%.
After 2010, power plants often use coal burning technology in fluidized bed furnaces. Raw coal is crushed to 5 ÷ 8 mm and fed into the furnace, the combustion efficiency of the combustion chamber is higher than 90%.
In the process of burning organic matter, about 20% of non-combustible inorganic clumps into large particles and falls to the bottom of the furnace called slag (or bottom ash). and recovered by many methods of dust collection (xiclon, bag filter, electrostatic filter, ...) called ash (fly ash).
The time when fuel is stored in the combustion chamber is not long, so no matter which coal burning technology is used, a coal blast furnace or a fluidized bed furnace, in the ash and slag, there are still coal particles that have not been burned or have not burned. Therefore, the amount of loss when burned (MKN) mixed in ash and slag is still quite high, usually 20 ÷ 30% (Pha Lai, Uong Bi, ...) sometimes up to 45 ÷ 50% (Yen Phu, Dam Bac Giang, Ninh Binh before). .
The chemical composition of the original ash depends mainly on the type of coal used in the thermal power plant. Currently, power plants use coal or brown coal. These differences are shown in Table 3. These properties guide the processing and use of raw ash.
From the above figures can be seen:
+ When using coal source (Quang Ninh), ash and slag contains a large amount of silicon and alumina with a very small ratio of lime and SO3, called silica-alumina ash. This is an active inorganic substance similar to natural polygon. If prepared (separate organic part in ash), can be used as an additive to cement. If it is processed deeply (thoroughly separates the organic part of the ash), it can be an additive to the concrete.
+ When using brown coal source (Thai Nguyen, Lang Son, ...), the content of silicon and alumin is much lower, but the ratio of lime and SO3 is much higher, called sulfate-lime ash. Due to the relatively high lime content, these ash are themselves a solid binder in water. Therefore, this ash can be used directly to reinforce the rocky pavement of the road transport system.
3. Research and development of ash and slag recycling in Vietnam
3.1 Initial Research
In the world, for a long time, people have recruited fly ash to get selected coal for reuse, and collected ash used as construction materials (in 1914 used fly ash to replace a part of porland cement of concrete).
In Vietnam, in power plants such as Yen Phu (Hanoi), Cao Ngan (Thai Nguyen), Viet Tri (Phu Tho), Dam Ha Bac (Bac Giang) and Ninh Binh - due to the high amount of unburnt coal, the burning heat of the ash reaches 3000 ÷ 4000 kcalo; Local people have exploited ash and slag, used manual sieve to separate the leftover coal to burn bricks, calcify lime and make civil fuel (almond coal). The remaining ash is mixed with the mud as a binder for direct use, or lime is added to make the slag bricks.
In the 80s of the last century, Power Company 1 in conjunction with the Department of Mineral Recruitment, University of Mining and Geology studied and recruited ash and slag in Cao Ngan thermal power plant (Thai Nguyen) and Pha thermal power plant. Lai (Hai Duong). The study was only conducted in a laboratory and the scale of the selection line consisted of only 4 small-volume machine compartments (in Cao Ngan, Pha Lai and Duc Giang). At that time, the research goal was only to recover the amount of unburnt coal still contained in the ash. Therefore, it has limited the ability to deploy the application on an industrial scale.
In the early 90s, the Union of Cement Enterprises applied research results of the project 26A-07-01 on "Research on using Pha Lai thermal ash and slag as an additive for cement production", group of engineers mineral sorting (Sub-Association of Mineral Selection Course 7), researching, designing and installing the ash and slag sorting line of Pha Lai Thermal Power Plant with a capacity of 80,000 tons / year (calculated according to the ash selection). On March 25, 1993, the first 200 tons of selected slag was transported to Hoang Thach Cement Factory to put into production. After that, the Ministry of Construction decided to allow the use of Pha Lai ash and slag additives in cement production in Hoang Thach, Bim Son and Hai Phong rotary kiln cement factories. The mixing ratio is not more than 15 %, provided that the Pha Lai ash additive must satisfy the requirement of MKN (unburnt coal) content <11%. The lower the MKN content, the faster the strength-forming reaction and the higher the binder strength obtained.
Since 1997, meeting the requirements of using ash to produce mass concrete in the construction of irrigation dams, hydroelectric projects and some other civil construction projects, this sorting workshop has provided dozens of thousand tons of qualified ash (with MKN content <6%, humidity Wa <3%) used for projects such as Bai Thuong dam (Thanh Hoa), Tan Giang dam (Ninh Thuan), Long river (Binh Thuan province) ) ...
In 1995-1997, the Institute of Materials Science, the Vietnam Academy of Science and Technology also conducted a study on the possibility of recycling and using ash in Ninh Binh thermal power plant to recover coal for domestic and production needs. bricks, while ash and slag as materials for roads and construction bricks. Having established a construction investment project with ash and slag collection facilities; But because of objective difficulties, it has not been implemented.
3.2 Development of investment and construction of ash and slag sorting factories
Due to the outstanding advantages and economic efficiency of roller compacted concrete technology using additive ash and slag (RCC) in the construction of hydroelectric dams, Vietnam has decided to apply this technology to dam projects. electricity, first of all is Son La hydroelectricity. Son La hydropower project has a huge demand for ash, averaging 20,000 tons / month. So just in a short time, a series of thermal ash and slag processing factories were built and put into operation.
In 2006, Bac Son Joint Stock Company and Cao Cuong Joint Stock Company carried out 02 projects to build Pha Lai ash-sorting workshop at Binh Giang Lake. These workshops are currently operating with a capacity of 5,000-6,000 tons / month of ash products. The raw materials of these factories are ash from Binh Giang waste reservoir, Pha Lai thermal power plant.
In 2007, Son La Hydropower Project Management Board built a sorting workshop adjacent to the Pha Lai II thermal power plant, with a capacity of 10,000 tons / month. In early 2008, this workshop was put into operation. The raw material source is fly ash, taken directly from silo of the Pha Lai II thermal power plant.
In 2008, the Pha Lai Concrete Additive Company (PHALAMI) built the Pha Lai thermal fly ash sorting workshop with a capacity of 5,000 tons / month. May 8-2009, officially put into operation. The material source of this sorting workshop is also directly taken from the cylinders of the Pha Lai II thermal power plant.
At the end of 2009, Ninh Binh Thermal Power Joint Stock Company has invested in building a ash and slag sorting workshop, with a capacity of 20,000 tons / year. In September 2011, the workshop started operation. The source of raw materials is ash and slag stored at the landfill No. 1, near Ninh Binh Thermal Power Plant.
In 2011, at Uong Bi Thermal Power Plant, it is under construction of an ash and slag sorting workshop with a capacity of 50,000 tons / year. Expected, early 2012 will put into production.
There are also studies of application of ash and slag from Pha Lai thermal power to other but ineffective methods such as: Song Da 12 Company has invested in building a concrete additive factory from Pha Lai thermal fly ash. by electrostatic selection method. And Hai Son Joint Stock Company has invested in building a concrete additive factory by the method of burning in a tunnel kiln.
The flotation factories are operating using flotation method that allows the collection of ash, meeting the quality requirements for the production of building materials (cement additives, concrete additives), and collecting coal for recruitment. for households for civil and industrial consumption.
The thermal power ash and slag sorting factories are operating, basically the same technology scheme of the same sorting: In the motorized flotation machine compartments, the selection is petroleum products and pine oil. Diagram of recruitment technology as shown in Figure 4.
The differences of these workshops are in terms of feed source and product dehydration plan:
+ The feed source is raw ash and slag stored in the landfill: use pump to suck, transfer from the landfill, pour directly into the spiral classifier, separate the slag before stirring to contact flotation.
+ The feed source is primitive fly ash stored in silos: Use a pipe, lead to an anti-sediment tank and then pump straight into the mixing tank to contact flotation. If you do not use a pipeline, you must use a truck to transfer it to the storage bunker of the sorting workshop, and then feed the materials into the contacting flotation tank.
+ The ash needs to be dried, bagged and stored before being loaded and transported to the consuming place. When financial conditions allow, the ash is dehydrated in a concentration tank, filtered and dried - fully implementing these stages, ensuring industrial hygiene in the selection workshop. In the case of the building of a spacious sorting workshop, the ash is dehydrated in the sedimentation tank, then loaded into the yard to sprinkle with natural water before being fed to the dryer.
The development process mentioned above shows that it is necessary to invest in the construction of ash and slag sorting workshops in thermal power plants, both making use of waste resources and minimizing environmental impacts. Vietnamese science and technology staff can research, design, manufacture equipment, construct, install and transfer technology to ash and slag sorting workshops with modules of 10 tons / h ÷ 30 tons / h.
Figure 4: Schematic principle of ash and slag sorting factories in Vietnam
4. Use ash and slag after recycling
4.1 Use of ash as a concrete additive
Thermal fly ash is used as a bulk concrete additive for hydroelectric dam projects using roller compacted concrete pouring technology (RCC).
The total number of hydroelectric projects expected to be put into operation with medium capacity (over 30 MW) and large is about 70 projects with a total capacity of 12,341 MW. In which, the number of projects expected to apply RCC technology is 10 projects with a total volume of concrete for dam embankment is 12.6 million m3 and require ash admixture from 1.7 to 2.6 million m3.
Some large projects that need to use ash as a concrete additive for RCC technology are shown in Table 4.
The ash of coal-fired thermal power plants in Quang Ninh is silicon-alumina ash, meeting the quality requirements as an additive for RCC concrete according to ASTM-C618. In fact, four ash and slag processing factories in Pha Lai have provided sufficiently recruited ash as concrete admixtures and met the construction progress of Son La, Ban Ve, Song Tranh 2 hydropower plants ... and in In the coming time, Lai Chau Hydropower is being deployed.
4.2. Use ash and slag as a cement additive
The ash loss content when calcined <11% can be used to mix into cement with the average rate of 10 ÷ 20 %. Therefore, the cement company also has a large demand for ash and slag as an additive. With the aim of reducing costs and improving some of the properties of cement (making large concrete, waterproof, anti-thermal expansion, lighter than plain concrete), it is possible to mix the amount of ash to 20 ÷ 40%, depending on the depends on the type of cement to be produced.
A medium-sized cement production plant like Kansai Ninh Binh with a capacity of 1.4 million tons / year can use about 280,000 tons of additives / year. Thus, with the total capacity of the cement plants in Vietnam to 2015 is about 75 million tons, the demand for ash used as an additive is huge (Table 5).
4.3. Use ash and slag to make concrete
Our country is currently in the process of developing bridges and culverts, hydroelectric projects, and embankments. According to the survey, the concrete companies supplying the market about 15% are prefabricated concrete, the remaining 85 % is sold by cement factories directly to construction investors.
There are many infrastructure constructions that require some special additives such as fly ash from thermoelectric fly ash to increase technical solutions and increase the time of use. But because the amount of fly ash that meets the quality standards (sorting ash) is small, its use is not widely used. All concrete companies such as Vinh Tuy Concrete Company, Ha Tay Construction Concrete Company, Viet Duc Concrete Company ... currently have to import some chemicals as additives from abroad at very high prices.
4.4 Demand for sulfate-lime ash to construct roads
Many countries around the world have used ash and slag from thermal power ash to build roads, such as embankment roads through soft soil; as a drainage layer, the foundation layer of the structure of the road surface of a motor vehicle, the lower layer of the surface layer of sandstone material reinforced with lime ash - gypsum or ash - cement; making concrete pavement and making mineral powder of asphalt concrete ...
In Vietnam, the Department of Roads, the University of Transport has conducted research on using fly ash as a binder to reinforce sand and stone materials for road surface. The results showed that when a mixture of 80 % fly ash and 20 % of lime used as the binder for sugar reinforcement, it will achieve quite high mechanical strength.
When making pavement, use the following mixtures: Limestone + lime + moist fly ash; Wet fly ash + cement or moist fly ash + lime + gypsum. Dry fly ash is used as a mineral powder of asphalt concrete and for making concrete pavement. The use of fly ash as roads is very promising. There is a pilot project to build rural roads in Kim Dong district, Hung Yen.
4.5 Demand for production of bricks, light concrete, autoclaved concrete
Unburnt brick (laterite brick, lime slag brick) has been used for a long time, but still has many limitations and currently accounts for only about 8 % of total brick cover.
To produce 40 billion bricks from now to 2020, it will take 60 million tons of coal, and in 2020 alone, 6.3 million tons of coal must be used. The production of fired bricks, especially manual furnaces, emits a large amount of harmful CO2 and SO2, affecting the living environment, human health, and reducing crop yields.
Recently, the Prime Minister issued Decision No. 121/2008 dated August 29, 2009, approving the master plan for building materials development up to 2020. In which, the planning orientation for development of unburnt construction materials to 2020 as shown in Table 6.
Table 6: Demand for baked and unburnt building materials
Standard brick ratio
|2||Fired clay bricks||22,5||24 ÷ 25.6||25.2 ÷ 29.2|
|3||Unburnt construction materials||2,5||6,4 ÷ 8||12.6 ÷ 16.8|
|4||Unburnt rate, %||10,0||10 ÷ 25||30 ÷ 40|
Source: Decision 121/2008 / QD-TTg of the Prime Minister, dated August 29, 2008.
The advantages and economic and technical efficiency in production, using unburned materials will change consumers' habits of using building materials. Certainly unburnt brick products will develop strongly in the coming time.
Using unburnt brick (light concrete, autoclaved concrete produced from ash and slag) for high-rise buildings has high economic efficiency. In this mixture (ash, cement, lime, gypsum, aluminum powder), the ash from thermal fly ash is the main ingredient, accounting for up to 70 % by weight. Therefore, the demand for quality ash to supply to the non-fired brick production market, light concrete bricks and autoclaved concrete is huge.
5. Development direction of recycling and using ash and slag in Vietnam
1. Vietnam is industrializing the country towards sustainable development. Therefore, the sustainable development of industries related to mining and processing and using coal is an indispensable requirement.
In the coal burning process, there are two main sources of waste: gas and solid. In the exhaust gas source, in addition to CO2, NxOy and SO2 ... there may be some volatile metals. Solid waste sources are mainly ash, plaster, and some rare and scattered metals.
Therefore, the development of innovation and the application of scientific and technological advances to the collection, recycling and use of waste is of great importance, not only reducing the volume of waste but also recovering one more. the number of secondary materials and reducing the source of environmental pollution.
2. The use of energy coal without washing with high ash content (Ac≥ 35-40%) now not only creates great transportation costs but also creates more ash and slag of the burning process, increases the load and causes pollution. environment.
It is necessary to soon implement the planning of coal sorting factories to increase the output of clean coal through the cleaning with the ash content Ak ≤ 8-10% from 30% to 80 ÷ 90% of coal production. In the world, there are clean energy centers using coal with Ak ≤ 3%.
3. In order to improve the combined economic efficiency of the coal processing, transport and use process, it is necessary to consider investing in the construction of thermal power plants using high quality coal in accordance with the standard of energy coal. of the World (Ak ≈ 8 ÷ 10%).
At the same time, it is also necessary to research and improve the technology and equipment of operating thermal power plants to improve the efficiency of waste collection and recycling.
4. In recent years, the construction of thermal power ash and slag plants has been a spontaneous investment movement, so there are unavoidable problems in technology, equipment, indicators, energy, water, economy and conservation. environmental protection ...
In addition, a number of power plants, chemical plants have not paid attention to the recycling and use of ash. This is especially important when using coal sources with toxic impurities that easily pollute the environment, including imported coal.
5. It is necessary to develop a program for rational exploitation and processing, efficient and economical integrated use of raw coal, with the coordination of coal mining, thermal power, cement production and chemical fertilizer industries. In the program, special attention should be paid to recycling and waste use because Vietnam's coal resources are quite poor and Vietnam needs to use many different types of construction materials in the period of infrastructure construction. , industrial and civil projects ...
6. It is necessary to propagate, promote and adopt sanctions and measures to encourage research, development and investment in the construction of recycling and use of coal ash as fuel (including: sorting ash and selected coal). Attention should be paid to the non-fired brick production facilities, the concrete manufacture and the additive for other materials industries (heat and sound insulation…).
Using coal for energy production is one of the causes of global climate change. It also creates a large amount of ash and slag and is a source of environmental pollution on soil, water and air.
The recycling and use of ash when burning coal in Vietnam has achieved certain results.
To sustainably develop the energy industry in Vietnam in the coming time, it is necessary to have measures to continue to innovate and apply scientific and technological advances as well as to invest in the construction of recycling facilities and use of ash waste. slag industries.
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