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Unifrax have been supplying PUREFRAX® filter elements in to successful hot gas filtration applications for 20 years. The below article describes a recent case study and outlines how we have built on the success of these products to progress in to new application fields, as well as develop 3 metre length PUREFRAX® filters and the associated extra benefits such a candle can provide.

In 2013, emissions monitoring was undertaken on a gasification system processing Municipal Solid Waste (MSW). Gasification is a well-established process dating from the early 1800s when it was first used to produce town gas from Coal. The gasification system here is integrated with a high temperature air filter, using 1.25 metre long (60mm OD) PUREFRAX® filter candles with a Sodium Bicarbonate (NaHCO3) dry sorbent injection system. The gasification of the MSW produces a combustible synthetic gas (‘syngas’ – typically containing a mix of CO, CO2, H2, CH4 & H2O) which is subsequently combusted in a combustion chamber. The hot exhaust gases from the process are extracted via an induced draft (ID) fan. These are cooled from ≈900oC to >400oC at the filter inlet. By maintaining the temperature at above 400oC, the de-novo synthesis of Dioxins/Furans is prevented. The de-novo synthesis mechanism is the main one by which Dioxins form in the flue gases and involves the formation of Dioxins from organic and inorganic compounds (known as ‘precursors’) at temperatures between 200-400oC (known as the de-novo temperature ‘window’). The filter operates at temperatures above 400oC and by removing these precursors (such as Carbon in fly ash and metal chlorides) as particulate at such temperatures, the main mechanism by which Dioxins are formed is prevented. Dioxins and Furans are poly-chlorinated compounds and it has been shown that major reduction of Dioxin/Furan formation can be brought about with upstream sorbent injection for Hydrogen Chloride (HCl). The filter incorporates Sodium Bicarbonate injection in order to neutralise acid gases such as HCl, HF & SO2. Figure 1 provides a diagrammatic representation of the process.

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Figure 1: Process Diagram

As can be seen from figure 1, the combustion gases are cooled through simple radiant heat loss through a suitable length of ducting to achieve the required temperature loss from ≈900oC to >400oC at the filter inlet. However, at other facilities operating the same gasification and Unifrax PUREFRAX® filtration technology principle, this temperature loss is achieved by passing the gases through a waste heat boiler in order to generate steam which is fed to a steam turbine in order to generate electricity or the steam is passed through a heat exchanger to heat water which can then be pumped to any available district heating networks. Typically, a tapping on the Steam Turbine Generator is made in order to provide steam at a suitable pressure for such a purpose – these systems known as CHP (Combined Heat and Power) systems. The interest and development of these systems in the UK is increasing substantially, particularly as gasification technologies can receive government support under the Renewables Obligation Directive through the receipt of Renewable Obligation Credits.

In order to maximise the energy recovery in such systems, the combustion gases are cooled to lower temperatures (≈200oC) and as such will have passed through the de-novo temperature ‘window’ as described above. As such, Powdered Activated Carbon (PAC) is added along with the Sodium Bicarbonate (SB) in order to adsorb Dioxins/Furans. By injecting directly in to the filter, the sorbents create a fully reactive coating on the surface of the filter candles thus maximising contact time between the acid gases and the sorbent particles due to the necessity of the incoming gases to pass through the filter cake that contains the sorbent reagent. The injection of the sorbent as a ‘secondary dust’ also aids this filtration as it enables the particulate to agglomerate. This allows sub-micron particulate to be more easily filtered. Unifrax ceramic filters are almost 100% efficient and allow particulate down to 0.1 micron dia. to be filtered.

Typically, through waste composition analysis i.e. through knowledge of waste Chlorine%, Fluorine% and Sulphur% – estimates of the HCl, HF and SO2 concentrations can be calculated and when coupled with the required Emission Limit Values (ELVs) the required Sodium Bicarbonate additions can be determined. This is further enhanced through pre-abatement flue gas sampling for HCl, HF & SO2. Addition rates are based on the following reactions given below (and a stoichiometric of 1 – 1.5) –

NaHCO3 + HCl -> NaCl + H2O + CO2

2NaHCO3 + SO2 + 1/2 O2 -> Na2SO4 + 2CO2 + H2O

NaHCO3 + HF —–> NaF + H2O + CO2

Furthermore, on many systems – the addition rates are linked to continuous emission monitors (CEMs) that measure the acid gas concentrations and control the addition rates accordingly – thus maximizing efficient use of the sorbent (and thus enabling a financial saving benefit) and optimizing the ability of the system to meet the required ELVs. For example, the system mentioned here has the sorbent injection system coupled controlled to an HCl analyser located in the stack.

Recent emissions testing from the above facility showed that the ELVs for ALL pollutants regulated under the Industrial Emissions Directive (Chapter IV and Annex VI) were being met and therefore demonstrated the efficiency of the filtration system employing Unifrax filtration technology.

Unifrax are now involved in the development of filtration systems incorporating the PUREFRAX® filter candles to filter the syngas rather than the combustion gases as is the case above. The syngas can be cleaned using wet/chemical scrubbers in order to remove particulate, HCl, HF, H2S and NH3. However, waste water treatment from the scrubbers can be very expensive – especially with regards to the removal of metals. By installing a PUREFRAX® filter prior to the wet scrubber, particulate (and thus metals in particulate form) can be removed beforehand thus reducing this expense. Acid gases such as HCl and HF can also be removed using a dry sorbent system as described above – also reducing the expense of having to remove these at the wet scrubber which can now be used to scrub H2S and NH3. By producing a clean syngas, the syngas can be fed to a gas engine or a gas turbine thus increasing electricity production efficiency significantly when compared to the waste heat boiler utilizing combustion gases and the steam turbine route as described previously. These systems are being developed, particularly with the new 3 metre candles that Unifrax have developed in mind.

Space constraints are often a real concern for such developments, particularly because of the size and amount of equipment involved. Therefore, filtration systems employing 3 metre PUREFRAX® candles can significantly reduce the required footprint when compared to systems employing the shorter/smaller diameter candles. Ensuring the equivalent surface area also enables the cleaning systems to be much simplified e.g. reducing the number of reverse jet ‘blast’ valves etc.

Unifrax have been at the forefront of hot gas filtration for 20+ years and it is evident that our success has enabled us to help our customers achieve their objectives – not just in terms of being able to easily meet their environmental responsibilities but also in terms of providing them with equipment capable of allowing them to develop their own technologies to achieve greater financial reward. For more information regarding Unifrax PUREFRAX® filter candles contact your local Unifrax sales office