| A-weighted sound pressure levels | | Sound pressure level indicated when the incident sound pressure is weighted to reflect the frequency response of the human ear |
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| AAS | | Acronym - Atomic Absorption Spectrometry |
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| Absorption | | The penetration of a substance into the mass of another substance by molecular or chemical action. |
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| Absorption coefficient | | The fractional attenuation of a beam of radiation per unit distance through a gaseous medium per unit pressure (atmosphere) of absorbing gas component. Units are m-1atm-1. Term often used to mean Extinction Coefficient |
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| Absorptivity | | The fraction of radiation incident on a surface that is absorbed. |
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| ac-coupled | | In signal amplification, an ac-coupled amplifier offsets the dc-component of the signal to zero while amplifying the ac-component of the signal. |
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| Acidification | | Acidification is a complex chemical and atmospheric phenomenon that occurs when emissions of sulphur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall to earth as rain, snow, or fog. The dry forms are acidic gases or particulates |
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| Activated carbon | | Carbon, usually in a ground form, that has been treated to have extra functional groups, used widely for adsorption of air and water pollutants, due to it’s adsorptive ability and high surface area. Carbon sources for activated carbon production include coconut shells, coal, wood and lignite. |
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| Activation energy | | The external energy that must be provided to reactants in order to initiate a reaction. The combination of this energy with the internal enthalpy of the reactants is sufficient to break the reacting molecules into their constituting atoms. |
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| Acute | | Medical. Having a rapid onset and following a short but severe exposure |
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| Adiabatic flame temperature | | Refers to the theoretical flame temperature assuming no heat losses. It is computed by equating the lower heating value of the fuel to the enthalpy of combustion products corresponding to a unit mass of fuel and to a known excess air (but assuming no recirculation) |
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| Adsorption | | A process in which a substance, usually a gas, accumulates on the surface of a solid. The amount of gas adsorbed is a function of temperature and pressure as well as the nature of the gas itself. Adsorption may be used to store gases or to separate gas mixtures by varying the pressure or the temperature. |
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| Aerodynamic mixing factor | | Compares the mass ratio between comburent and fuel at a point in a flame to the mass ratio supplied through the burner |
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| Agglomerate | | A cluster of individual particles in which the particles are held together by surface forces resulting from a change of environment. Opposite of agglomeration is dispersion. |
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| Air | | So called “atmospheric” air - the atmosphere at the Earth’s surface which provides the essential oxygen content of industrial comburents. |
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| Air staging | | Design adopted in certain combustion devices where it is desirable to deliver progressively the air. This technique is often used to reduce the combustion temperature |
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| Air-fuel stoichiometry | | The quantity of air required to completely combust a given quantity of a fuel - may be expressed on a volume or a mass or a mixed volume/mass basis |
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| alkali metals | | Normally refers to sodium and potassium hydroxide, but may include lithium hydroxide. Traces of sodium and potassium compounds are present in both feedstock materials and fuels for many processes. Their hydroxides are formed at high temperature and set up thermally and operationally detrimental volatile cycles |
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| Alkylation | | A reaction in which a straight-chain or branched-chain hydrocarbon group, which is called a radical or alkyl group, is united with either an aromatic molecule or a branched-chain hydrocarbon. |
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| Amino-type structures | | Compounds containing an –NHi group. Organic compounds which have this group are called amines |
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| amorphous | | Non crystalline, without form or texture |
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| Anaerobic digestion | | Anaerobic digestion is the bacterial fermentation of organic material in the absence of air. |
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| Analyte | | The target compound for analysis |
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| Anthracite | | The highest rank coal characterised by low volatile matter - always less than 10% - and high carbon content it has a semi-metallic lustre and is capable of burning, relatively easily, without smoke – see also Semi-anthracite. |
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| Anthropogenic | | Related to human activities |
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| Araldite® | | Commercially available synthetic resin |
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| Aromatic | | Of, relating to, or containing one or more six-carbon rings characteristic of the benzene series and related organic groups. |
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| Aromatic structures | | Planar (or nearly linear) cyclic ring systems having a sequence of alternating double (or triple) and single chemical bonds |
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| Artificial neural network | | A computational model inspired by the structure of a biological brain. Therefore it is typically composed of a large number of highly interconnected processing elements, that are analogous to neurons, and these are linked together with weighted connections that are analogous to synapses in the brain. |
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| Ash | | Ash is the non-combustible material that is contained by a liquid or solid fuel, which is left as a residue after the completion of the combustion process. (See also Fly ash). |
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| Ash fusibility | | The ability of coal ash to gradually soften and melt with an increase in temperature as a result of chemical reactions and the melting of its components |
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| ASM | | Acronym - Algebraic Stress Model - CFD turbulence sub-model |
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| associated gas | | Gas that is produced at the same time as black oil. In the reservoir, it may be dissolved in the oil, or if sufficient gas is present and depending on reservoir temperature and pressure, may form a separate gas layer or cap. |
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| Atomisation | | In relation to the firing of liquid fuels, atomisation is the process of transforming the fuel flow in the burner into a stream of droplets thus increasing the surface to volume ratio to aid evaporation in the root of the flame |
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| Attenuation Coefficient | | The fractional attenuation of a beam of radiation per unit distance through a transparent medium. |
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| Auto-ignition temperature | | The lowest temperature required to initiate self-sustained combustion in the absence of a spark or a flame |
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| Automatic burner | | A burner that can operate without manual intervention including start up and shut down processes. |
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| available lime | | An analytical term for the calcium oxide content of quicklime of hydrated lime that is able to react with sucrose solution (available lime test standard) |
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| Axial diffusion flame | | The flame produced by two distinct but coaxial jets of fuel and air injected along the axis of a combustion system |
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| Axial turbulent diffusion flame | | The flame produced by two distinct but coaxial jets of fuel and air injected along the axis of a combustion system |
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