AWJ13: Alstom

Novel Anode Bake Furnace Gas Cleaning

By Bo Herrlander, Alstom

Fig.1 The integrated Alstom AHEX FTC

Introduction In November 2011 Alstom started the novel gas cleaning plant for anode bake furnace at Alcoa Mosjøen, Norway. This Fume Treatment Centre (FTC) comprises a new gas cooling principle replacing the conventional conditioning tower with a heat exchanger. The “AHEX” heat exchanger has the dual purpose of cooling the flue gas and simultaneously works as a reactor for capturing tar and hydrogen fluoride (HF) on alumina. The heat exchanger is integrated into the filter and will thus constitute a very compact FTC design.

The novel Fume Treatment Centre concept Even though improved state-of-theart firing technologies on today’s open anode bake furnaces have significantly reduced the emissions, there is still a need for further gas cleaning to meet regulations. The main furnace emissions are compounds in the flue gas such as PAH (Polycyclic Aromatic Hydrocarbons), HF, SO2 and carbon particulate. HF emissions originate

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from the recycled butts used in the anode production and PAH from the green anode material. There are PAHs known for their carcinogenic, mutagenic and teratogenic properties such as benz[a]anthracene*, chrysene*, benzo[b]fluoranthene*, benzo[j]fluoranthene, benzo[k] fluoranthene*, benzo[a]pyrene*, benzo[ghi]perylene, dibenz(a,h) anthracene* and indeno(1,2,3-cd) pyrene* (*classified by the US EPA as probable human carcinogens). Some of these compounds are subject to emission levels set by government authorities. These are typically expressed as subsets of the various PAH’s such as PAH-16 and OSPAR 11, which includes 16 or 11 different PAH. The abatement of PAH is temperature dependent in such a way that the removal efficiency increases by lowering the flue gas temperature. (see fig.1) This novel FTC with AHEX solves a number of problematic issues related to the traditional conditioning towerfilter combination. The conditioning towers evaporative cooling principle increases the flue gas moisture

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content, typically by some 6 - 7 %, to reach the acceptable operation temperature levels around 100 – 110 oC. However, a drawback of this method is that the increased moisture content may cause corrosion in the conditioning tower and hydrolysis of the bag polyester material in the fabric filter. By contrast the gas cooled in the AHEX avoids this humidity increase and thus prevents the above problems.

In order to prevent these from fouling the AHEX tubes, alumina is injected upstream. Through this measure there is a controlled condensation of tar on the alumina. Since the tar will end up on the alumina there is no need for disposal of hazardous material. Conventional conditioning tower cooling may occasionally see wet bottom and tar rich effluents, and it will inevitably result in tar deposits in the ducts connecting to the filters, from where it must be removed and and safely disposed of as hazardous material. With AHEX no such ducts are required, as AHEX is integrated in the fabric filter.

This novel FTC with AHEX is built on Alstom’s DDS (Distributed Dedicated Scrubber) concept. The DDS is based on the well proven Abart dry scrubbing technology, which features a two stage counter current gas cleaning process.

It is developed for green field as well as for retrofit/modernization projects, applications where space is limited, or where it is desired to keep the resources used for site installation/ erection to aluminium. The DDS invention has been granted a patent in all major aluminium producing countries. The DDS integrates fresh alumina storage and is equipped with an internal alumina handling system powered by one high pressure fan.

Enriched alumina from the DDS is distributed back to the pots via Alstom’s Alfeed system. There is one exhaust fan per filter compartment, which operates on medium voltage (440 V). This allows full flexibility

Fig.2 The Årdal DDS for tuning the DDS for optimal performance. The DDS is supplied in modules, which makes the DDS easy to transport and install. The DDS may be fully shop-manufactured, as the size of a DDS compartment meets road transportation requirements.

Shop fabrication ensures a uniformly high quality of work. Several DDS can be erected simultaneously and independently. The DDS system may integrate a SO2 scrubber on the top, which enables it to comply with more stringent emission limit values. The available reagent choice is between an alkaline solution or seawater.

This DDS/SO2 technology will be in operation in 2013 at a smelter in Europe. (see fig.2) The AHEX is integrated to the DDS, upstream of the filter stage. The hot gas, containing the condensable fumes, is cooled inside multiple water cooled steel tubes in the AHEX and enters the cooling tubes from the top. The fumes are mixed with alumina in the plenum upstream of the tubes inlets.

The hot fumes include condensable tar components, which condense on the alumina surface during the gas

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cooling. Simultaneously HF and to some extent SO2 is adsorbed. Due to the efficient mixing of alumina and gas inside the heat exchanger tubes, more than 95% of the HF and tar is being adsorbed on the alumina inside the tubes. The efficient collection of tar aerosols on the alumina particles reduces the risk for tar depositing on the heat exchanger surfaces. In addition, the injected alumina will clean the surfaces from possible deposits as demonstrated in the earlier trials in the Middle East, which were the basis for this patented design. Control or elimination of heat exchanger surfaces fouling has been the main driver behind Alstom’s development of this new fire tube heat exchanger.

Alstom has long-term experience with fire tube heat exchangers on similar or more difficult flue gases, such as from Fe/Si- and Si-metal furnaces. Over the last 3 years this technology has also been proven for potgas in full-size demonstration units (EHEX, MHEX, IHEX) at Alcoa Mosjøen in Norway.

The adsorption process is enhanced by the even gas/particle distribution, relatively long retention time and short mixing length within the confined

space of the multiple parallel tubes.

The dry process of the novel AHEX FTC, allows the gas to be cooled to temperatures below 105°C, possibly even below 80°C. This allows for further condensation of PAH and improved cleaning efficiency. After leaving the heat exchanger the cooled gas enters directly into the dry scrubber where the main part of the injected alumina is separated into the filter hopper and re-circulated directly back to the heat exchanger inlet. Primary alumina is injected in the filter compartment, and collects on the bags in a final polishing stage, to adsorb any trace components of tar fumes and HF. The re-circulated or spent alumina leaves the system through an overflow device inthe filter hopper to be sent to the pots.

The new AHEX FTC can efficiently handle a larger variation of the flue gas flow than today’s systems. There is no need for gas re-circulation, which is common for the conditioning tower based FTC. The heat energy recovered in the AHEX may be used or disposed into the environment. One example of efficient use of the heat energy is

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Fig.4 AHEX The combined heat exchanger and tar condensation system the use in district heating, another to use it for seawater desalination.

Electricity production is also possible by deploying an Organic Rankine Cycle (ORC) machine. For the AHEX plant at Mosjøen, the hot water will be used for both district heating and for driving an ORC for electricity production. During the cold season, an extension from the plant’s (and thus also the town’s) district heating system to the AHEX is planned.

Validation of the novel AHEX FTC concept The full scale AHEX concept is demonstrated at the existing Alcoa Mosjøen FTC delivered by Alstom.

This includes six filter compartments downstream the conditioning tower.

One compartment is retrofitted with the AHEX heat exchanger. Thus the gas bypasses the existing conditioning tower and flows directly into the top of the heat exchanger and further on to one filter compartment, which operates on gas from the heat exchanger only. This compartment is therefore conveniently benchmarked with the other five compartments running on flue gas from the conditioning tower.

The measurements on the gas from these compartments are references in the full scale validation of the AHEX

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performance. The ingoing water temperature to the AHEX is usually 60°C and the outgoing is 80-90°C. The inlet gas temperature normally varies between 160 to 190°C and the corresponding outlet gas temperature reads 90-100°C. (see fig.3) The heat recovered in the AHEX heats up the 50% glycol water mixture to about 90°C. This fluid flows in a closed loop between the AHEX and the heat delivering heat exchanger. Here it is normally cooled down to about 60°C. The heat flow is calculated from measuring the fluid mass flow and corresponding temperatures in and out of the AHEX, deploying a specific heat value of approximately 3300 KJ/ kgK for the heat transfer fluid. The heat transferred to the fluid is in the range of 0.8 to 1 MW. This indicates a total heat recovery potential of about 5 MW for the complete anode bake plant at Alcoa Mosjøen. (see fig.4) A fifty percent higher gas flow is estimated to flow through the AHEX compartment, compared to the remaining compartments. The reason for the higher gas flow to the AHEX compartment is the lower pressure drop across the AHEX compared to the conditioning tower. The gas flow is estimated within +/-10% accuracy

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assuming a gas specific heat value of about 0,37 Wh/Nm3. This is based on the fact that the heat absorbed in the fluid will be equal to the heat recovered from the gas (neglecting the small heat loss to the environment).

The total gas flow to the remaining compartments is measured in a venturi duct.

To validate that there is no excessive dust deposits on the AHEX tubes, a heat transfer coefficient is calculated from the measured data and divided by a theoretically calculated heat transfer coefficient from the literature.

The stable quota curve indicates that the heat transfer coefficient is not degrading due to e.g. excessive dust deposits. Even if there are some fluctuations in the measured pressure drop, it is evident that the pressure drop is not increasing over time.

This is also verified by several visual inspections of the heat exchanger surfaces. It demonstrates that the heat exchanger is clean and not fouled by tar residue.

Extractive PAH samples from the flue gas were collected with standard methods from the inlet to the conditioning tower, and from the AHEX compartment outlet as well as from the other compartments outlets.

gas will allow for even higher removal efficiency. The AHEX offers recovery of approximately 1 MWth heat per compartment.

Fig.3 The installed full scale demo AHEX FTC at Alcoa Mosjøen Based on these measurements the removal efficiency is calculated.

During the measurements, the outlet gas temperature from the AHEX was set identical to the conditioning tower outlet temperature, to simplify the comparison of the AHEX concept with a conventional filter compartment.

The measured removal efficiency of the AHEX can therefore be considered to be conservative, since more PAH will be collected in an AHEX FTC, should it be operated at the lowest gas temperatures possible. An external laboratory analyzed the samples for the different PAH compounds (Gas chromatography–mass spectrometry method). It is evident that the AHEX compartment has similar or better removal efficiency compared to the reference compartment. Overall the removal efficiency for the PAH16 gas compounds was 18% higher for the AHEX compartment. As the gas flow through the AHEX compartment is in the order of fifty percent higher compared with the reference compartment, the AHEX compartment collects about 70% more PAH (kg/h) than the reference compartment. An ocular inspection of the enriched alumina from the AHEX compartment revealed a much darker color compared with the reference compartment. As the primary alumina

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flow to all of the compartments is identical, it supports the higher collection efficiency. HF emissions from the filter compartments were measured by portable HF analyzer.

It showed that the HF emissions from the AHEX compartment were significantly lower than from the reference compartment.

Conclusion A novel Fume Treatment Centre (FTC) concept has been developed. The core of this concept is the integrated heat exchanger reactor combination, simultaneously cooling the flue gas and adsorbing PAH, condensed tars and HF on alumina. This novel FTC concept is a further development of the Alstom’s DDS (Decentralized Dedicated Scrubber) Technology. This new AHEX concept integrated into the filter eliminates the need for a conditioning tower with water injection.

All the operational challenges related to the conditioning tower (corrosion, tar deposits, bag hydrolysis) are reduced or removed. The fumes flow directly into the filter, without the need for a duct from the conditioning tower to the filter. The AHEX concept allows cooling the gas to below 100°C without the risk of corrosion of the duct and the filter. The improved cooling of the

The concept has been validated on a full scale demo-plant at Alcoa Mosjøen, which has been in operation since November 2011. The performance of the compartment with AHEX has been compared with another compartment running on conditioning tower at the same gas temperature. The emission measurements show that the AHEX has an improved removal efficiency, compared to the compartment with the conditioning tower. This higher efficiency is achieved even though the AHEX compartment handles fifty percent more gas compared with the compartment downstream of the conditioning tower.

The novel AHEX FTC is more compact, compared with a conditioning tower cooled FTC, and allows for improved removal efficiency, reducing emission of carcinogenic tars and gaseous fluorides. It recovers heat which when used reduces the carbon footprint of the smelter. It eliminates handling of carcinogenic residues from tar drop outs in conditioning towers and ducts, and it adds “renewable” energy to the smelter. It secures lower operational and capital costs compared with the conventional conditioning tower cooled FTC. AHEX is flexible, allows integration in existing FTCs and will add many benefits, both to green field and brownfield smelter projects.

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