3.2 Air

Definitions

"Air Pollution" is defined as the introduction by man, directly or indirectly, of substances into the air which results in harmful effects of such nature as to endanger human health, harm living resources and ecosystems, cause material damage, interfere with amenities and other legitimate uses of the environment.

Air pollutants can be solid (dust and particulates), liquids or gases. Aerosols are produced when gases and liquids are mixed . The changes of the pollutants from one state to another is a function of the temperature, pressure, chemical and physical circumstances that give rise to them. In addition, air pollutants abundance in air is related to the abundance at source.

Quantitatively, the three important life cycles, namely, the sulphur cycle, the nitrogen cycle and the carbon cycle, play a big role in contributing to air pollutants and also as sinks of excess of these gases.

Anthropogenic sources is a term to describe man-made sources such as industries, chemical usage, and transport. They are usually the regulated sources. 3

3.2.1 Importance of Air Pollution Control

Air in its natural state is life supporting. It is harmful only when its composition has been changed by the introduction of harmful substances to it.

In Kenya, the main cause of high incidence of diseases of the upper respiratory tract is suspected to be due to the presence of pollutants in the atmosphere, in residential, and working places. It has been observed that there is also a correlation between levels of air pollution, social, economic, technological and the capacity of the country to enforce its laws.

Morbidity patterns in Kenya closely relate to causes of mortality. Over the years, malaria and acute respiratory infections (ARI) accounted for almost half of the reported visits to the outpatient facilities. In-patient morbidity depicts the same trends as outpatient where malaria leads with 26% of the total reported in patients closely followed by ARI which account for 22% of the cases. From this stand-point lone, it is apparent that the country's medical expenditure goes to air pollution related diseases Reducing this pollution can be achieved by enforcing air quality standards.

3.2.2 Objectives of Air Quality Standards

Air quality management aims at ensuring that an environmentally safe level of air pollutants is maintained so as to protect human health and the environment. For health the effects of some air pollutants are more threatening than others. They can be grouped as below:

Group I:
Life threatening injury, where injured persons may have intense irritant-induced cough, respiratory insufficiency, and systemic effects.

Group II:
Severe injury where injured persons will have strong irritant-caused cough, respiratory difficulties, but no systemic effects.

Group III:
Mild injury, where persons may have moderate or mild irritant caused cough, eye-symptoms/signs and possibly headache.

The air pollutants to be discussed and for which standards need to be developed are those that fall under all of the above groups.

3.2.3 Composition of Natural Unpolluted Air

Other constituents found in normal air include cosmetics such as perfumes; solid particles such as dusts; chemical gases and their intermediaries; aerosols and dust, Their addition to air makes it unclean or polluted.

3.2.4 Priorities for Air Pollution Control in Kenya

This is essential that the issues of air pollution are identified with a view to solving environmental problems; and the proposed logical steps to follow are as outlined below.

Priority 1: Those with local impacts to structures, health, soil, and water. These are generally environmental impacts such as acid-rain.

Priority 2: Those with global impacts such as global warming and ozone layer depletion.

(i) Acid Rain

Both "wet deposition" acid rain, snow, fog, and cloud vapours and "dry" deposition-acidic particulate and aerosols, are formed when large volumes of sulphur dioxide(SO 2 ) and nitrogen oxides(NO X ) are released from the combustion of fossil fuels.

Stationary sources such as coal burning power plants, ore smelters and industrial boilers, are responsible for nearly all human caused SO x emissions and for about 35% of human caused NO x emissions.

Smoke stacks emit gases into the atmosphere where most are converted to sulphate and nitrate particulates and distributed down-wind. The phenomenon of acid deposition is largely associated with highly industrialised regions of Europe and North America. In Kenya, it is localised to specific industrial environs.
Localisation of acid-rain in Kenya is mainly related to sulphuric and sulphonic acid manufacturing plants.
Indicators of acid-rain normally include ecosystem damage, damage of physical structures like corrosion of marble and metal building materials.

Increasing attention is now being paid to the health effects of acid aerosols derived from chemical transformation of SO x and NO x in the atmosphere. counting evidence suggests that aerosols may damage human health by contributing to respiratory problems such as bronchitis and asthma.

(ii) Global Air Pollution Issues

The global concern comes about when the impact of pollution of air is extended locally, regionally, and globally in that order.

As a rule of thumb, most local impacts have health and aesthetic negative impact while the regional and global changes may in addition have impacts on climate, affect the structure, ecosystems and other life support systems. The air pollution issues of global significance are therefore, ozone layer, depletion global warming and climate change.

Ozone Layer Depletion
Particles, when released into the atmosphere are removed within hours to weeks by one of three general processes: by absorption of sunlight (photolysis), by dissolution in water droplets (rain-out) or by reaction with hydroxyl radical, HO, or Ozone O 3 (oxidation). a group of chlorinated halogenated hydrocarbons, namely, chlorofluorocarbons (CFCs) and bromochlorocarbons (halons) are stable for the above processes. CFCs, halons, carbon tetrachloride, and their intermediaries are transparent, insoluble, and unreactive to atmospheric oxidising agents; so none of the above processes affect them and they can last for a long time in the atmosphere. Their molecules are carried into the stratosphere by great storms in the equatorial region. There, 25 to 35 kilometres, they are destroyed by intense, very energetic solar ultraviolet radiation. This UV-C radiation is not present in the lower earth surface because it is strongly absorbed by ozone in the stratosphere and the CFC molecules have to rise above most of this before they encounter it. Ozone, the triatomic form of oxygen (O 3 ), mostly exists where it is created, high overhead in the stratosphere, when ordinary diatomic form of oxygen O 2 is broken into two oxygen molecules by absorbing the C-radiation which reacts with the strong halogenated molecules commonly known as the ozone depleting substances (ODSs). Most of the ODSs are consumed in refrigeration, foam blowing, degreasing, as solvents, in agriculture, and dry-cleaning.

Chlorofluorocarbons (CFCs)

There are two CFCs currently in major use in Kenya namely CFC-11 and CFC-12. The CFC-11 is mainly used in polyurethane foam blowing for insulating panels for domestic refrigeration. Consumption of CFC and hence, its
importation was 100 metric tonnes per year. It is expected that this level has gone up because the major user is still using the same technology. There has been a move in other sectors of foam blowing to move away from CFC-11 and
use methylene chloride instead.

Other users of foam blowing are importing foam blowing chemicals already pre-mixed and data on current consumption is not very accurate. The aerosol sector was the major user of CFC-12 (150 tonnes in 1989). By February, 1993, practically all the users of CFC-12 in aerosols had converted to using butane as propellant and hence, this precipitated a drop in the use of CFC-12. The main use of CFC-12 now is in refrigeration and air conditioning of which 100 metric tonne are estimated to be in use per year.

Methyl bromide is one of the most important ODSs currently used in the agricultural sector. The current consumption is mainly in cut-flowers, fumigation, pre-shipment and quarantine purposes. Current imports stand at 400 metric tonnes.

Policy measures to reduce the use of ozone depleting substances include the ratification of the Vienna Convention and Montreal Protocol.

The phasing out of ODS in Kenya according to the country programme, envisages the following:
- conservation of existing stocks;
- introduction of alternatives to new installations and equipment;
- legislating against their imports, and
- recycling.

Climate Change

There are two types of sources of air pollutants that can cause global warming and consequently climate
change, namely, point sources such as industries and vehicles, and non-point sources such as land use and forests.

CO 2 is the most common greenhouse gas produced by anthropogenic activities.

By far the largest source of CO 2 emissions is from oxidation of carbon when fossil fuels are burnt which accounts for 70-90 percent of the total anthropogenic emissions. When fuels are burned, most carbon is emitted as CO 2 immediately during combustion process. Some carbon is released as
CO, CH 4 , or non methane hydrocarbons which oxidize to CO 2 in the atmosphere within a period ranging from a few days to 10-11 years. It is possible to get an accurate estimate of national.

Since fuel combustion is widely dispersed throughout most activities in the national economy, it is possible to obtain an accurate estimate of CO 2 emissions by accounting for the carbon in fuel supplied to the economy.

3.2.5 Indicators of Air Pollution in Kenya

In Kenya, pollution from anthropogenic sources fall in the following order in terms of distribution of contributors:
- land use;
- transport;
- animals/agriculture;
- mining and transport of mined products;
- solid waste disposal;
- building and construction;
- industries; and,
- electrical appliances.

3.2.6 Scientific Knowledge on Priority Pollutants

The pollutants discussed here below are chosen by virtue of their abundance in air and their potential impact to the health of people, animals and the general ecosystem.

(a) Oxides of sulphur
(b) Oxides of Nitrogen
(c) Oxides of Carbon
(d) Dust Particles
(e) Vaporous organic Compounds
(f) Inorganic air pollutants

3.2.7 History of Emission Monitoring
(i) Total Suspended Particulate
The early incidence of survey of air pollution level in Nairobi and other parts of the country were conducted in 1982.The industrial areas of the city had the highest levels of concentration of total suspended particulate pollutants. The level fell with increasing distances from the industrial area.

Particulate is a major cause of respiratory diseases acting synergistically with other pollutants. The Directorate of Occupational Health and Hygiene (DOHSS) has tried to link particulate in work places with various disorders. One study carried in 1983-84 involving 5 textile industries and two cotton ginneries was intended to relate sisal dust in textile industries and cotton dust ginneries to upper respiratory tract infections (URTI). One textile factory had 19.7% of the workers examined with clinical and radiological evidence of byssinosis. 24.2% of the workers in a ginnery had similar findings. In another study in a sisal industry the prevalence rate was 16.3% in the crushing section and 5.7% in the decorting section. These are areas with the highest dust measurements.

(ii) Acidic Precipitation

In 1982 measurements of acidity of rain water in selected stations in Kenya revealed that there has been acid rain in Kenya. By acidic it is meant the rain water whose pH is less than 5.6. The acidity was higher in the vicinity of Nairobi than in other remote areas. This was to be expected since Nairobi had a larger number of emission sources of acidic gas compared to the rest

(iii) Carbon Monoxide (CO)

Average ambient concentration levels of carbon monoxide in Nairobi associated with emissions from motor vehicles showed that concentrations of CO can reach as high as 15 ppm using the criteria for vehicles without emission controls. In Kenya, as will be seen later, invisible gases are not monitored. On the average, the concentration levels were found to be around 5 ppm.

(iv) Hydrocarbons and Nitrogen Oxides

In a study reported in December, 1993, N0x and hydrocarbons were estimated for Nairobi hydrocarbons. The highest computed concentrations was 517 mg/m 3 on Moi Avenue during morning traffic peak. The lowest values were for City Hall way 16.2 mg/m 3 and Parliament Road 33 mg/m 3 . Moi Avenue had the highest computed afternoon concentration of 158 mg/m 3 .

(v) Lead

In 1992, a study indicated that lead levels in Nairobi fall with WHO guidelines 0.5 - 15 mg/m 3 . This lead comes mainly from motor vehicles.

3.2.8 Emission Reduction Techniques

If air quality standards are set without the development and implementation of measures directly aimed at emission reduction, the standards will fail to obtain clean air. Currently, various mission control methods or processes
can be used, separately or together, to meet air quality standards.

a) Emission Control

The Public Health Act, assign a significant role to the height of smoke-stacks (the source of emissions) to reduce the ground level concentration of pollutants in the vicinity of plants. This measure is in itself economical and
technologically simple. In most, but not all, meteorological conditions, it could effectively protect the environment around the plant. In recent years, however, heightened awareness about the problem of long-range transport of air pollutants and, in particular, the increasingly serious problem of acid deposition as evidenced at Thika due to Kel Chemicals Ltd, has turned the issue of stack height into a reason for concern rather than a method of control. Height of stacks without consideration to the nature and concentration of pollutants in place of emission control devices must today be considered an obsolete abatement mechanism for air pollutants.

b) Fuel Control

Specification of the characteristics of fuel, ash, or sulphur content, is an effective pollution control measure. Restrictions should be imposed on specific types of industries or in particular parts of Kenya especially with due regard to the synergism of the industries.

c) Technology Control

Control technologies have been tried in industries in Kenya. For example the Kenya Refineries Limited is able to reduce sulphur content in fuel. To what degree is not possible to know without consistent monitoring and enforcement of whatever standards will be adopted; however, it is true standards or no standards, emission abatement during combustion can effectively reduce sulphur and nitrogen oxides.

d) Use of Emission Limits

Emission limits can be defined as "the maximum amount of pollutant that is permitted to be discharged from a single polluting source", and also as the maximum admissible concentration of pollutant present in the fumes at the
chimney outlet. Quantitative emission standards commonly fall into one of two categories: either an energy-input-related standard, such as nanogram of pollutant discharged per Joule of energy supplied to the plant (ng/J), or a
standard expressed as a concentration of pollutant present in the flue gases, such as, milligrams of pollutant per cubic metre of discharge gas (mg/Nm 3 )

Oil Fuel Fired Boilers and Their Efficiency

Majority of the industrial users of boilers use cost first and, then, production capabilities as prime criteria for the selection of such equipment while that factors such as energy management and environmental issues are normally
relegated to minor roles. The high cost of fuel today together with the high rate of environmental degradation demands greater awareness of the management technics which should be incorporated in the designs of such plants.

In most applications of boilers heat is produced from the combustion chamber. The fuel is introduced into the combustion chamber at the burner, simultaneously mixed with air and ignited. The common fuel used are fuel oil, saw-dust, pulverized coke, and used oil. The heat transfer in the boiler, which could be categorised either as fire- tube boiler or water-tube boiler, takes place through a heat exchanger so that the product is not directly exposed to the combustion gases

The efficiency of the boilers depends greatly on the type of fuel used and by extension on the cost of the fuel. The selection of the fuels also depends on the type of burners employed. The burner system is designed to provide
adequate mixing of fuel and combustion air with a flame shape as required by the combustion chamber design. Multiple burners may be installed to provide a temperature pattern within the boiler combustion chamber.

The appearance of the burner flame can be a guide to correct combustion conditions. Setting up the burner requires some experience. The appearance of the flame should be checked for future reference after an experienced boiler engineer has performed this task. In general terms, an oil flame should be a light brown or yellow colour. A short blow-torch shaped flame indicates too much air, whereas a long lazy smoky flame indicates too little air.

The quantity of fuel needed to generate a specific amount of heat depends on the heating value of the fuel. Heating value is the amount of heat generated when a fixed quantity of fuel is completely burned.

Combustion Air System

Stoichiometric air represents the amount of air required for complete combustion with the perfect mixing of the fuel and air. Stoichiometric air is some times called theoretical air. If perfect mixing is achieved, every molecule of fuel and air takes part in the combustion process. Excess air must be supplied to ensure complete combustion of the fuel and air does occur. Percentage excess air is defined as the total amount of combustion air supplied in excess of the stoichiometric air, expressed as a percentage of the stoichiometric air.

The minimum amount of excess air required varies with the fuel used and the efficiency of mixing the air and the fuel. If less than the minimum quantity of air is supplied, some of the fuel will not burn completely and there is a waste of fuel energy and emission of unwanted combustion products. Evidence of incomplete combustion usually shows up as CO in the flue gas. A continuous gas analyzer, can be used to check for CO in the fuel gas.

Too much air also wastes energy. The gases leaving the boiler are hot and contain heat energy. If excessive amount of are supplied to the boiler furnace, the excess will also be heated. The minimum losses occur when mount supplied is slightly greater than the stoichiometric amount.

Combustion air can be supplied to the chamber by natural or forced draft system. Natural draft uses the negative pressure (draft) produced by the furnace stack to draw combustion air into the furnace and the resulting flue gases out of the furnace.

There are several disadvantage related to natural draft firing. The amount of combustion air drawn into the furnace cannot be controlled accurately and the fuel air mixing is inefficient. This means that higher levels excess air must
be maintained to ensure that complete combustion is achieved all conditions. The furnace pressure is always negative which allows air to leak into the furnace, and create additional flue gas volume and heat losses.

Forced draft firing uses a fan to supply combustion air to the furnace chamber. Air flow is regulated by use of dampers so that accurate control of the proportion of air to fuel for various firing rates is possible. A common method used to achieve this is to operate the fuel valve and the damper with a common mechanical linkage. Some of the adjustable cam is used to vary the relative positions of the fuel valve and the damper to provide proper fuel/air ratios at all firing rates.

The combustion air fan also provides better mixing of the fuel and the air. The air is introduced into the furnace chamber around the burners and turbulence can be created by vanes which produce a swirling motion in the air as it enters the chamber. A high pressure drop between the air supply and the furnace chamber is required to produce turbulence, and this can only be achieved with a forced draft system. These advantages mean that the excess air for a forced draft system can be lower than for natural draft firing, with resulting lower heat losses to the flue gas.