Industrial Pollution

1. Industrial Pollution • Industrial smokestacks - tall smokestacks do not remove pollutants but simply boost them higher into the atmosphere - reducing their concentration at the site. Pollutants are then transported over large distances - adverse effects far from site -SO2 and NO emissions cause acid rain. Subsequent formation of sulphuric acid attacks limestone,marble and buildings in general. Main contributor to global warming is CO2 from burning of coal and oil. • Source emission standards are specified to limit the discharge of hazardous substances into air - in form of conc. levels that are believed to be low enough to protect public health. Source emission standards are also specified to limit discharge of pollutants into air so that air quality standards achieved. • US amendments to US Clean Air Act - identified ozone, CO, particulates and air toxins as major air pollution problems. • Montreal Protocol, 1985 - 49 countries agreed on a UN convention to protect the ozone layer. Renegotiated in 1990, the convention calls for certain chlorocarbons and flourocarbons by 2000 and provides aid to developing countries in making transition.

2. Pollution Control Technology (S & N2 Emissions) • Technologies for control of emissions - industrially emitted particles can be trapped in cyclones, electrostatic precipitators and fabric filters. Pollutant gases can be collected in liq’s or on solids, or incin’ted into harmless subst’ces. H2Cl requires some form of scrubbing for efficient removal. • Over 180 diff. systems of Flue Gas Desulphurisation (FGD) exist w’wide -at least 30 used commercially. • Reducing Sulphur and Nitrogen Emissions - limestone/gypsum process mixes wet limestone with the hot gases - Calc. sulphate (gypsum) is formed, removing up to 90% of sulphur. Other systems do not need large amounts of limestone and produce usable pure sulphur or acid rather than waste. The UK market for gypsum could be supplied by 6 FGD-fitted large power stations. • Power station NOxcan be red’d with low NOx burners - increasingly fitted at large stations - different type of flame can reduce by 30-40% -low cost. • For NOx reductions of 80% or more, selective catalytic reduction (SCR) can be used - ammonia is injected into the superheated region of a boiler or outside it in a catalyst bed. Around half the cost of FGD (USA, Japan, Ger.). One process (‘NOx Out’) uses urea and achieves 80% red’n at 25% cost of SCR.

3. Pollution Control Technology (S & N2 Emissions) • As of 1988 the UN ECE Conv. included a NOx Protocol under which UK emissions must not rise above 1987 levels will force uptake at power station controls as well as traffic sources. • Environmental Protection Act - gives Sec. of State for Env. additional powers in the field of pollution control. Act covers the control of air pollution from combustion and incineration processes - Scheduled Processes under HMIP (Sch. A) or Local Auth. (Sch. B) control and enforced using BATNEEC principle. • Further, for a Sch. A process a more holistic approach taken of overall process through the concept of Integrated Pollution Control (IPC) - this takes into consideration the distribution of pollutants between land, air and water to ensure the Best Practical Env. Option (BPEO) is used to minimise the overall impact on the environment. • Under this legislative framework, responsibility for ‘control’ will be with the operator who will have to demonstrate that BAT is being employed (or BPEO for Sch. A). • If BAT/BPEO has already been demonstrated the onus on operator to invoke NEEC as the basis for less stringent control standards. • In parallel EC aims to tighten standards through Directives which must be implemented in national law by Member States.

4. Gas Cleaning Technology • Removal of Particles: - • Main reasons - (i) Health of operators and surrounding population - main danger is inhalation of particles between 0.5 and 3 µm (ii) eliminate explosion risks(iii) prevent wastage of valuable materials (iv) gas itself may be required for use in further process - e.g. blast furnace gas used or firing stoves. • Choice of equipment - particle size, quantity of gas to be handled, conc’n of dust or mist and physical properties of particles. The efficiency of system to remove particles varies with particle size - need to match the size to technology for peak performance. • Types: - Cyclones (e.g. Med. / High Efficiency) Precipitators (e.g. Tubular or Irrigated) Fabric Filter Spray Tower Scrubbers (Wet Impingement or Venturi) Disintegrator

5. Gas Cleaning Technology • Removal of Particles: - • Separation Equipment: - Depends on one or more of the following principles and in some plant the relative importance of each is difficult to assess. Gravitational Settling Centrifugal Separation Inertia or momentum processes Filtration Electrostatic Precipitation Washing with a Liquid Agglomeration of solid particles and coalescence of liquid droplets

6. Gas Cleaning Technology Absorption of Gases: - • The removal of one or more selected components from a mixture of gases by absorption into a suitable liquid. • Can be considered in two groups - those where process is solely physical and those where a chemical reaction is occurring. Physical • Two phases are brought into contact with each other - thus water in contact with air evaporates until the air is saturated with water vapour and the air is absorbed by the water until it becomes saturated with the individual gases. the degree of absorption is determined by its partial pressure (dependant on its temp. and con'c.n. • Wetted-wall columns/packed towers/spray towers Chemical Reaction • The gas reacts chemically with a component of the liquid phase - in the cleaning of CO2 it reacts directly with caustic soda or ethanolamine solution. General An expensive exercise

7. Controlling Emissions in Industry - Some Examples • Non-Ferrous Metal Smelting : Main focus is on problem of SO2 control - (i)FGD, modification of furnaces to produce a strong gas stream, replacement of sources giving rise to the weak SO2 stream with alternative, modern technology producing strong SO2 streams controllable by acid plants. • Cement Manufacturing : predominantly SO2 from cement kilns - 75 - 90% removed by the limestone in the raw material feed (lime, silica, alumina and iron) produces calcium oxide and other alkaline oxides in the kiln - react with SO2to form calc. sulphate - used again in the solid clinker. Fabric filters can also be fitted - removing 50% SO2. • Metallurgical Industry (e.g. prod’n of ferro-alloys): SO2 produced in combustion of fuels for process heat and the roasting of sulphide containing ores - emissions depend on the sulphur content of the ore as well as the prod’n technology. Examples of control technology - scrubbing of the SO2/SO3 containing gases with addition of nitric acid and nitrogen oxides produces sulphuric acid (70%) and removal of sulphur is 99%.Important step is to use low sulphur fuels • Other important industries : Pulp and Paper Industry and Refineries

8. Pollution Control Systems I Particle Control SystemsProcess • Settling chambers Gravity • Cyclones Inertial separation • Filtration Inertial separation & diffusion • Electrostatic precipitators Electrostatic forces • Wet scrubbers Inertial separation & diffusion II Gas and Vapour Control Systems • Wet scrubbers Absorption • Activated charcoal Adsorption • Thermal destruction Chemical oxidation:direct flame or catalytic • Biological oxidation Biofiltration and bioscrubbers • Advance oxidation Chemical reactions initiated by UV light augmented by ozone and hydrogen peroxide

9. I Particle Control Systems 1. Cyclones • Description • Method: centrifugal force to separate particulate from a gas or liquid flow stream. • Simple design, most efficient and most economical form of an inertial separator - most commonly used dust removal devices within industry. • For particles with diameters of 2μm and larger. • Usually no moving parts - low maintenance, current designs - collection η>90% for 2μm particles. • Low initial capital costs and low running costs - usually the first devices considered for use in separation of particulate from gas. • Operation Particulate is separated from gas stream by centrifugal force - particulate thrown towards outside of a spinning column of gas, while relatively clean gas exhausts from centre of the spinning vortex.

10. 1. Cyclones(cont’d) • Performance • 2 factors affecting performance (i) velocity at which a particulate is moving toward the wall of cyclone, (ii) length of time available to move particle into a region where it will be collected before the gas exits the device (residence time). • 2 measures of cyclone performance (i) energy consumption in moving particle through cyclone (ii) fractional or size efficiency curve. • Applications • Air Pollution Control - as primary emission control device - woodworking/ processing, metal grinding, plastics manufacturing. • Process Cyclone Applications - one of most common applications is in conjunction with fluidised beds in combustion processes, chemical reactors, petroleum refining, etc. Collects entrained solids from fluidisation gas and returns to bed. • Precleaners - one of most common general ind. uses is as precleaner for other air pollution control equipment (filtration, scrubbers, ESPs). • Liquid entrainment separators - Renewal of entrained liquid droplets from a gas stream.

11. Cyclone Separator

12. Inertia or Medium Separators

13. Gravity Separators

14. 2. Media Filtration • Wide variety of styles and types of industrial media filtration equipment. Three general groups of application:- (i) Ambient Air Filtration • Air from general atm. or building - removal of contaminants in order to protect people, equipment or processes. HVAC systems in almost every ind. building utilise some from of media filtration. Filter elements are thrown away when dust cake forms - a flow cannot be maintained. • Most common types - • roll media (loose non woven media treated with an oil tackifier to increase efficiency. • Bag filters (commonly fibreglass multi-compartment filters (: 30-95%). • pleated filters. (ii) Nuisance Dust Protection of the environment, workers and machinery in the industrial work-place. Includes static filters of all kinds - cabinet envelope filters, baghouses, cartridge filters and multi-stage systems. (iii) Process Dust Collection Similar to nuisance dust application however dust is not a nuisance but a product of the system process intended for sale (e.g. flour and starch milling)

15. Media Filtration

16. 3. Particle Scrubbing • Particulate scrubbing using water - possibly oldest method of air pollution control used with industry since early 1900’s. Relatively high particulate collection efficiency achievable. Simple process in its basic form - spraying water into a gas stream. • Basic principle - to generate easily collected large particles by combing liquid droplets with relatively small dust particle. Dust particles are ‘grown’ into larger particles by several methods - combining them with relatively large droplets, absorption of water by the dust particles and subsequent increases in mass, or formation and growth of condensable particles by rel. cold temps within the scrubber (first of these is most significant method used within scrubbers in most applications). • Inertial Impaction Scrubbing - Venturi - most common mechanical design to achieve inertial impaction - high speed airborne dust particles impact and adhere to liquid droplets in the throat section. • Packed bed scrubbers - airborne, pollutants transferred to the scrubbing liquid as the two streams encounter one another in the packing. Packing creates a large surface area per unit volume. Pollutants must be removed by independent means before the liquid is recycled to top of the tower. • Bubblecap scrubbers - use numerous trays through which the gas is bubbled to achieve the gas - liquid interaction needed to remove particle and gaseous pollutants.

17. Particle Scrubbing

18. 4. Electro-Static Precipitators • High efficiency particle collectors that remove small particles from an airstream due to an electric charge placed on the particles. Plate-wire ESP consists of 100’s of vertically mounted small diameter wires placed midway between dozens of large vertically mounted plates that are vertically grounded. • High voltage applied to wire - generates electrons that are transferred to airborne particles. The electrically charged wire and plates also establish an electric field causing charged particles to attach themselves. Plates are rapped mechanically - fragments of the deposited dust fall to hoppers below. • Performance sensitive to variations in the gas volumetric flow rate and electrical resistance of the collected dust. • Uses - cleaning large steady volumetric flow rates of gas e.g. coal fired electric utility boilers, lime and cement kilns.

19. Electro-Static Precipitator

20. Conventional F.F. Plant with Pollution Control

21. II Gas and Vapour Control Systems 1. Wet Scrubbers - Absorption • Gaseous industrial pollutants include acid gases (HCl2, H2, SO4, HBr, HCN, H2S, HF, etc), other inorganic vapours (SOx, NOx, NH3, Cl2, etc) and organic vapours (formaldehyde, ethylene, benzene, etc). • Absorption (literally means “taking in”, material that absorbs is called the solvent, gas being absorbed is called the solute) is one of main mechanisms utilised within the industrial air pollution control industry to remove gaseous pollutants. • Operation - needs to be a sufficient contacting area between gas and solvent and sufficient contact time. Most common device for absorption - packed tower. Two basic methods of absorption - chemical and physical. • physical absorption - gas reaches equilibrium with a solvent without changing its chemical properties (e.g. collection of gaseous HCl into water. HCl gas is very soluble in water and readily absorbed. Within water, liquid HCl is formed. • Chemical absorption - the solute is chemically reacted with solvent to form a new compound. Rate of absorption is directly related to the difference between the concentration of the solute within the bulk gas and the concentration of the solvent will contain at equilibrium. The chemical reactions used in absorption are relatively rapid when compared to time required for mass transfer - usually assumed to be instantaneous.

22. II Gas and Vapour Control Systems(cont’d) 2. Adsorption Devices • Like absorption is a mass trans. op. (contaminant - adsorbate; collecting media adsorbent) - captures instead of chemically altering or destroying the adsorbate. Often used where recovery of adsorbate is desired. • Process - one in which a porous solid brought into contact with either a liquid or gaseous fluid stream to selectively remove unwanted contaminants by depositing them on the solid (e.g. removal of moisture by using a silica gel is one type of adsorption). • Ads. devices commonly used in ind. air poll. control for removal of certain organic and inorganic compounds and elements:- • purifying intake, circulation, or exhaust gases from toxic gases, odours and other noxious gases. • solvent (VOCs) recovery from air leaving an evaporation chamber or process (spray painting and dry cleaning). • Fractionisation of gases. • Adsorbent may capture the adsorbate by both physical and chemical means. • physical adsorption - adsorbate molecules adhere to the adsorbent material (physical bonding force) - condense on surface of adsorbent, releasing heat. If adsorbent is highly porous - greater surface area. • chemisorption - usually occurs at elevated temps. where there is adequate available energy to make or break chemical bonds. • Most common adsorbent is activated carbon, also act. alumina, silica gel and clays.

23. II Gas and Vapour Control Systems(cont’d) 3. Thermal Oxidisers • Primarily used for control of emissions that require destruction of a pollutant (e.g. toxic or haz. gases). Process used in ind’s. where VOCs have to be oxidised primarily to CO2 and H2O. • Typical applications - chemical, printing and painting (where solvents containing hydrocarbons are used) and pharmaceutical industries. Industrial hydrocarbons will react in sunlight  ozone (causes resp. problems, corrosion and damage to vegitation). Title III of US Clean Air Act (1990) classifies these organic hydrocarbon emissions as Hazardous Air Pollutants (HAPs). • Process - pollutants react with O2 under high temperature where changed by oxidation to a different chemical compound. Most common reaction products are H20 and CO2 (plus heat). Ox. Reaction: CxHy+ (x + y/4) O2xCO2 + (y/2) H20 (x and y are coefficients which define the hydrocarbon being oxidised) • Thermal oxidisers cause oxidation reaction to occur in 2 ways:- • by raising temp. of the process gas until reaction occurs spontaneously • by preheating the flue gas and then exposing it to a catalyst which promotes the oxidation to occur at a lower temp. (and much higher rate)

24. Miscellaneous Particle and Gaseous Separation Technologies • Particulate Separation - in most cases the following are for pre-cleaning for other downstream equipment (prevent damage to secondary devices e.g. cyclones and scrubbers). (i) Settling Chambers - use gravity to settle out rel. large particles (>100m) particles from low velocity gas stream, particles drop to bottom of the collection hopper while clean gas continues. (ii) Inertial separators - uses inertia of particles to assist in or as main mechanism for collection. Basic example - addition of a baffle plate to a gravity settling chamber. As gas changes direction particulates continue in original path. • Gaseous Pollution Control (i) Biofiltration - gaseous pollutants removed from a process gas stream by aerobic digestion or consumption by microbes. Effective on certain VOCs and some organic compounds. Generally most viable for treating large volumes of gas with low levels of certain appropriate contaminants. Originally developed for odour control but numerous control applications since. • Biofilter consists of simple bed of material conducive to support of microbe growth - gas passes through at low velocity. To promote microbe growth - bed kept moist and gas humidified before entry. • Materials - compost, peat, wood, chips, soil, polystyrene, fibreglass wool, act. carbon. Effective on alcohols and many aromatic hydrocarbons. Successfully controlled contaminants include hydrogen sulphide, benzene, xylene, phenol, toluene, etc. Removal ’s > 90% achieved.