Incinerating waste produces exhaust gases laden with diverse pollutants. Different processes are used to remove these pollutants before the purified flue gas leaves the plant safely through the stack.
For decades HZI has been doing research into flue gas treatment and developing new and effective treatment systems. We harness internal synergies to optimally match and combine our combustion and flue gas treatment equipment. The numerous processes that have emerged can be used individually or together to create high-performance integrated systems.
The appropriate flue gas treatment approach is chosen in close consultation with the client. The selection criteria include regulatory air pollution limits, the client’s expectations regarding the energy efficiency of the plant, restrictions on waste water discharge, and the space available on site – to name only a few.
Various pollutants are introduced into the combustion process by the waste, including oxidised combustion products such as SO2and NOxproducts, as well as substances such as hydrogen chloride and hydrogen fluoride, heavy metals, dioxins and furans. We use specific types of treatment to separate these pollutants out during the EfW process and then dispose of them appropriately. Our comprehensive range of processes enables us to cover a broad spectrum of requirements.
Thanks to many years of experience, HZI can now offer a wide range of high-performance processes for reducing emissions. On the basis of the individual requirements and circumstances of each plant, we develop tailored integrated flue gas treatment approaches. When putting together the right overall approach we take account of the following factors:
HZI’s fabric filter is a product developed by Hitachi Zosen Inova to remove solids (dust) from the flue gas. It is a physical separation process where the solids are filtered out at the surface of a gas-permeable fabric.
If the fabric filter is used in a dry sorption (the XeroSorp process) or semi-dry sorption (HZI SemiDry) process, a filter layer of reagents and fly ash forms on the filter bags, where the separation of pollutants takes place.
The Main Advantages of HZI’s Fabric Filters:
In an electrostatic precipitator particles are ionised – in other words electrostatically charged – and then separated out under the influence of an electric field. The electrostatic precipitator, comprising two or three electric fields, is usually placed directly downstream of the boiler.
The Main Advantages of the Electrostatic Filter:
Dry Sorption (XeroSorp)
In HZI’s XeroSorp dry scrubbing process, corrosive acid gases (for example HCl, HF and SO2) are neutralised with the help of adsorption additives.
The term “xero” is a Greek prefix derived from the word “ξηρός” (xērós), meaning “dry”. As the name suggests, the process does not involve the introduction of any water at all. Instead we use two different adsorption additives for neutralisation: hydrated lime and sodium hydrogen carbonate (commonly known as sodium bicarbonate).
Depending on the additive used for neutralisation we call the process:
The main advantages of the XeroSorp process:
Semi-Dry Sorption Process (HZI SemiDry)
The HZI SemiDry system involves a semi-dry sorption process employing the principle of the circulating fluidised bed.
Hydrated lime is injected into the fluidised bed reactor to neutralise corrosive acid gases. The temperature in the reactor is key to the sorption process. To achieve the ideal reaction temperature (typically 145°C), water is sprayed into the reactor simultaneously. In addition to regulating the temperature, the water reactivates the recirculated residues, optimising the separation efficiency of the process.
The Main Advantages of HZI’s SemiDry Process:
Wet scrubbing is the most effective method for removing acid gases from even heavily burdened flue gases. It achieves the lowest emissions, and is also excellent when it comes to removing particulates and aerosols.
Wet scrubbers consist of several stages, each of which fulfils a particular purpose: separating out specific pollutants, saturating flue gases, or extracting heat. Each scrubber stage therefore features specific equipment, for example nozzles, liquid distributors, packed beds, ringjets, droplet separators, etc. Thanks to different forms of intensive contact of the flue gases with water, pollutants are captured and separated extremely thoroughly.
The individual scrubber stages have their own water circuits. An overarching water distribution system ensures that all stages are supplied with sufficient water and that the separated pollutants are separated via the scrubber blowdown. The scrubber blowdown products are treated in a waste water treatment unit before final discharge.
HZI basically offers three different types of wet scrubber:
The Main Advantages of the HZI Wet Scrubber:
In the SNCR (selective non-catalytic reduction) process, nitrous oxides (NOx) react with ammonia and are chemically converted into nitrogen (a natural constituent of air) and water vapour.
For efficient denitrification (DeNOx), it is crucial to operate in the right temperature ranges. If the temperature is too high, the ammonia is “burned”. If the temperature is too low, the reaction slows and a large portion of the ammonia reaches the stack unused. The ideal temperature range is to be found in the post-combustion chamber, although there it is subject to fluctuation because of variations in load and fuel. To prevent this, the reducing agent (ammonia water or urea) is injected into the post-combustion chamber at various points.
But even within the optimum temperature range, the NOx separation rate with a conventional SNCR process is limited. Achieving higher NOx removal efficiency requires injecting more ammonia than is consumed. The excess can be removed downstream in a wet scrubbing process. In our patented process, the ammonia is recovered from the effluent and can be re-injected into the post-combustion chamber.
The Main Advantages of the HZI SNCR Process:
DyNOR® Dynamic NOx Reduction
The DyNOR® system developed by Hitachi Zosen Inova’s engineers is an improved SNCR process. It enables nitrogen oxide to be reduced to very low levels with minimal ammonia slip, something that was previously only possible with an SCR process. The non-catalytic DyNOR® process closes the gap between the costly SCR process and the conventional SNCR process. It is an investment that pays off.
The Main Advantages of the DyNOR® Process:
Selective catalytic reduction (SCR) is the most effective DeNOx process. Nitrous oxides (NOx) are converted into nitrogen and water vapour at a catalytic surface, with ammonia water or urea used as a reducing agent.
Using a catalytic converter allows the reduction of nitrous oxides at significantly lower temperatures, and is considerably faster than the SNCR process. DeNOx rates of more than 90% can be achieved. Other advantages of the process are low consumption of ammonia water (almost stoichiometric) and low ammonia slip. If the catalytic surface is sufficiently large, part of the dioxins and furans are destroyed at the same time.
The SCR process is flexible in terms of operating temperature, and can be deployed at various points along the flue gas treatment process chain. On this basis a distinction is made between the following approaches:
Raw gas catalytic converter:
Clean gas catalytic converter:
Low temperature catalytic converter:
The Main Advantages of the SCR Process:
Adsorption onto Activated Carbon or Lignite Coke
Both nitrogen and dioxins/furans can be adsorbed onto activated carbon or lignite coke and then removed from the flue gas.
Depending on the overall flue gas treatment approach, the activated carbon or lignite coke can be introduced and subsequently ejected at different stages of the process:
Injected into the flue gas duct before the fabric filter and removed at the fabric filter (residue discharge)
The main advantages of adsorption onto activated carbon or lignite coke: