A Presentation OnCirculating Fluidized Bed Combustion Technology

Submitted By

1. V. VAMSI KRISHNA2. Y. VENKATRAMAYYA3. A. KARTHIKEYAN4. ABHISHEK KUMAR SINGH

Under the Course of Training at STEAG Power Plant Learning Center

STEAG Energy Services (India) Pvt. Ltd.A – 29, Sector – 16, Noida 201 301, U.P

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The worldwide green house emissions, limited resources of fossil fuel, and international environmental issues drive the scientists and technologists to think, analyze and act so as to make the world sustainable.

CFBC Technology characterizes compatibility with wide range of low grade fuels, high combustion efficiency and low pollution. It was adopted in thermal power plants three decades ago and has taken an upward trend in power utilities and process industries.

During the last two decades there have been tremendous developments in this technology. France is having 250MW CFBC units running since 90’s and 400MW CFBC boilers have been commissioned recently.

ABSTRACTGroup-7 Slide-2

Industrial Applications of FBC Technology

Transportation of solids Mixing of fine powders for heat exchange Coating of plastic materials on metal surface Drying of solids Carryout synthetic reactions Cracking of hydrocarbons Catalytic reforming of hydrocarbons Carbonization and gasification Calcining and clinkering

FBC Technique is often utilized for reduction of iron ore to produce pig iron

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Process Flow of Pulverized Fuel Type BoilerGroup-7 Slide-4

Typical Fuel Pipe Routing from Bowl-MillsGroup-7 Slide-5

Simple Flow Diagram of CFBC Boiler

FEATURES OF CFBC BOILER:

Low temperature control kinetic combustion. High velocity, high density and high flow circulating fluidization process of solid materials. High intensity of heat, mass and momentum transmitting process.

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Typical Process Flow of PFBC Based CCPPGroup-7 Slide-7

Atmospheric Fluidized Bed Combustion Boiler

The bed depth is usually 0.9 to 1.5 meter deep and the pressure drop averages about 1 inch of WC per inch of bed depth.

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Principle of Fixing, Bubbling and Fast Fluidized BedsGroup-7 Slide-9

Comparison of BFBC & CFBC BoilersGroup-7 Slide-10

General Bed Material Requirement for CFBC

Al2O3 = 30 to 40% SiO2 = 50 to 60% Alkalis (Na2O + K2O) = < 3% Iron Oxide = < 3.5% Moisture = <1% Initial Deformation Temperature = > 1300oC

CHEMICAL COMPOSITION(% BY WEIGHT):

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Circulating Fluidized Bed Combustion BoilerGroup-7 Slide-12

Flow Diagram of Circulating Fluidized Bed Power StationGroup-7 Slide-13

Total Heat Loss in Boiler with LIGNITE (18%)Group-7 Slide-14

Total Heat Loss in Boiler with Imported Coal (11%)Group-7 Slide-15

Advantages of Circulated Fluidized Bed Combustion Boilers

1. High Boiler Efficiency Higher Boiler effi ciency even on part loads due to better burn ability. FBC boilers can burn fuel with a combustion effi ciency of over 95% irrespective of ash content. FBC boilers can operate with overall effi ciency of 84% (+/- 2%). 2. Reduction in Boiler Size High heat transfer rate over a small heat transfer area immersed in the bed result in overall size reduction of the boiler hence compact plant design is possible. 3. Fuel Flexibility FBC boilers can be operated effi ciently with a variety of fuels. Even fuels like Lignite, Bio-mass, flotation slimes, washer rejects, agro waste and other diffi cult fuels can be burnt effi ciently.

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Advantages of CFBC Boilers Contd..4. Ability to Burn Low Grade Fuel FBC boilers would give the rated output even with inferior quality fuel. The boilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500kcal/kg. 5. Ability to Burn Fines Coal containing fines below 6 mm can be burnt effi ciently in FBC boiler, which is very diffi cult to achieve in conventional firing system. 6. Pollution Control SO2

formation can be greatly minimized by addition of limestone or dolomite for high sulphur coals. 3% limestone is required for every 1% sulphur in the coal being feed. Low combustion temperature results in NOX formation level below 200mg/mm3

S + O2 --------->SO2

CaCO3 ----------> CaO + CO2 (Calcination)2CaO + 2SO2 + O2 -----------> 2CaSO4 (Sulphation)

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Advantages of CFBC Boilers Contd..7. Low Corrosion and Erosion The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (in the order of 1 m/sec). 8. Easier Ash Removal – No Clinker Formation Since the temperature of the furnace is in the range of 750 – 900o C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation. 9. Less Excess Air & Higher CO2 in flue gas

The CO2 in the flue gases will be of the order of 14 – 15% at full

load. Hence, the FBC boiler can operate at low excess air, which is only 20 – 25%. 10. Simple Operation, Quick Start-Up High turbulence of the bed facilitates quick start up and shut down. Full automation of start-up and running operation is possible using reliable equipment.

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Advantages of CFBC Boilers Contd..11. Fast Response to Load Fluctuations Excellent operational flexibility & ideally suited to wide load fluctuation. Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates. 12. No Slagging in the Furnace-No Soot Blowing In FBC boilers, volatilization of alkali components in ash does not take place and the ash is non sticky. This means that there is no slagging or soot blowing. 13. Provisions of Automatic Coal and Ash Handling System Automatic systems for coal and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas fired installation. 14. Provision of Automatic Ignition System Control systems using micro-processors and automatic ignition equipment give excellent control with minimum manual supervision.

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Advantages of FBC Boilers Contd..15. High Reliability The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs. 16. Reduced Maintenance Routine overhauls are infrequent and high effi ciency is maintained for long periods. 17. Quick Responses to Changing Demand A fluidized bed combustor can respond to changing heat demands more easily than stoker fired systems. This makes it very suitable for applications such as thermal fluid heaters, which require rapid responses. 18. High Efficiency of Power Generation By operating the fluidized bed at elevated pressure, it can be used to generate hot pressurized gases to power a gas turbine. 19 Simplified Fuel Preparation & feedingNo Pulverizing of fuel. Only crushing is required.

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This can be combined with a conventional steam turbine to improve the effi ciency of electricity generation and give a potential fuel savings of at least 4%.

Variants of CFBC BoilersBased on the method of solid circulation and heat exchange...

LURGI one of the pioneer in FBC technology, developed CFBC boiler based on hot cyclone technology. In the combustion chamber heat is transferred to the membrane water tube walls through the circulation of solid material. External re-circulation of hot solid material is carried out via cyclone. External heat exchanger is provided in the external recirculation path.

1. Hot cyclone design:

2. Cold cyclone design:

Foster Wheeler uses cold cyclone design. The cyclone is steam cooled and it is the first pass of super heater. The major advantage is that it is now a part of the furnace system, thickness of refractory is reduced thus reducing start-up times.

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3. Internal Re-Circulation, U Beam Type:

Variants of CFBC Boilers Contd..

Internal Re-circulation Type (IR-CFBC) boiler has been developed by Babcock and Wilcox. This type employs a patented two-stage separation system.

The primary stage consists of U-beam impact separators and the secondary stage consists of multi-cyclone dust collector which work together to provide a particle collection efficiency in excess of 99.8%.

The U-beam separator system forms a part of the furnace water wall enclosure reducing the amount of refractory required. This also reduces the start-up time.

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S.No. Particulars AFBC PF Boiler CFBC

1 Flue gas velocity in furnace 1.5 – 2m/s 6m/s 5-6m/s

2 Bed material Required Not required Required

3 Bed temperature 700 – 950OC 1100 – 1300OC 700 – 950OC

4 Refractory Small quantity Small quantity Very high

5 Fuel flexibility Low grade fuel can be used

Only high grade fuel should be used

Low grade as well as multi grade fuel

6 Combustion Efficiency 97% 99.5% 99%

Comparison of AFBC, PF & CFBC BoilersGroup-7 Slide-23

S.No. Particulars AFBC PF Boiler CFBC

7 Boiler Size Small & medium Big Big or small

8 Fuel with alpha quartz Can be used Can be used Cannot be used

9 Operation with variation of load

Frequent load variation can lead to

clinker formation

Fuel will be controlled from mill

according to the load variations

Frequent & Large load variations can be

sustained

10 Make-up of bed material Regular make-up to maintain bed height No bed Only at the time of

starting

11 Response time Lower Faster Better than AFBC but less than PF Boiler

12 Start-up processManual start-up

(Because of risk of clinker formation)

Manual start-up Auto start-up with burner management

Comparison of AFBC, PF & CFBC BoilersGroup-7 Slide-24

Practical Problems Faced with CFBC Boilers

Bottom ash bucket chain conveyer problem. High differential pressure & under performance of ash cooler. Frequent damage of expansion bellows. Combustion problem causing clinkering and cooler inlet pipe chocking problem & cooler inlet line/valve failure. Underperformance of air pre heater.

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Practical Problems Faced with CFBC Boilers

ESP high ash disposal problem causing poor ESP performance. Heavy furnace leakage causing under performance of boiler & ESP. ESP support & shaft insulator support failure causing non availability of ESP fields. Unbalance of ID, PA & SA fans. Frequent boiler tube leakages. Refractory failure.

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ConclusionCFBC Boiler provides efficient approach even in

poor fuel quality leading to good aspect for developing countries. It is very much essential for countries like India.

India has about 226 Billion Tonnes of Coal reserves as per the estimates of Geological Survey of India-out of which about 100 Billion Tonnes is extractable and most of them are of Low grade coal, which can be effectively burnt in CFBC system.

Further CFBC has the best and suitable control over environment issues. It meets with the present and future growing demands with economical viability.

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