Commission Implementing Decision (EU) 2017/1442Show full title

8. DESCRIPTION OF TECHNIQUES U.K.

8.1. General techniques U.K.

Technique	Description
Advanced control system	The use of a computer-based automatic system to control the combustion efficiency and support the prevention and/or reduction of emissions. This also includes the use of high-performance monitoring.
Combustion optimisation	Measures taken to maximise the efficiency of energy conversion, e.g. in the furnace/boiler, while minimising emissions (in particular of CO). This is achieved by a combination of techniques including good design of the combustion equipment, optimisation of the temperature (e.g. efficient mixing of the fuel and combustion air) and residence time in the combustion zone, and use of an advanced control system.

8.2. Techniques to increase energy efficiency U.K.

Technique	Description
Advanced control system	See Section 8.1
CHP readiness	The measures taken to allow the later export of a useful quantity of heat to an off-site heat load in a way that will achieve at least a 10 % reduction in primary energy usage compared to the separate generation of the heat and power produced. This includes identifying and retaining access to specific points in the steam system from which steam can be extracted, as well as making sufficient space available to allow the later fitting of items such as pipework, heat exchangers, extra water demineralisation capacity, standby boiler plant and back-pressure turbines. Balance of Plant (BoP) systems and control/instrumentation systems are suitable for upgrade. Later connection of back-pressure turbine(s) is also possible.
Combined cycle	Combination of two or more thermodynamic cycles, e.g. a Brayton cycle (gas turbine/combustion engine) with a Rankine cycle (steam turbine/boiler), to convert heat loss from the flue-gas of the first cycle to useful energy by subsequent cycle(s).
Combustion optimisation	See Section 8.1
Flue-gas condenser	A heat exchanger where water is preheated by the flue-gas before it is heated in the steam condenser. The vapour content in the flue-gas thus condenses as it is cooled by the heating water. The flue-gas condenser is used both to increase the energy efficiency of the combustion unit and to remove pollutants such as dust, SOX, HCl, and HF from the flue-gas.
Process gas management system	A system that enables the iron and steel process gases that can be used as fuels (e.g. blast furnace, coke oven, basic oxygen furnace gases) to be directed to the combustion plants, depending on the availability of these fuels and on the type of combustion plants in an integrated steelworks.
Supercritical steam conditions	The use of a steam circuit, including steam reheating systems, in which steam can reach pressures above 220,6 bar and temperatures of > 540 °C.
Ultra-supercritical steam conditions	The use of a steam circuit, including reheat systems, in which steam can reach pressures above 250–300 bar and temperatures above 580–600 °C.
Wet stack	The design of the stack in order to enable water vapour condensation from the saturated flue-gas and thus to avoid using a flue-gas reheater after the wet FGD.

8.3. Techniques to reduce emissions of NO_X and/or CO to air U.K.

Technique	Description
Advanced control system	See Section 8.1
Air staging	The creation of several combustion zones in the combustion chamber with different oxygen contents for reducing NOX emissions and ensuring optimised combustion. The technique involves a primary combustion zone with substoichiometric firing (i.e. with deficiency of air) and a second reburn combustion zone (running with excess air) to improve combustion. Some old, small boilers may require a capacity reduction to allow the space for air staging.
Combined techniques for NOX and SOX reduction	The use of complex and integrated abatement techniques for combined reduction of NOX, SOX and, often, other pollutants from the flue-gas, e.g. activated carbon and DeSONOX processes. They can be applied either alone or in combination with other primary techniques in coal-fired PC boilers.
Combustion optimisation	See Section 8.1
Dry low-NOX burners (DLN)	Gas turbine burners that include the premixing of the air and fuel before entering the combustion zone. By mixing air and fuel before combustion, a homogeneous temperature distribution and a lower flame temperature are achieved, resulting in lower NOX emissions.
Flue-gas or exhaust-gas recirculation (FGR/EGR)	Recirculation of part of the flue-gas to the combustion chamber to replace part of the fresh combustion air, with the dual effect of cooling the temperature and limiting the O2 content for nitrogen oxidation, thus limiting the NOX generation. It implies the supply of flue-gas from the furnace into the flame to reduce the oxygen content and therefore the temperature of the flame. The use of special burners or other provisions is based on the internal recirculation of combustion gases which cool the root of the flames and reduce the oxygen content in the hottest part of the flames.
Fuel choice	The use of fuel with a low nitrogen content.
Fuel staging	The technique is based on the reduction of the flame temperature or localised hot spots by the creation of several combustion zones in the combustion chamber with different injection levels of fuel and air. The retrofit may be less efficient in smaller plants than in larger plants.
Lean-burn concept and advanced lean-burn concept	The control of the peak flame temperature through lean-burn conditions is the primary combustion approach to limiting NOX formation in gas engines. Lean combustion decreases the fuel to air ratio in the zones where NOX is generated so that the peak flame temperature is less than the stoichiometric adiabatic flame temperature, therefore reducing thermal NOX formation. The optimisation of this concept is called the ‘advanced lean-burn concept’.
Low-NOX burners (LNB)	The technique (including ultra- or advanced low-NOX burners) is based on the principles of reducing peak flame temperatures; boiler burners are designed to delay but improve the combustion and increase the heat transfer (increased emissivity of the flame). The air/fuel mixing reduces the availability of oxygen and reduces the peak flame temperature, thus retarding the conversion of fuel-bound nitrogen to NOX and the formation of thermal NOX, while maintaining high combustion efficiency. It may be associated with a modified design of the furnace combustion chamber. The design of ultra-low-NOX burners (ULNBs) includes cmbustion staging (air/fuel) and firebox gases' recirculation (internal flue-gas recirculation). The performance of the technique may be influenced by the boiler design when retrofitting old plants.
Low-NOX combustion concept in diesel engines	The technique consists of a combination of internal engine modifications, e.g. combustion and fuel injection optimisation (the very late fuel injection timing in combination with early inlet air valve closing), turbocharging or Miller cycle.
Oxidation catalysts	The use of catalysts (that usually contain precious metals such as palladium or platinum) to oxidise carbon monoxide and unburnt hydrocarbons with oxygen to form CO2 and water vapour.
Reduction of the combustion air temperature	The use of combustion air at ambient temperature. The combustion air is not preheated in a regenerative air preheater.
Selective catalytic reduction (SCR)	Selective reduction of nitrogen oxides with ammonia or urea in the presence of a catalyst. The technique is based on the reduction of NOX to nitrogen in a catalytic bed by reaction with ammonia (in general aqueous solution) at an optimum operating temperature of around 300–450 °C. Several layers of catalyst may be applied. A higher NOX reduction is achieved with the use of several catalyst layers. The technique design can be modular, and special catalysts and/or preheating can be used to cope with low loads or with a wide flue-gas temperature window. ‘In-duct’ or ‘slip’ SCR is a technique that combines SNCR with downstream SCR which reduces the ammonia slip from the SNCR unit.
Selective non-catalytic reduction (SNCR)	Selective reduction of nitrogen oxides with ammonia or urea without a catalyst. The technique is based on the reduction of NOX to nitrogen by reaction with ammonia or urea at a high temperature. The operating temperature window is maintained between 800 °C and 1 000 °C for optimal reaction.
Water/steam addition	Water or steam is used as a diluent for reducing the combustion temperature in gas turbines, engines or boilers and thus the thermal NOX formation. It is either premixed with the fuel prior to its combustion (fuel emulsion, humidification or saturation) or directly injected in the combustion chamber (water/steam injection).

8.4. Techniques to reduce emissions of SO_X, HCl and/or HF to air U.K.

Technique	Description
Boiler sorbent injection (in-furnace or in-bed)	The direct injection of a dry sorbent into the combustion chamber, or the addition of magnesium- or calcium-based adsorbents to the bed of a fluidised bed boiler. The surface of the sorbent particles reacts with the SO2 in the flue-gas or in the fluidised bed boiler. It is mostly used in combination with a dust abatement technique.
Circulating fluidised bed (CFB) dry scrubber	Flue-gas from the boiler air preheater enters the CFB absorber at the bottom and flows vertically upwards through a Venturi section where a solid sorbent and water are injected separately into the flue-gas stream. It is mostly used in combination with a dust abatement technique.
Combined techniques for NOX and SOX reduction	See Section 8.3
Duct sorbent injection (DSI)	The injection and dispersion of a dry powder sorbent in the flue-gas stream. The sorbent (e.g. sodium carbonate, sodium bicarbonate, hydrated lime) reacts with acid gases (e.g. the gaseous sulphur species and HCl) to form a solid which is removed with dust abatement techniques (bag filter or electrostatic precipitator). DSI is mostly used in combination with a bag filter.
Flue-gas condenser	See Section 8.2
Fuel choice	The use of a fuel with a low sulphur, chlorine and/or fluorine content
Process gas management system	See Section 8.2
Seawater FGD	A specific non-regenerative type of wet scrubbing using the natural alkalinity of the seawater to absorb the acidic compounds in the flue-gas. Generally requires an upstream abatement of dust.
Spray dry absorber (SDA)	A suspension/solution of an alkaline reagent is introduced and dispersed in the flue-gas stream. The material reacts with the gaseous sulphur species to form a solid which is removed with dust abatement techniques (bag filter or electrostatic precipitator). SDA is mostly used in combination with a bag filter.
Wet flue-gas desulphurisation (wet FGD)	Technique or combination of scrubbing techniques by which sulphur oxides are removed from flue-gases through various processes generally involving an alkaline sorbent for capturing gaseous SO2 and transforming it into solids. In the wet scrubbing process, gaseous compounds are dissolved in a suitable liquid (water or alkaline solution). Simultaneous removal of solid and gaseous compounds may be achieved. Downstream of the wet scrubber, the flue-gases are saturated with water and separation of the droplets is required before discharging the flue-gases. The liquid resulting from the wet scrubbing is sent to a waste water treatment plant and the insoluble matter is collected by sedimentation or filtration.
Wet scrubbing	Use of a liquid, typically water or an aqueous solution, to capture the acidic compounds from the flue-gas by absorption.

8.5. Techniques to reduce emissions to air of dust, metals including mercury, and/or PCDD/F U.K.

Technique	Description
Bag filter	Bag or fabric filters are constructed from porous woven or felted fabric through which gases are passed to remove particles. The use of a bag filter requires the selection of a fabric suitable for the characteristics of the flue-gas and the maximum operating temperature.
Boiler sorbent injection (in-furnace or in-bed)	See general description in Section 8.4. There are co-benefits in the form of dust and metal emissions reduction.
Carbon sorbent (e.g. activated carbon or halogenated activated carbon) injection in the flue-gas	Mercury and/or PCDD/F adsorption by carbon sorbents, such as (halogenated) activated carbon, with or without chemical treatment. The sorbent injection system can be enhanced by the addition of a supplementary bag filter.
Dry or semi-dry FGD system	See general description of each technique (i.e. spray dry absorber (SDA), duct sorben injection (DSI), circulating fluidised bed (CFB) dry scrubber) in Section 8.4. There are co-benefits in the form of dust and metal emissions reduction.
Electrostatic precipitator (ESP)	Electrostatic precipitators operate such that particles are charged and separated under the influence of an electrical field. Electrostatic precipitators are capable of operating under a wide range of conditions. The abatement efficiency typically depends on the number of fields, the residence time (size), catalyst properties, and upstream particle removal devices. ESPs generally include between two and five fields. The most modern (high-performance) ESPs have up to seven fields.
Fuel choice	The use of a fuel with a low ash or metals (e.g. mercury) content.
Multicyclones	Set of dust control systems, based on centrifugal force, whereby particles are separated from the carrier gas, assembled in one or several enclosures.
Use of halogenated additives in the fuel or injected in the furnace	Addition of halogen compounds (e.g. brominated additives) into the furnace to oxidise elemental mercury into soluble or particulate species, thereby enhancing mercury removal in downstream abatement systems.
Wet flue-gas desulphurisation (wet FGD)	See general description in Section 8.4. There are co-benefits in the form of dust and metals emission reduction.

8.6. Techniques to reduce emissions to water U.K.

Technique	Description
Adsorption on activated carbon	The retention of soluble pollutants on the surface of solid, highly porous particles (the adsorbent). Activated carbon is typically used for the adsorption of organic compounds and mercury.
Aerobic biological treatment	The biological oxidation of dissolved organic pollutants with oxygen using the metabolism of microorganisms. In the presence of dissolved oxygen — injected as air or pure oxygen — the organic components are mineralised into carbon dioxide and water or are transformed into other metabolites and biomass. Under certain conditions, aerobic nitrification also takes place whereby microorganisms oxidise ammonium (NH4 +) to the intermediate nitrite (NO2 –), which is then further oxidised to nitrate (NO3 –).
Anoxic/anaerobic biological treatment	The biological reduction of pollutants using the metabolism of microorganisms (e.g. nitrate (NO3 –) is reduced to elemental gaseous nitrogen, oxidised species of mercury are reduced to elemental mercury). The anoxic/anaerobic treatment of waste water from the use of wet abatement systems is typically carried out in fixed-film bioreactors using activated carbon as a carrier. The anoxic/anaerobic biological treatment for the removal of mercury is applied in combination with other techniques.
Coagulation and flocculation	Coagulation and flocculation are used to separate suspended solids from waste water and are often carried out in successive steps. Coagulation is carried out by adding coagulants with charges opposite to those of the suspended solids. Flocculation is carried out by adding polymers, so that collisions of microfloc particles cause them to bond thereby producing larger flocs.
Crystallisation	The removal of ionic pollutants from waste water by crystallising them on a seed material such as sand or minerals, in a fluidised bed process
Filtration	The separation of solids from waste water by passing it through a porous medium. It includes different types of techniques, e.g. sand filtration, microfiltration and ultrafiltration.
Flotation	The separation of solid or liquid particles from waste water by attaching them to fine gas bubbles, usually air. The buoyant particles accumulate at the water surface and are collected with skimmers.
Ion exchange	The retention of ionic pollutants from waste water and their replacement by more acceptable ions using an ion exchange resin. The pollutants are temporarily retained and afterwards released into a regeneration or backwashing liquid.
Neutralisation	The adjustment of the pH of the waste water to the neutral pH level (approximately 7) by adding chemicals. Sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH)2) is generally used to increase the pH whereas sulphuric acid (H2SO4), hydrochloric acid (HCl) or carbon dioxide (CO2) is generally used to decrease the pH. The precipitation of some pollutants may occur during neutralisation.
Oil-water separation	The removal of free oil from waste water by gravity separation using devices such as the American Petroleum Institute separator, a corrugated plate interceptor, or a parallel plate interceptor. Oil-water separation is normally followed by flotation, supported by coagulation/flocculation. In some cases, emulsion breaking may be needed prior to oil-water separation.
Oxidation	The conversion of pollutants by chemical oxidising agents to similar compounds that are less hazardous and/or easier to abate. In the case of waste water from the use of wet abatement systems, air may be used to oxidise sulphite (SO3 2–) to sulphate (SO4 2–).
Precipitation	The conversion of dissolved pollutants into insoluble compounds by adding chemical precipitants. The solid precipitates formed are subsequently separated by sedimentation, flotation or filtration. Typical chemicals used for metal precipitation are lime, dolomite, sodium hydroxide, sodium carbonate, sodium sulphide and organosulphides. Calcium salts (other than lime) are used to precipitate sulphate or fluoride.
Sedimentation	The separation of suspended solids by gravitational settling.
Stripping	The removal of purgeable pollutants (e.g. ammonia) from waste water by contact with a high flow of a gas current in order to transfer them to the gas phase. The pollutants are removed from the stripping gas in a downstream treatment and may potentially be reused.