Air pollution is a matter of great concern worldwide; the disastrous results have already being noticed at several parts of the globe. While most of the air pollutants add up to the existing pollution level, a few of the components in the air pollutant can cause an immediate effect on the surroundings or the environment into which these are emitted. Among these Particulate Matter (PM) present in the emission or exhaust from automobiles, industries or any form of human activity are the major ones.
So the mini-project "Wet Scrubbers for diesel engines" aims at reducing the harmful effects of these Particulate Matter, by removing them from the exhausts of diesel engines using the principle of Venturi apparatus. The basic idea of the project is to remove or extract these Particulates by making them come in contact with a suitable liquid spray hence absorbing these pollutants to form a slurry which can be collected and disposed or recycled safely as per required.
The entire apparatus, principle involved, design and working procedure which will be discussed in detail in the coming chapters.
2. WET SCRUBBER
The term wet scrubber describes a variety of devices that remove pollutants from a furnace flue gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or by some other contact method, so as to remove the pollutants.
The design of wet scrubbers or any air pollution control device depends on the industrial process conditions and the nature of the air pollutants involved. Inlet gas characteristics and dust properties (if particles are present) are of primary importance. Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases by dissolving or absorbing them into the liquid.
Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device referred to as a mist eliminator or entrainment separator (these terms are interchangeable). Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant. There are numerous configurations of scrubbers and scrubbing systems, all designed to provide good contact between the liquid and polluted gas stream.
Figures 1 and 2 show two examples of wet scrubber designs, including their mist eliminators. Figure 1 is a venturi scrubber design. The mist eliminator for a venturi scrubber is often a separate device called a cyclonic separator.
Figure 2 - Packed bed tower
Figure 2 has a tower design where the mist eliminator is built into the top of the structure. Various tower designs exist.
A wet scrubber's ability to collect small particles is often directly proportional to the power input into the scrubber. Low energy devices such as spray towers are used to collect particles larger than 5 micrometers. To obtain high efficiency removal of 1 micrometer (or less) particles generally requires high energy devices such as venturi scrubbers or augmented devices such as condensation scrubbers. Additionally, a properly designed and operated entrainment separator or mist eliminator is important to achieve high removal efficiencies. The greater the number of liquid droplets that are not captured by the mist eliminator the higher the potential emission levels.
Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas-to-liquid contact is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.
If the gas stream contains both particle matter and gases, wet scrubbers are generally the only single air pollution control device that can remove both pollutants. Wet scrubbers can achieve high removal efficiencies for either particles or gases and, in some instances, can achieve a high removal efficiency for both pollutants in the same system. However, in many cases, the best operating conditions for particles collection are the poorest for gas removal.
In general, obtaining high simultaneous gas and particulate removal efficiencies requires that one of them be easily collected (i.e., that the gases are very soluble in the liquid or that the particles are large and readily captured) or by the use of a scrubbing reagent such as lime or sodium hydroxide.
3. CATEGORIZATION OF WET SCRUBBERS
Since wet scrubbers vary greatly in complexity and method of operation, devising categories into which all of them neatly fit is extremely difficult. Scrubbers for particle collection are usually categorized by the gas-side pressure drop of the system. Gas-side pressure drop refers to the pressure difference, or pressure drop, that occurs as the exhaust gas is pushed or pulled through the scrubber, disregarding the pressure that would be used for pumping or spraying the liquid into the scrubber.
Scrubbers may be classified by pressure drop as follows:
- Low-energy scrubbers have pressure drops of less than 12.7 cm (5 in) of water.
- Medium-energy scrubbers have pressure drops between 12.7 and 38.1 cm (5 and 15 in) of water.
- High-energy scrubbers have pressure drops greater than 38.1 cm (15 in) of water.
However, most scrubbers operate over a wide range of pressure drops, depending on their specific application, thereby making this type of categorization difficult.
Another way to classify wet scrubbers is by their use - to primarily collect either particulates or gaseous pollutants. Again, this distinction is not always clear since scrubbers can often be used to remove both types of pollutants.
Wet scrubbers can also be categorized by the manner in which the gas and liquid phases are brought into contact. Scrubbers are designed to use power, or energy, from the gas stream or the liquid stream, or some other method to bring the pollutant gas stream into contact with the liquid.
The main concern when designing this scrubber was indeed the immediate effect of PM on the environments especially Diesel Particulate Matter or otherwise DPM, these particulates are emitted from all diesel engines especially automobiles like huge trucks, buses, auto-rickshaws and old engines used mainly in small scale industries. DPM primarily consist of unburnt diesel soot and ash particles, metallic abrasion particles, sulfates, and silicates.
When released into the atmosphere, DPM can take the form of individual particles, with most in the invisible sub-micrometer range of 10 micrometer to even 100 nanometers, due to which they remain afloat in the air for days and even weeks, while the heavier particle may settle due to gravitation the lighter particle gets settled only due to precipitation or fog.
The hazardous effects of DPM on human beings vary according to the exposure. Because of their small size, inhaled particles may easily penetrate deep into the lungs. Exposures can lead to acute short-term symptoms such as headache, dizziness, light-headedness, nausea, coughing and irritation of the eyes, nose and throat. Long-term exposures can lead to chronic health problems such as cardiovascular disease, cardiopulmonary disease, and lung cancer.
By Continuity equation when a fluid flows through a closed section the mass flow rate is always constant at any point in the flow, that is the product of density, area of cross section and the velocity of the flow is always constant, now for an incompressible fluid only area and velocity are the variables, which form the basis of a venturi apparatus, which comprises of a divergent section, a throat and a convergent section. Now when the fluid enters the divergent section the area decreases as approaching the throat so as per continuity equation the velocity of the flow will increase for an incompressible fluid hence the maximum velocity occurs at the throat.
Assuming the exhaust gas used in this mini-project to be an incompressible fluid as per continuity equation when an the gas is allowed to flow into such an apparatus a high velocity obtained at the throat can be used to atomize the liquid droplets which are introduced at the beginning of the throat, these atomized liquid droplets very well absorb and dissolve the Particulates in the diesel exhaust, the end product in turn exits through the divergent section of the venturi apparatus. Then the spent liquid settles down into the collecting chamber while the particle free exhaust gases are ejected out from above the chamber through the exhaust gas outlet port.
6. BERNOULLI'S PRINCIPLE
A flow of air into a venturi meter. The kinetic energy increases at the expense of the fluid pressure, as shown by the difference in height of the two columns of water.
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Bernoulli's principle is named after the Dutch-Swiss mathematician Daniel Bernoulli who published his principle in his book Hydrodynamica in 1738.
Bernoulli's principle can be applied to various types of fluid flow, resulting in what is loosely denoted as Bernoulli's equation. In fact, there are different forms of the Bernoulli equation for different types of flow. The simple form of Bernoulli's principle is valid for incompressible flows (e.g. most liquid flows) and also for compressible flows (e.g. gases) moving at low Mach numbers. More advanced forms may in some cases be applied to compressible flows at higher Mach numbers (see the derivations of the Bernoulli equation).
Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. If the fluid is flowing out of a reservoir the sum of all forms of energy is the same on all streamlines because in a reservoir the energy per unit mass (the sum of pressure and gravitational potential ρ g h) is the same everywhere.
Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.
Incompressible flow equation
In most flows of liquids, and of gases at low Mach number, the mass density of a fluid parcel can be considered to be constant, regardless of pressure variations in the flow. For this reason the fluid in such flows can be considered to be incompressible and these flows can be described as incompressible flow. Bernoulli performed his experiments on liquids and his equation in its original form is valid only for incompressible flow. A common form of Bernoulli's equation, valid at any arbitrary point along a streamline where gravity is constant, is:
is the fluid flow speed at a point on a streamline,
is the acceleration due to gravity,
is the elevation of the point above a reference plane, with the positive z-direction pointing upward – so in the direction opposite to the gravitational acceleration,
is the pressure at the chosen point, and
is the density of the fluid at all points in the fluid.
where Ψ is the force potential at the point considered on the streamline. E.g. for the Earth's gravity Ψ = gz.
The following two assumptions must be met for this Bernoulli equation to apply:
- the fluid must be incompressible – even though pressure varies, the density must remain constant along a streamline;
- friction by viscous forces has to be negligible.
By multiplying with the fluid density ρ, equation (A) can be rewritten as:
is dynamic pressure,
is the piezometric head or hydraulic head (the sum of the elevation z and the pressure head) and
is the total pressure (the sum of the static pressure p and dynamic pressure q).
The constant in the Bernoulli equation can be normalised. A common approach is in terms of total head or energy head H:
The above equations suggest there is a flow speed at which pressure is zero, and at even higher speeds the pressure is negative. Most often, gases and liquids are not capable of negative absolute pressure, or even zero pressure, so clearly Bernoulli's equation ceases to be valid before zero pressure is reached. In liquids – when the pressure becomes too low – cavitation occurs. The above equations use a linear relationship between flow speed squared and pressure. At higher flow speeds in gases, or for sound waves in liquid, the changes in mass density become significant so that the assumption of constant density is invalid.
In many applications of Bernoulli's equation, the change in the ρ g z term along the streamline is so small compared with the other terms it can be ignored. For example, in the case of aircraft in flight, the change in height z along a streamline is so small the ρ g z term can be omitted. This allows the above equation to be presented in the following simplified form:
where p0 is called total pressure, and q is dynamic pressure. Many authors refer to the pressure p as static pressure to distinguish it from total pressure p0 and dynamic pressure q. In Aerodynamics, L.J. Clancy writes: "To distinguish it from the total and dynamic pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure."
The simplified form of Bernoulli's equation can be summarized in the following memorable word equation:
static pressure + dynamic pressure = total pressure
Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own unique static pressure p and dynamic pressure q. Their sum p + q is defined to be the total pressure p0. The significance of Bernoulli's principle can now be summarized as total pressure is constant along a streamline.
If the fluid flow is irrotational, the total pressure on every streamline is the same and Bernoulli's principle can be summarized as total pressure is constant everywhere in the fluid flow. It is reasonable to assume that irrotational flow exists in any situation where a large body of fluid is flowing past a solid body. Examples are aircraft in flight, and ships moving in open bodies of water. However, it is important to remember that Bernoulli's principle does not apply in the boundary layer or in fluid flow through long pipes.
If the fluid flow at some point along a stream line is brought to rest, this point is called a stagnation point, and at this point the total pressure is equal to the stagnation pressure.
Applicability of incompressible flow equation to flow of gases
Bernoulli's equation is sometimes valid for the flow of gases: provided that there is no transfer of kinetic or potential energy from the gas flow to the compression or expansion of the gas. If both the gas pressure and volume change simultaneously, then work will be done on or by the gas. In this case, Bernoulli's equation – in its incompressible flow form – can not be assumed to be valid. However if the gas process is entirely isobaric, or isochoric, then no work is done on or by the gas, (so the simple energy balance is not upset). According to the gas law, an isobaric or isochoric process is ordinarily the only way to ensure constant density in a gas. Also the gas density will be proportional to the ratio of pressure and absolute temperature, however this ratio will vary upon compression or expansion, no matter what non-zero quantity of heat is added or removed.
The only exception is if the net heat transfer is zero, as in a complete thermodynamic cycle, or in an individual isentropic (frictionless adiabatic) process, and even then this reversible process must be reversed, to restore the gas to the original pressure and specific volume, and thus density. Only then is the original, unmodified Bernoulli equation applicable. In this case the equation can be used if the flow speed of the gas is sufficiently below the speed of sound, such that the variation in density of the gas (due to this effect) along each streamline can be ignored. Adiabatic flow at less than Mach 0.3 is generally considered to be slow enough.
7. CONSTRUCTIONAL DETAILS
For the construction of the Wet scrubber some mechanical components and some machining processes were required.
1. Convergent section
This section consist of a conical section of 1:4 tapper with decrease in cross-sectional area as approaching the throat section this section is the inlet for the exhaust gas.
2. Throat section
The cylindrical throat section also bears the pores for the inlet water spray, for which four pores with adequate spacing are provided on the throat in all the directions.
3. Water jacket
A water jacket is provided around this throat so that water can be inputted into the pores from a common water inlet port.
4. Divergent section
Similar to the convergent section, the divergent section is also conical but with increase in area as moving away from the throat. And the tapper ratio is of 1:6.
5. Collecting tank
The collecting tank is a cylindrical container on top of which the entire wet scrubber apparatus is welded vertically, and sealed with only a exhaust port on top of the sealed surface. At the bottom most portion of the container there is a slurry outlet for the exit of the spent water.
6. Hose and pipes
For connecting the exhaust of the engine to the apparatus a hose of 1 meter is used and is of 1.5 inch, while flexible pipes of 0.5 inch was used for connecting the water inlet to the throat and disposing the spent water outlet.
Fig 5 Different Component
7.2 MACHINING PROCESSES
1. Sheet metal cutting
The different components involved were made from sheet metal, a plain sheet on which the development of these components were initially drawn and then were cut out using sheet metal cutting tools.
2. Gas welding
The cut out pieces were then bent into the desired ahspes and the joints were the welded to each other by means of a gas welding torch.
3. Filing and finishing
The different surface of the welded structure was then finished by the filing process.
8. VENTURI SCRUBBER
Figure 6 - Venturi scrubber
A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.
Venturi devices have also been used for over 100 years to measure fluid flow (Venturi tubes derived their name from Giovanni Battista Venturi, an Italian physicist).
About 35 years ago, Johnstone (1949) and other researchers found that they could effectively use the venturi configuration to remove particles from gas streams. Figure 1 illustrates the classic venturi configuration.
A venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section and, as the area decreases, gas velocity increases (in accordance with the Bernoulli equation). Liquid is introduced either at the throat or at the entrance to the converging section.
The inlet gas, forced to move at extremely high velocities in the small throat section, shears the liquid from its walls, producing an enormous number of very tiny droplets.
Particle and gas removal occur in the throat section as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is forced to slow down.
Venturis can be used to collect both particulate and gaseous pollutants, but they are more effective in removing particles than gaseous pollutants.
Figure 8- Wetted throat venturi scrubber
Liquid can be injected at the converging section or at the throat. Figure 2 shows liquid injected at the converging section. Thus, the liquid coats the venturi throat making it very effective for handling hot, dry inlet gas that contains dust. Otherwise, the dust would have a tendency to cake on or abrade a dry throat. These venturis are sometimes referred to as having a wetted approach.
Since it is sprayed at or just before the throat, it does not actually coat the throat surface. These throats are susceptible to solids buildup when the throat is dry. They are also susceptible to abrasion by dust particles. These venturis are best used when the inlet stream is cool and moist. These venturis are referred to as having a non-wetted approach.
Venturis with round throats (Figures 2 and 3) can handle inlet flows as large as 88,000 m³/h (40,000 cfm) (Brady and Legatski 1977). At inlet flow rates greater than this, achieving uniform liquid distribution is difficult, unless additional weirs or baffles are used. To handle large inlet flows, scrubbers designed with lon
Figure 9 - Rectangular throat venturi scrubber
Water sprays help prevent solids buildup. The principal atomization of the liquid occurs at the rods, where the high-velocity gas moving through spacings creates the small droplets necessary for fine particle collection. These rods must be made of abrasion-resistant material due to the high velocities present.
All venturi scrubbers require an entrainment separator because the high velocity of gas through the scrubber will have a tendency to entrain the droplets with the outlet clean gas stream.
Cyclonic, mesh-pad, and blade separators are all used to remove liquid droplets from the flue gas and return the liquid to the scrubber water. Cyclonic separators, the most popular for use with venturi scrubbers, are connected to the venturi vessel by a flooded elbow (Figure 8). The liquid reduces abrasion of the elbow as the outlet gas flows at high velocities from the venturi into the separator.
9. PARTICLE COLLECTION
Figure 10 - Adjustable-throat venturi scrubber with plunger
Venturis are the most commonly used scrubber for particle collection and are capable of achieving the highest particle collection efficiency of any wet scrubbing system. As the inlet stream enters the throat, its velocity increases greatly, atomizing and turbulently mixing with any liquid present.
The atomized liquid provides an enormous number of tiny droplets for the dust particles to impact on. These liquid droplets incorporating the particles must be removed from the scrubber outlet stream, generally by cyclonic separators.
Particle removal efficiency increases with increasing pressure drop because of increased turbulence due to high gas velocity in the throat. Venturis can be operated with pressure drops ranging from 12 to 250 cm (5 to 100 in) of water.
Most venturis normally operate with pressure drops in the range of 50 to 150 cm (20 to 60 in) of water. At these pressure drops, the gas velocity in the throat section is usually between 30 and 120 m/s (100 to 400 ft/s), or approximately 270 mph at the high end. These high pressure drops result in high operating costs.
The liquid-injection rate, or liquid-to-gas ratio (L/G), also affects particle collection. The proper amount of liquid must be injected to provide adequate liquid coverage over the throat area and make up for any evaporation losses. If there is insufficient liquid, then there will not be enough liquid targets to provide the required capture efficiency.
Before designing the wet scrubber there are few design factors to be considered for obtaining the maximum efficiency of the scrubber and they are mentioned below.
10.1 Design factors to be considered
1. The diameter of the throat
Throat diameter of the scrubber is of primary importance, because the velocity of the gas stream depends on the maximum possible reduction in cross sectional area since higher velocity is required to atomize the scrubbing liquid, but care should be taken concerning this dimension because an optimum diameter is required for obtaining this velocity without loading the engine.
2. The volume of the inlet gas
The overall size of the unit depends on the volume of the exhaust inputted to the wet scrubber for purification purpose, hence the volume of exhaust gas coming out of the engine should me known.
3. Inlet gas characteristics
Wet scrubbers are designed on the basis of the industrial process conditions and the nature of the air pollutants. Inlet gas characteristics and dust properties are of primary importance and they can be designed to collect particulate matter and/or gaseous pollutants.
10.2 Calculation of throat diameter
The wet scrubber in this mini project was designed, tested and intended to be used on a stationary 416 cc Bajaj diesel engine, so hence to get the minimum possible diameter for the throat the area through which the exhaust gas exits the engine was calculated by considering the stroke length of the exit valve and the area of the exit port. The dimensions considered here can be better understood from the fig 5.1.
Fig 11: Valve dimensions
Area through which exhaust gas escapes engine cylinder = pi * diameter of valve * length of valve opening
= 22/7 *28*6.8
Radius of throat = square root of (above area/pi)
Hence diameter = 27.58mm.
The length of the throat was taken to me 60 mm considering a 10 mm gap between the four pores (provided on the throat for the inlet water) and the boundaries of the throat
10.3 Dimensions of the Converging and Diverging sections
The dimensions of the Converging section was obtained by drawing a 1:4 tapper from the throat diameter obtained, along 100mm and was obtained to be 77.5mm and now using this dimension 1:6 tapering section was created for the divergent section and the dimension obtained from the drawing was 150 mm.
Fig12: Dimensions involved
Note: Dimensions of all figures shown in this chapter are in mm
The Wet Scrubber's exhaust gas inlet hose is initially connected to exhaust or the silencer of the engine, then the water inlet to the throat of the Scrubber was connected to a suitable water supply from the over head tank, now make sure the water outlet pipe is placed a bit above the bottom surface of the tank so as to prevent the purified gas from coming out of this pipe, also care should be taken to place this outlet pipe in a safe disposal area for recycling or disposing the spent water.
Now when the engine is started, it can be observed that the soot or ash containing exhaust gas just circulates through the apparatus and exits from the exhaust port above the collecting tank. The water supply is turned on hence, and the water jacket around the throat gets filled up providing water into the four pores placed in different directions on the throat. The exhaust gas from the engine enters the convergent section at the top most portion of the wet scrubber, and as the area goes on decreasing towards the throat by Continuity equation the velocity will increase to a maximum value at the throat, this added velocity will then atomize the liquid or water spray at the throat to form a fine mist or fog.
The atomized droplets will easily absorb the Diesel Particulate Matter and also dissolve water soluble gases, the spent water then settles down into the collecting tank. Now as the water level in the tank rises, spent water exits the tank via the outlet pipe.
Fig 13: Working of the Wet Scrubber
1. Relatively high efficiency, almost 99% efficient in removing participate matter and dust particles from the exhaust gas than purification methods like chemical treatment, electrolytic treatment, dry treatment etc.. are.
2. Also has the added advantage of dissolving the water or liquid soluble gaseous pollutants.
3. The scrubbing liquid can be changed as per the characteristics of the exhaust gas for more serious treatment of pollutants.
4. Simple and easy construction, involving only a simple venturi apparatus, for heavy duty purpose depending on the load a scrubber with variable throat diameter can be easily fabricated too.
5. No maintenance cost once set and also relatively cheap to run compared to other means of treatment methods.
6. Since liquids are used as scrubbing media hot exhaust even containing flammable gases can be recycled without any hazardous results.
1. The main disadvantage is that the end product that is the spent water has to be disposed of safely or recycled.
2. A continues supply of the scrubbing liquid has to be inputted into the scrubber for the process.
3. The apparatus can only purify Particulate Matter and dust particles while other gaseous .pollutants like CO, CO2, SO4 etc. are not eliminated.
4. Right now the scrubber is a stationary unit since there is no source of water supply with the necessary head, so can only used with stationary engines.
5. High initial cost of setting the equipment also for a higher volume of exhaust a larger scrubber and so hence more floor space will be occupied
1. An efficient method for recycling or disposing off the end product or spent water can be in cooperated with this equipment.
2. If there is no issue of loading the engine, then the divergent section can be placed immersed in the spent water itself to make sure the entire gas comes in contact with the liquid, hence efficiently dissolving all water soluble pollutants.
3. To make the entire unit mobile a source of water with suitable head can be innovated with this project.
Industrially wet scrubbers are found to have up to 99% efficiency in successfully eliminating Particulate Matter from exhaust gas. Although wet scrubbers were never in cooperated on such small scale applications like diesel engine, this mini-project "Wet Scrubber for Diesel Engines" has proved it can efficiently remove DPM that is diesel particulate matter from the exhaust. The results have been proved using the Government authorized smoke test certificate published previously in this report. The difference have been found to be more than 60% reduction in DPM emission on fitting the project apparatus.