Semi-dry Circoclean® Flue Gas Cleaning
Our semi-dry Circoclean® process is a well-proven, robust, wastewater-free process for the removal of various pollutants such as SO2, SO3, HCl, HF, dioxins, and furans as well as heavy metals such as mercury from flue gas.
The process can be used for plants burning biomass, refuse derived fuels (RDF) or domestic and industrial waste, in the industrial sector and downstream of coal- and oil-fired boilers. Furthermore, this process replaces a spray dryer for the treatment of wastewater from other flue gas cleaning stages.
Process
Before entering the circulating fluidised bed reactor (Circoclean® reactor), the absorbents are injected into the flue gas. Hydrated lime (Ca(OH)2) and activated carbon (AC) are usually used.
The flue gas flows through the reactor from below, causing the fly ash contained in the raw gas and the additives to fluidise and form a fluidised bed. Acid gases, dioxins, furans, and heavy metals are separated in this fluidised bed. Due to an intensive mass transfer and a high velocity in the circulating fluidised bed, a high separation efficiency is achieved. The optimum flue gas temperature and moisture content for the operating conditions are set by the additional injection of water into the Circoclean® reactor.
The gas then enters the fabric filter. After the separation of solid particles in the filter, a large part of the additives is returned to the Circoclean® reactor to achieve efficient use of the absorbents.
Low investment, operating and maintenance costs
Our Circoclean® flue gas cleaning process benefits from an optimised design that significantly reduces the required footprint. The design eliminates the use of high wear rotating parts, which not only reduces investment costs but also maintenance expenditures. This design ensures a high degree of plant availability. The hydrated lime used as an absorbent can be obtained on site from burnt lime to further reduce operating costs.
The advantages of our semi-dry Circoclean® flue gas cleaning at a glance:
Dry FER-DI® flue gas cleaning
As a simple and cost-effective alternative for applications with moderate concentrations of pollutants, we offer the dry FER-DI® process (Flexible Economic Reagent Direct Injection). Acid gases such as SOx, HCl, HF as well as dioxins and furans and heavy metals such as mercury are reliably removed.
Installations are possible in plants for the thermal treatment of waste, in biomass incineration plants, in the industrial sector and downstream of coal- and oil-fired boilers.
Process
Depending on your individual requirements, we use either sodium bicarbonate (NaHCO3) or hydrated lime (Ca(OH)2) as absorbent in combination with activated carbon (AC).
The absorbent is added directly to the hot flue gas at the outlet of the boiler. The pollutants contained in the raw gas react with the additives and are then separated in a downstream particle filter system (fabric filter). A large proportion of the solid particles separated in the fabric filter are returned to the duct area in order to make optimum use of the separation potential of the absorbent used. A part is withdrawn from the process for disposal.
Reduced costs
The FER-DI® process benefits from an even more compact plant design with minimal space requirements, which also reduces your investment costs. At the same time, the simple concept enables minimal maintenance and operating costs.
The advantages of our dry FER-DI® flue gas cleaning at a glance:
Solid particle removal systems are an essential element of modern flue gas cleaning and are critical for securing compliance with applicable emission regulations. Depending on specific project requirements, we offer state-of-the-art fabric filters or electrostatic precipitators for your process.
Fabric filter
Our fabric filters are used to separate the finest particulate pollutants. In addition, they act as a fine filter stage for gaseous acidic substances such as SOx, HCl and HF as well as heavy metals, dioxins, and furans in conjunction with our Circoclean® and FER-DI® system. Clean gas values of 5 mg/Nm³ particle concentration and also significantly below can be achieved permanently.
Applications are possible in the field of waste, sewage sludge and biomass incineration, downstream of boilers fired with refuse derived fuels (RDF), coal or oil, as well as in the industrial sector. Low dust emissions are reliably maintained even with changing fuel qualities and load conditions.
The fabric filters are usually designed with several chambers so that the plant components can be easily separated from the flue gas flow for inspection and maintenance purposes. The particle-laden flue gas enters the filter chamber from below via the raw gas inlet. The low inflow velocity promotes the pre-separation of coarse particles and ensures uniform flow distribution. The gas flows through the filter hoses, in which the solid particles stick, from the outside to the inside. The filter cake that forms on the hoses is removed by means of compressed air cleaning, falls into the filter funnels, and can be discharged.
We have a reference list of more than 50 fabric filters installed in different types of plants worldwide.
Low-pressure pulse jet (LPPJ) fabric filter
For installations of a certain minimum size, we usually use our low-pressure pulse-jet (LPPJ) fabric filter.
The main advantage of this type of filter is that the low purge air supply pressure is less than 1 bar (g), as opposed to 2 to 7 bar (g) for conventional systems, which means that significantly less energy is required for the cleaning process. In addition, the required compressed air can be generated with a simple rotary blower.
High-pressure pulse jet (HPPJ) fabric filter
For smaller flue gas volumes, we typically use our high-pressure pulse-jet (HPPJ) fabric filter which in this case can be constructed more cost-effectively from small chambers.
Electrostatic precipitator
With our proven electrostatic precipitators, we can remove particulate matter from the flue gas and achieve clean gas values below 8 mg/Nm³. Applications are possible in sewage sludge, waste and biomass incineration plants, downstream of steam generators fired with refuse derived fuels (RDF), coal or oil, and in the industrial sector.
In mono-sludge incineration plants, electrostatic precipitators usually form the first stage of flue gas cleaning downstream of the boiler. The pre-separation of particulate matter in the electrostatic precipitator ensures low-pollutant ash, which serves as the basis for the phosphorus recovery required by law.
After the flue gas enters the electrostatic precipitator, dust particles are negatively charged using spray electrodes (cathodes). The particles pass through a strong electric field where they are attracted by positively charged collecting electrodes (anodes) and stick. Periodic tapping cleans the precipitation electrodes, causing attracted dust to fall into the ash hoppers of the electrostatic precipitator. The gas cleaned of dust particles leaves the electrostatic precipitator via the clean gas hood.
We have over 100 electrostatic precipitator references worldwide.
We have decades of experience in the design, construction, and optimisation of wet scrubbers. To date, over 200 wet flue gas cleaning systems based on Doosan Lentjes technology have been installed in various plants around the world.
Wet scrubbers can be applied in sewage sludge incineration plants, in power stations as well as in industrial facilities and plants for the thermal treatment of waste. Due to the optimal utilisation of additives, wet scrubbers are used when operating costs for additives and the amount of residual material are to be kept low. If lime-based additives are used (e.g. limestone or hydrated lime), the valuable material gypsum can be produced instead of residual material to be landfilled. Furthermore, wet scrubbers can be used as the second stage of a flue gas cleaning system if the aim is to achieve particularly low emission values.
Depending on individual project requirements and goals, we offer acidic and alkaline systems. If the pollutant load in the wastewater needs to be separated, acidic and alkaline wet scrubbers are designed as separate cleaning stages. This is done either in a sequential design with separate scrubbing towers or in a combined scrubber design. The combined scrubber provides for integration of the acid scrubber into the alkaline one, with both systems separated by a separating floor. This design reduces both space and resource requirements and optimises investment and operating costs.
If the separation of the pollutant load is not necessary, the toxic substances (SOx, HCl, HF, ammonia (NH3) and mercury (Hg)) can in principle be separated in an alkaline system.
Acidic wet scrubbers
Our acid wet scrubber systems are primarily used to absorb the pollutant components hydrogen chloride (HCl), hydrogen fluoride (HF), ammonia (NH3) and mercury (Hg) from the flue gas. An acid scrubber is usually operated in combination with a separate alkaline SOx scrubber. More cost-efficient is the Doosan Lentjes option of integrating both scrubber stages in one scrubbing tower. Here, the flue gas enters the acid scrubber above the absorber sump. It flows upwards through the absorption zone in a counterflow procedure before entering the alkaline stage through a separating floor.
The flue gas quenching takes place in the acidic stage. The missing amount of liquid can be compensated by blowdown water from the alkaline stage. Since the scrubbing solution used contains recirculated absorbent, the acidic stage usually does not require additional sorbent if neutralisation of the effluent takes place externally.
To increase process efficiency, a fluidised bed generator – a so-called tray – is installed below the nozzle level. A fluidised bed forms on the tray as an additional absorption zone, which intensifies the contact between flue gas and wash suspension. Our patented technology with a variable tray allows the geometry to be adjusted during operation, thus ensuring optimum separation performance across all load ranges.
For additional separation of mercury (Hg), precipitant is dosed into the absorber sump, which chemically binds Hg.
Alkaline wet scrubbers
If the alkaline scrubber is preceded by an acidic treatment stage, it mainly absorbs sulphur dioxide (SO2) and sulphur trioxide (SO3) and partially absorbs the remaining pollutants. If your project does not require separation of the pollutant load in the wastewater, the alkaline wet scrubber can be used as a stand-alone solution to absorb all toxic substances contained in the flue gas (SO2, SO3, HCl, HF, ammonia (NH3) and mercury (Hg)).
The flue gas enters the alkaline scrubber from below. The absorbent is distributed in the flue gas via nozzle levels using the counterflow principle. Depending on the requirements and project conditions, limestone, milk of lime, caustic soda, or seawater, for example, can be used as a sorbent. If lime-based additive is used (e.g. limestone or hydrated lime), the valuable material gypsum can be produced instead of residual materials to be landfilled.
The washing solution used is recirculated by means of pumps in order to optimise the use of the absorbent. In the process, a reserve spray level with stand-by pump enables high plant availability.
For the denitrification of flue gases – i.e. the separation of NOx emissions – we offer you customised solutions depending on the required emission limits. Our portfolio includes systems integrated into the combustion process (primary measures for nitrogen oxide reduction) and separate (secondary) applications.
Selective Non-Catalytic Reduction (SNCR)
The selective non-catalytic reduction (SNCR) process separates NOx emissions by injecting a reagent into the first pass of the boiler. The reducing agent can be either ammonia water or urea (NH2CONH2), which reacts with the nitrogen oxides (NO, NO2) to form nitrogen (N2) and water.
The SNCR process reduces the NOx¬ emissions in the flue gas to values in the upper range of the BREF requirements.
Selective Catalytic Reduction (SCR)
If your project requires compliance with stricter NOx emission limits, we can offer a separate selective catalytic reduction (SCR) system to help you achieve NOx limits in the lower range of the BREF documents.
The SCR system essentially consists of the reducing agent injection, a metering and mixing section, and the catalytic reactor, which, depending on the requirements, is equipped with several honeycomb catalyst layers. The layers are made up of individual catalyst modules.
Before the flue gas enters the upper section of the SCR reactor, the reducing agent (either NH3 produced from urea or ammonia water) is finely atomised and evaporated in the flue gas channel. To optimise the reaction conditions, a static mixer is installed downstream of the ammonia water injection. This improves the mixing of the reducing agent with the hot flue gas. The amount of ammonia water to be injected depends on the quantity and NOx content of the flue gas.
While ensuring a low NH3 slip, the NOx emissions are separated in the reactor. Selective reduction at the catalyst produces nitrogen (N2) and water vapour (H2O) from the nitrogen oxides (NO and NO2) by adding ammonia water (NH4OH). To control the intensity of the reactions, the catalyst used is specially tailored to the requirements of your process in terms of its chemical, physical, and geometric properties.
Depending on the project-specific conditions, the SCR can be designed as a High-Dust, Low-Dust or Low-Temperature application. The High-Dust SCR is integrated directly into the boiler passes, where the flue gases still have the temperature of about 300-400 C° required for the catalytic reaction. In the Low-Dust variant, the SCR is installed downstream of the flue gas cleaning system. Pollutants such as SOx or dust are already removed from the gas, which has a positive effect on the service lifetime of the catalytic converter. In order to reach the temperature required for denitrification, the flue gas is (re)heated e.g. with a gas/gas heat exchanger, steam heat exchanger or gas burner. The design of a Low-Temperature application also provides for the integration of the SCR downstream of the flue gas cleaning system, but without the installation of heat exchangers.
Heat recovery
For optimised heat recovery, both gas-gas and gas-liquid heat exchangers are integrated, depending on the requirements. This offers the possibility of increasing efficiency through combustion air preheating, feedwater preheating or district heat extraction.
Flue gas condensation
The extraction of district heat can be further increased by flue gas condensation. In this process, the flue gas is cooled well below the dew point and the resulting condensation heat is transferred to the cooling circuit. This can be done in a separate heat exchanger in the clean gas or integrated into a wet scrubber.