Design of a Wet Flue Gas Desulfurization Process
Summary
This report describes the design of a wet flue gas desulfurization plant. The design starts from the flue gas duct after the particle precipitator and induced draft fan and ends before the stack. The wet flue gas desulfurization process is used for removal of sulfur dioxide from flue gas. The removal takes place in a scrubber where the sulfur dioxide is absorbed into water droplets that contain ground limestone. The sulfur dioxide reacts with the limestone and forms calcium sulfite which is further oxidized to calcium sulfate with oxygen. The end product is dried gypsum calcium sulfate dehydrate. The design is based on a description of the process presented in chapter 35 in the book Steam/its generation and use. (Steam/its generation and use, 1992) The limestone for the scrubber is prepared by grinding the limestone in a wet ball mill. The ground limestone is mixed with water into a slurry. The product from the scrubber is dewatered in a hydrocyclone and further dried in a vacuum filter. The flue gas leaving the scrubber is heated in a regenerative heat exchanger with hot flue gas entering the scrubber.
Table of Content
Summary 2
Process Plant Design 2008 – Project specification 3
Introduction 4
Absorber Tower 5
Mist elimination 6
Tray System 7
Spray tower design 8
Reaction tank 9
Process Flow Diagram and Layout 10
Mass Balance 11
Mass Balance for flue gas in 11
Mass Balance for limestone side 12
Sizing 13
Design of Absorption Tower 13
Design of Hydroclone 14
Ball Mill 17
Vacuum filter 17
Recirculation Pumps 17
Pump that pumps recycled water, limestone mixture. 20
Cost Estimation 22
References 26
Process Plant Design 2008 – Project specification
Design a wet flue gas desulfurization (WFGD) plant for cleaning the flue gas from a power plant that uses coal as fuel. The power plant is designed to produce 500 MW electricity at full load with 35 % overall electrical efficiency based on the calorimetric (higher) heating value of the fuel. The fuel, coal, contains 73.2 % C, 4.5 % H, 1.0 % N, 9.1 % O and 1.2 % S (% dry weight). The coal contains about 0.1 t H2O/t dry fuel and has a higher heating value of 29.8 GJ/t dry fuel. The flue gas temperature after the electrostatic filter and the induced draft fan is 150 °C and the pressure is 115 kPa. The flue gas flow at this point is about 830 m3/s with a composition of 12.95 % CO2, 0.08 % SO2, 3.09 % O2, 70.59 % N2 and 13.29 % H2O by volume. The current limit for new power plants in Finland using coal is 200 mg SO2/m3(n) calculated with an combustion air flow that gives 6 % O2 in the flue gas. The minimum SO2 removal efficiency needed in this case is about 88 %. The desulfurization process will be designed for 90 % removal. The desulfurization plant should use limestone as sulfur dioxide absorbent with in situ forced oxidation and regenerative heat exchangers for heating the flue gas after the desulfurization. The used limestone contains 95 % reactive CaCO3. References: Steam – its generation and use. 40th edition. The Babcock & Wilcox Company, 1992, ISBN 0-9634570-0-4.
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Introduction
Wet FGD technology, which is based on using limestone or lime as a reagent, is a wet scrubbing process and has been the FGD technology most frequently selected for sulfur dioxide (SO2) reduction from coal-fired utility boilers. The cleaned gas is discharged to the stack. This type of FGD system removes SO2 by scrubbing the flue gas with either a limestone or lime (reagent) slurry. The wet FGD process is considered a commercially mature technology and is offered by a number of suppliers.
Flue gas is treated in an absorber by passing the flue gas stream through a limestone or lime slurry spray. In typical absorber designs, the gas flows upward through the absorber countercurrent to the spray liquor flowing downward through the absorber. In a typical design, slurry is pumped through banks of spray nozzles to atomize it to fine droplets and uniformly contact the gas. The droplets absorb SO2 from the gas, facilitating the reaction of the SO2 with reagent in the slurry. The desulfurized flue gas passes through mist eliminators to remove entrained droplets before the flue gas is sent to the stack.
In most wet FGD systems, SO2 collection efficiency is controlled by selecting appropriate design features for the system. For example, the quantity of liquid sprayed relative to flue gas is related to the SO2 collection efficiency needed and is referred to as liquid-to-gas (L/G) ratio. Higher L/G ratios improve SO2 removal by exposing the gas to more absorbing liquor. However, higher L/G ratios also consume more power, and this design feature must be factored against other important design features, including type of reagent.
After contacting the gas, the slurry collects in the bottom of the absorber in a reaction tank. The slurry is agitated to prevent settling. Limestone or lime consumed in the process is replenished by adding fresh limestone or lime slurry to the reaction tank. [5]
Absorber Tower
The absorber is an "open spray tower", e.g. without internals to enforce the mass transfer between flue gas and liquid. The absorber slurry is pumped from the absorber sump to the spray nozzles and is sprayed counter current to the flue gas flow. To limit the droplet entrainment to the clean gas, a mist eliminator is installed before the absorber outlet duct.
To protect the absorber from corrosion, the inner surface is rubber lined. To avoid settlement of solid gypsum and other components, the absorber is equipped with a special designed conical bottom (Bischoff System). This type of bottom requires no additional agitator. Also integrated in the absorber bottom is the in situ forced oxidation system. [24]
Fig 1 – wet scrubber
Mist elimination
Mist eliminators are installed in the upper cross section of the absorber to remove entrained slurry droplets from the flue gas. In order to keep the downstream ductwork clean and to assure reliable operation of downstream reheat devices (if used), mist must be removed from the flue gas.
After leaving the absorber spray zone, the scrubbed gas flows upward through a two-stage mist elimination system. The stages consist of multi-pass chevron vanes that remove entrained slurry droplets by inertial impaction. The first stage chevron acts as a barrier to keep the major portion of process slurry droplets entrained in the gas stream from leaving the absorption zone. The small fractions of entrained droplets that pass through the first stage mist eliminator are removed by the second stage.
The faces of the mist eliminators are washed intermittently and in zones with fresh process water. The mist eliminator wash cycles, flux rates and pressures have been designed to provide effective rinsing of any solids and chemically active liquids. [23]
Fig 2 – Mist eliminator
Tray System
Absorber tray system serves the dual purpose of providing gas/liquid contact for SO2 absorption and more uniform distribution of flue gas across the tower. The gas rises through the absorber, contacting a froth of slurry on the tray. This action results in efficient contact of gas and reagent throughout the absorber. These absorbers use one or two trays depending upon the fuel and specified requirements for SO2 removal and operating parameters.
The tray provides uniform gas distribution and effective gas/slurry contact. The slurry and the flue gas must be uniformly distributed across the absorber tower’s cross-section for optimum SO2 removal. An efficient spray header and nozzle system is used to evenly distribute the liquor, while the tray distributes the gas flow. [22]
Fig 3 – Scrubber Trays
Spray tower design
Open spray tower absorbers, ranging in size from 20 to 65 feet in diameter, are used to provide intimate contact between flue gas and scrubbing slurry. These absorbers are constructed with various types of alloys, stainless steels, and mild steel with various corrosion/erosion resistant linings depending on the specific application.
The absorbers generally have 2 or 4 installed spray banks (levels). Each spray level is fed through an individual riser by a dedicated recycle pump. The requirement for spare spray pumps/levels is decided on a case-by-case basis depending on regulatory, customer, and availability requirements.
Each spray level consists of tangential-inlet, hollow cone spray nozzles comprised of nitride-bonded silicon carbide. This nozzle type provides the proper sized droplets for optimum SO2 absorption. In addition, the hollow cone SiC nozzles are durable, long-lasting, and have low clogging potential.
Design, selection and placement of spray nozzles are of critical importance in high-efficiency wet FGD systems. Spray nozzles are positioned via computer modeling to achieve the maximum gas to liquid contact and scrubbing efficiency. [23]
Reaction tank
The recycle slurry falls from the spray zone into the reaction tank built at the base of the absorber. This tank is sized to provide sufficient residence time for all of the gypsum precipitation and limestone dissolution reactions to take place. Fresh reagent slurry is added to the reaction tank where it dissolves and reaches equilibrium with the bulk of the recycle slurry prior to being returned to the spray banks by recycle pumps.
Gypsum bleed pumps discharge slurry to the primary dewatering system to maintain the desired scrubbing slurry concentration and reaction tank liquid level. [23]
Process Flow Diagram and Layout
Fig 4 - Layout for Flue Gas Desulfurization Unit
Fig 5 - Process Flow Diagram
Mass Balance
Mass Balance for flue gas in
Flue Gas / O2 / SO2 / N2 / CO2 / H2OComposition, % / 3.09 / 0.08 / 70.9 / 12.95 / 13.29
Density, Kg/m3 / 1.046399 / 2.092799 / 0.915599 / 1.438799 / 0.5886
Molecular Weight, g/mol / 32 / 64 / 28 / 44 / 18
Volumetric flow rate, m3/s / 25.647 / 0.664 / 588.47 / 107.485 / 110.307
Mass flowrate, Kg/s / 26.837 / 1.389618 / 538.8028 / 154.6493 / 64.92666
Molar flowrate, mole/s / 0.838656 / 0.021713 / 19.24296 / 3.514757 / 3.607036
In this process 90% of SO2 is removed in the scrubber. Mass balance for SO2 will be:
Mass flowrate of removed SO2 / 1.250656Molar flowrate of removed SO2 / 0.019542
Then, with this reaction formula CaCO3 consumes and produces CaSO4:
CaCO3 + SO2 + ½ O2 + 2H2O CaSO4.2H2O + CO2
Mass Balance for this reaction will be:
O2 / CaCO3 / CaSO4 / CO2 / H2O / CaSO4.2H2OMass flowrate / 0.009771 / 1.954151 / 2.657645 / 0.859826 / 0.703494 / 3.361139119
Molar flowrate / 0.312664 / 0.019542 / 0.019542 / 0.019542 / 0.039083 / 0.019541507
Mass Balance for limestone side
Mass Flowrate (Kg/s) / CaCO3 / water / CaSO4.2H2O / TotalStream no.1 / 1.954151 / 4.559685 / 0 / 6.513835503
Stream no.1a / 2.057001 / 4.799668 / 0 / 6.85666895
Stream no.23 / 0 / 19.04646 / 3.361139119 / 22.40759413
Stream no.2 / 0 / 18.95508 / 3.345013908 / 22.30009272
Stream no.3 / 0 / 0.091376 / 0.016125212 / 0.107501411
Stream no.4 / 0 / 0.049144 / 0.002047646 / 0.051191148
Stream no.5 / 0 / 0.042233 / 0.014077566 / 0.056310263
Stream no.6 / 0 / 7.721329 / 0 / 7.721328597
Stream no.7 / 0 / 0.001564 / 0.014077566 / 0.01564174
Stream no.8 / 0 / 4.559685 / 0 / 4.559684852
Stream no.9 / 0 / 3.202312 / 0 / 3.202312268
Stream no.10 (wash water) / 0 / 3.81396 / 0 / 3.81395792
Sizing
Design of Absorption Tower
Settling velocity of the liquid is given by Ut = 0.7(density of liquid -density of vapor)/density of vapor^1/2
Estimating density of limestone slurry 30%
density of flue gas / 0.6227density of limestone 100% / 2000 / 2
density of water 100% / 1000 / 1
take basis of 1000kg / 0.85
Estimated density of limestone slurry 30% = 1176.471
Ut = 3.041825
Minimum vessel diameter Dv = (4*V/3.14*Ut)^0.5 = 18.63798
Approximate diameter for absorber = 20 m
The height of the vessel outlet above the gas inlet should be sufficient to allow for disengagement of the liquid drops. A height equal to the diameter of the vessel = 20 m
Residence time at the reaction tank which is the lower side of the absorber is 10 minutes.
volumetric flow of limestone slurry / 0.018955volume held in the vessel / 11.37305
Liquid height / 0.041707
to be on safe side, the liquid height will be doubled / 0.083414
the height above the liquid volume / 10
the from top to the mist eliminator / 8
total height of the absorber / 38.08341
this is appproximately / 40m
Fig 6 – Wet Scrubber
Design of Hydroclone
Hydroclone No.1
efficiency of cyclone 95% / 0.95diameter of the particle 44micron (325mesh)
the particle diameter for which the cyclone is 50 per cent efficient, micron,
D50 / 30.60215
volumetric flow rate litre/min / 349.6901
viscosity of limestone slurry / 0.00085
Dc = D diameter of the cyclone chamber, cm,
Dc^3 / 9026940
Dc / 208.2157
Dc in metres / 2.082157
Dc approximate m / 3
Inflow diameter m / 0.428571
Overflow diameter / 0.6
underflow diameter / 0.3
angle at the apex 12 / 186
height of the slend section / 3.978968
Approxiate height m / 4
height rectangular side / 1.5
total height / 6
Fig 7 – Hydroclone design
Hydroclone NO.2
efficiency of the cyclone / 0.95diameter of the particle 15 microns / 15
volumetric flow rate litre/min / 6.298911
the particle diameter for which the cyclone is 50 per cent efficient, micron,
D50 / 10.48513
density of gypsum slurry kg/m^3 / 1024
diameter of the cyclone
Dc2 cubed / 25523285
Dc2 cm / 294.4278
diameter of the cyclone / 2.944278
From the diameter found we can assume the two hydroclones have same dimensions.