ASH HANDLING SYSTEMS FOR VARIOUS TYPES OF BOILERS

BROADLY SPEAKING, ALL ASH HANDING SYSTEMS HAVE THE FOLLOWING SEQUENCE OF PROCESSES:

  • Ash collection
  • Conveying
  • Storage
  • Dry or wet out-loading

Depending upon the quantity of fuel, type of boiler, usability of ash etc, the right ash handling system is chosen to ensure the highest efficiency and utilization. Below are different types of boilers that determine the type of ash handling system that needs to be used:

1. Stoker Fired Boilers

Stoker firing is a power-operated fuel feeding mechanism and grate. This self-cleaning system allows for burning large quantities of fuel with ease in, gives us greater control on the combustion process with higher efficiency. It also requires less labour of handling ash.

2. Atmospheric Fluidized Bed Combustor (AFBC)

FBC is atmospheric fluidized bed combustion, where the furnace pressure is atmospheric pressure. The combustion exhaust gas from the furnace goes through a cyclone, Bag Filters or ESP for arresting the ash particulates and are let out into the atmosphere. This kind of a system has comparatively lesser residence time, turbulence and lower combustion efficiency and lower desulfurization.

If there is less headspace under the boiler, then a mechanical dry bottom ash handling system is more suitable as it requires no water but still offers continuous ash removal, otherwise pneumatic ash handling may be used to convey ash out into an ash storage silo for interim holding prior to load out for disposal or reuse.FBC is atmospheric fluidized bed combustion, where the furnace pressure is atmospheric pressure. The combustion exhaust gas from the furnace goes through a cyclone, Bag Filters or ESP for arresting the ash particulates and are let out into the atmosphere. This kind of a system has comparatively lesser residence time, turbulence and lower combustion efficiency and lower desulfurization.

3. Circulating Fluidized Bed Combustor (CFBC) Boiler

In Circulating fluidized bed combustion boilers, the furnace is pressurized and the flue gas is re-circulated to capture the unburnt carbon, and to increase the thermal efficiency of the boiler, hence this is the next evolved version of the AFBC. CFBC boilers are particularly suitable for Indian coal as it has a higher percentage of ash than other types. Due to ash recirculation, the unburnt carbon percentage in exhaust gases can be reduced considerably, which in turn increases the quality of ash produced.

All fluidized combustors operate on either mechanical, pneumatic or a combination ash handling system, depending upon the type of fuel used, availability of ash disposal area, and the need for ash utilization.

4. Pulverized fuel Fired Boilers (PF Fired B)

In boilers with pulverized firing systems, about 80% of ash released is fly ash, and the rest gets collected as bottom ash. Since Indian coal has a significantly higher amount of ash, it needs an efficient ash handling system.

5. Waste Heat Recovery Boilers

WHRB boilers recover ‘waste’ heat, generated during the production process by way of fuel combustion or chemical reaction. This heat can be reused by supplementing it with additional heat to raise the boiler inlet gas temperature to a level commensurate with the desired steam conditions.

One of the biggest advantages of WHRB boilers is that they save a substantial amount of primary fuel that otherwise would have been consumed to generate the same amount of steam.

UTILIZATION OF FLY ASH IN CEMENT AND ALLIED INDUSTRIES

n the early years of thermal energy, coal fired power plants used to dispose-off entire fly ash in slurry form to ash dykes. A few years back, strict regulations on managing coal-combustion waste came into effect in an attempt to combat pollution, and hence people had to explore various commercial applications of fly ash.

Today fly ash is used in so many industrial and commercial applications this it has become a complete industry by itself.

1. Brick making

Interestingly, fly ash bricks were invented by 2 Indian inventors, who used lime, fly ash and gypsum to create a brick that did not need natural clay at all. Their process also did away with the requirement for kiln firing. Today, India produces over 170 million tonnes of fly ash bricks.

One of the best qualities of fly ash is that it has excellent insulation capability. Since it also absorbs much less heat than clay bricks, fly ash bricks are considered a more energy-efficient as well as eco friendly option. Fly ash bricks come in several load-bearing grades, and to encourage and ensure consistency of quality, the Indian Standard, IS 16720: 2018 has been formulated for the same. According to their official statement, “The standard lays down essential requirements of finish, dimensions, strength, water absorption, etc. with a view to achieving quality and uniformity in the manufacture of such bricks.”

2. Cement

Fly ash is a pozzolanic substance, which means it contains silicates and aluminum compounds that turns into cement when combined with water. Partial replacement of cement with fly ash is not only a more viable prospect commercially, it also saves on naturals resources, reduces CO2 emissions and ensures longer durability.

3. Roads and Embankments

If fly ash cement is used in road construction, it allows for much reduced permeation of concrete, which keeps the aggressive expansive minerals from damaging the concrete from the inside. Also, since it is lighter than traditional concrete, it causes lesser settlements. Fly Ash is particularly relevant in making stabilization projects and embankments over places with weak sub-soils, as it puts lesser pressure on the retaining walls.

4. Mine Backfilling

When areas are mined, and exhausted, backfilling is required to reclaim that land, otherwise such vast tracts of land will be rendered useless. Since mines require deep or shallow excavation, mostly in embedded carbonate rocks, they leave behind numerous hollows and shafts. If these gaps are not filled in properly they may open up large sinkholes or elongated fractures or other deformations in the future. To avoid such calamities that could possibly affect human lives, it is important to fill the mines with a material that is the ability to fill tight spaces, has a high resistance to water permeation or compression, and is eco-friendly.

Fly ash has all these qualities and lends itself beautifully as a filler for Mine Void Filling.

5. Fly ash Coagulant

Water pollution is one of the biggest ecological concerns today. If on one hand we encourage rapid industrialization, which results in greater production, and therefore more effluents and pollutants, then we must also have measures to manage these wastes so that we are able to reduce our carbon footprint and protect our environment.

Fly ash has proved useful in Wastewater Treatment, where modified or activated fly ash acts like a coagulant and traps and absorbs various types of organic compounds. Textiles, Handmade paper, Tanning & Dyeing etc. are all industries that benefit from using Fly ash in significantly reducing their contribution towards wastewater.

Indian economy has seen an enormous economic growth in terms of infrastructure development and manufacturing. Given the amount of natural resources that will be used in either creating this infrastructure or in the services provided by them, it is very important that environment-friendly, cost-effective options be considered very seriously. The Fly Ash industry has received a lot of impetus from the government, but if more and more people are educated about its benefits and enough attention is directed to its positive ecological impact, we are pretty sure that fly ash will see a lot of research and technological innovation in the coming years.

POTENTIAL FUTURE APPLICATIONS OF FLY ASH RE-USE

Across the globe, countries that have presence in coal based plants, and therefore have huge coal ash production, are expressing interest in extracting value out of this potential gold “dust” mine, instead of just dumping it in landfills. India, which ranks 4th in the world in coal ash production, may possibly quadruple the quantum of fly ash generated when it switches to super critical technologies for thermal power. With that situation looming large in our near future, it is critical that countries fast track research on fly ash utilisation.

Eventually, the choice for finding innovative uses for fly-ash wins out in the end because not only does one minimize the harsh impact of fly ash on the soil and ground water, but also helps in recycling an existing, and abundantly available resource that brings commercial benefits too.

Here are some of the ways that countries have found use of fly ash :

The use of one tonne of fly ash replacement in cement production typically saves one tonne of CO2. Perhaps the most important advantage for cement firms, though, is that fly ash is about one-third the price of cement – which is why some cement companies and power plants have entered into joint venture projects to recover fly ash.

A team from the UWC University in South Africa has been trying to completely replace cement with fly ash. Their aim is to create a prototype that can turn fly ash lower-cost roof tiles, bricks, paving stones, building elements like lintels, fire-retardant panels and insulation material, which could offer important solutions in low-cost housing to replace chipboard and gypsum board, or so-called drywall, which are highly flammable. This way, construction material will become cheaper and also result in huge energy savings.

One research from an agricultural university indicates that they have found that a certain composition of fly ash effectively increases yield of some crops and in reclaiming fallow land.

Universities and laboratories in India, South Africa, the USA and Australia etc. alike have been running funded research experiments for extraction of desirable rare earth metals from coal ash like Alumina, Vanadium etc. ‘Beneficiation’ is a term that refers to the process by which carbon is removed from fly ash to produce a higher-value material. Power plants can increase their revenue stream by adding the beneficiation step to their ash handling process.

In South Africa, CSIR researchers are converting fly ash, a by-product of the South African pulp and paper industry, into heat-resistant geo-polymers that can be used in the protection of metal surfaces against high temperatures in furnaces or kilns. The conversion of fly ash into valuable products such as heat resistant geo-polymers offers alternative options that could help the pulp and paper industry to reduce its environmental footprint and waste management costs.

In summary, it does seem that identifying applications of coal ash re-use will be extremely beneficial, offering an enhanced revenue stream for power plants and also be a means of significantly decreasing the carbon footprint.

FLY ASH RECOVERY AND DISPOSAL TRENDS AROUND THE WORLD

The World Energy Outlook 2017, Southeast Asia, cited that India, along with other developing economies in Asia, would helm the global demand for coal in years to come. In the next 20 years, power plants are expected to be responsible for almost 75% of the additional coal consumption.

And when there is so much coal, there is of course, a lot of coal ash. Globally, the most popular and the most economical solution for coal ash disposal is storage. Cost of disposal plays a big factor and can be curbed to a minimum if there are disposal sites available that are near a power plant.

Considering the ecological impact of coal ash on our environment, almost all countries have developed stringent regulations on its recovery, disposal and usage. In USA for example, the US Environmental Protection Agency (EPA) laid out the first-ever federal rules for coal ash disposal, where America’s coal-fired power plants are required to store coal ash only in landfills and disposal sites that meet minimum structural standards. In China, Fly ash is a Type II Class C general industrial solid waste. Currently, Chinese regulations allow for the fly ash to be stockpiled temporarily if it cannot be comprehensively utilized.

In Germany, where around 10 million tonnes of coal ash is produced per year, around 97% is re-used, and the rest stored only on a temporary basis. Other countries like the Netherlands are doing even better by having a zero landfill policy, which subsequently means that they are required to recycle 100% of the coal ash generated.

In the UK and US, the recent downward market trend in the coal industry has resulted in an increased interest by cement companies in mining landfill storage sites to recover the ash. There are recent studies and research papers that suggest that fly ash landfills have enormous untapped value and potential that, when unlocked, can bring tremendous ecology-friendly and economically viable opportunities for re-use applications.

In Malaysia, the coal consumption is set to increase by over three times. It already has very stringent pollution targets, therefore it prefers towards using HELE technologies, mainly USC, for new projects. In Vietnam, a recent resurgence of interest in nuclear power could offset coal’s share in the medium-term. “The availability of sufficient capacity from coal plants now being built means that it is only when plants are retired and additional power is needed from 2035 onwards, that AUSC units may be introduced and emissions of CO2 will fall.

WHAT ARE DIFFERENT TYPES OF ASH HANDLING SYSTEM?

In thermal power plants, an ash handling system is used to collect and dispose off discharged ash, once it has been cooled down to a manageable temperature, which is then used in various industries like construction, cement plants, and other allied industries.
Typically for a 2×500 MW plant based on Indian coal, the amount of ash generated is around 300 to 400 TPH depending on gross calorific value and ash content of the coal.
The amount of ash produced when coal is burnt in a thermal power plant is roughly thousands of tonnes. It could have an effect on other subjects too if proper ash handling methods are not followed.
Ash is conveyed mechanically, pneumatically and hydraulically. Mechanical systems typically include submerged or dry-flight conveyors, screw conveyors, and belt conveyors. Pneumatic systems may be positive or negative pressure Hydraulic systems are also known as sluice systems and may be used independently or in combination with pneumatic systems. The pneumatic and hydraulic systems utilise pipes.

ASH HANDLING SYSTEM:

About 80% of the ash that is generated in a thermal power plant is fly ash, with the remaining 20% being bottom ash. In the last 15‐20 years, it is a trend worldwide to use dense phase pneumatic conveying for fly ash collection and handling within a power plant For different environmental, economic and product benefits, coal ash is used by different types of industries in different ways of its necessity.

FLY ASH DISPOSAL

Fly Ash should be disposed of in dry mode (for normal continuous operation) and in wet slurry mode or high concentration slurry disposal (HCSD) mode (for initial operation period till 100% dry fly ash utilization is achieved and emergency operation when the dry disposal is interrupted). Fly Ash is a fine particulate that is collected via an economic hopper, air-preheated hopper and electrostatic precipitator (ESP).

BOTTOM ASH DISPOSAL

Bottom ash should be disposed of in a wet or semi-wet mode. Bottom Ash is mostly coarse and hence needs to be crushed before it is transported via an Ash Handling System.

In view of the above, the ash handling system covers evacuation of ash and its disposal in wet, semi-wet and dry form and consists of numerous equipment that works in a coordinated manner to achieve the ultimate functional need.

ASH HANDLING SYSTEMS ARE PRIMARILY BIFURCATED ON THE FOLLOWING GROUNDS:

1. Fly Ash Handling System: Fly Ash is captured and removed from the flue gases by an economizer, air-preheater, and ESP that are located at the outlet of the furnace and before the induced draft. The fly ash is pneumatically transported from collection hoppers of the economiser, air-preheater, and ESP to the storage silos.
2. Bottom Ash Disposal System: This system collects ash in the bottom hopper which is located directly under the furnace boiler. As this is coarse in nature, it has to be treated by multiple systems like clinker grinders which then ensure that the size of the bottom ash is manageable and can be easily transported to temporary storage or to an ash pond in slurry form.
3. Ash Slurry Disposal System: The bottom ash that is collected by the Bottom Ash Disposal System is often mixed with water and transported to an ash disposal area.

THERE ARE THREE MAIN KINDS OF ASH HANDLING SYSTEMS:

(1) Hydraulic system
(2) Pneumatic system
(3) Mechanical system

CLASSIFICATION OF ASH HANDLING SYSTEMS:

Hydraulic System: In this system, ash from the furnace grate falls into a system of water traveling at a high-pressure and is carried to the sumps.
It is generally used in large power plants. The hydraulic system is generally used for intermittent ash disposal figures.
In this method water at sufficient pressure is used to take away the ash to a sump. The ash is then dumped.
In this method, a low-pressure jet of water coming out of quenching nozzle is used to cool the ash. The ash falls into a trough and is then removed.Pneumatic System: In this system ash from the boiler ash outlet falls into a blow tank provided below the ash hopper. The fly ash is then pushed into pipelines and conveyed by the air stream to the point of delivery. Air leaving the ash separator is passed through a filter to remove dust etc and release clean air to the atmosphere.Mechanical System: In this system cooled ash falls on the belt conveyor and is carried out continuously to the bunker. The ash is then removed to the dumping site from the ash bunker with the help of trucks.Why Macawber Beekay Ash Handling System is the best: low investment and operating costs, operational safety, simplicity, flexibility and high transport capacity depending on channel size. It has no moving part and transports the material with low speed, in a protecting operating mode.

UTILIZATION OF FLY ASH IN INDIA

WHAT IS FLY ASH?

In thermal power plants, when pulverized coal is burned, we get a by-product called Fly Ash. This remnant can be responsible for serious environmental problems if let out incautiously and thus, Macawber Beekay extracts it using Vacuum System or Pressure Conveying System. For years, Fly Ash was considered an industrial waste and a larger part of it used to end up in ash dams & related landfills..

WHY FLY ASH UTILIZATION IS VITAL?

In the last few decades, in energy generation, coal has been a dominant player world-wide and is expected to maintain its dominion in coming years. India shows the largest growth in coal consumption, with over two-thirds of its electricity generation occurring using coal. With a surging utilization, The Indian Ministry of Power estimates that 1800 million tonnes of coal can be used every year leading to generation of 600 million tonnes of Fly Ash by 2030-31.

Loose Fly Ash can lead to severe health and environmental hazards, bringing attention to MoEF’s prescription for plants of achieving 100% utilization. Thus, having made it important to develop technical and economical solutions for fly ash utilization to reduce these fly ash related problems. It is very essential to set-up a Dry Fly Ash Handling System in the power plants.

UTILIZATION OF FLY ASH

The essential idea of reuse of this remnant is not only to control its effects on the environment, but also, to realize economic gain from the sale of this by-product and replace some limited or expensive natural resources with its employment. It can be used in the following areas-

CEMENT MANUFACTURING

Due to the presence of a significant amount of silica, alumina and lime in it, Fly Ash can be used as a sectional substitute (upto 30%) for Portland cement. The inclusion of Fly Ash as the blending material substantially improves the quality and durability of the concrete as Fly Ash reacts like a pozzolana with the lime in cement as it hydrates, creating a sturdy binder. In India, current production of conventional cement is similar to that of Fly Ash blended one. By using Fly Ash based cement, the cement industries can escalate their production with lesser resources.

CONSTRUCTION OF ROAD & EMBANKMENT

Using Fly Ash for road and embankment works gains an upper hand for its gradation, shear strength, compressibility and permeability. It’s also a lightweight material as compared to commonly filled conventional local soil thus, causing lesser settlements. Pozzolanic hardening property gives additional strength to the road pavements/ embankments and decreases the post construction horizontal pressure on retaining walls. It can replace a part of cement and sand in concrete pavements thus making them more economical than roads constructed using conventional materials.

FLY ASH BRICK

Fly ash can be used for manufacturing bricks used for building construction. These bricks are light in weight and stronger than common burnt clay bricks available otherwise. Fly ash can also be used for manufacturing pavement blocks or tiles normally used for strengthening and decorative flooring in pavements, walkways etc. The use of fly ash for manufacturing these will give an economical edge compared to the conventional bricks or blocks and also, help in preserving land area dug out for clay brick manufacturing.

CONCLUSION

The production of coal fly ash is expected to grow for coming years, as a result of the world’s increasing dependence on coal-based power generation. Understanding the characteristics of this by-product provides a foundation for its alternative uses. Having the idea of the diverse ways to use fly ash, such as in the cement manufacturing, road construction etc, is essential to meet the CEA’s guidelines of Fly Ash utilization and its better management to reduce environmental pollution. Macawber Beekay is joining hands with the society for a cleaner and greener India by manufacturing durable Ash Handling Systems..

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