Emissions in alumina calcination

The main emissions stemming from a calcination plant, which are usually regulated by local and international laws, are: NOx, CO, dust and SO2. Although the CO2 emissions are not directly regulated, there are clear ambitions among producers to reduce these as well. Other emissions from combustion processes include dioxins, furans, PAHs (polyaromatic hydrocarbons), VOCs (volatile organic compounds) and heavy metals however in this article we’ll focus on the main emissions.

There are two types of emission sources: 1) either emissions are a direct result of input into a calciner (be that feed, fuel or other raw materials) which can be called background emissions, or 2) they are generated by processes or conditions in the calciner itself. Fuel NOx, SO2 or Hg emissions would be typical background emissions in this context, whereas thermal NOx, CO or CO2 and dust/particulate emissions are inherent from the reactions occurring in the calciner or the technologies/equipment used in the process. The emissions prevention and reduction strategy may partly or fully depend on the source of the emission.

There are also two possible pollution prevention strategies: 1) pollution prevention, also called pre-control, or 2) add-on emissions control, or post-control. Pollution prevention are techniques that minimize, reduce or prevent creation of emissions at the source. By carefully designing production processes, using non-emission creating or lower emitting substances and implementing conservation techniques, the generation of air pollutants are eliminated as opposed to abating them after they are created. This could, for example, be the selection of low sulfur fuel instead of heavy fuel oil to reduce the SOx emissions. Post-control techniques reduce pollutants after they have been generated by the process. For example, using an electrostatic precipitator to reduce dust emissions to allowable levels or injection of ammonia to break down NOx would be typical examples of add-on emissions control measures.

Nitrogen Oxides (NOx)

NOx emissions are produced by two primary mechanisms during combustion. One is termed “fuel NOx” and is related to the nitrogen content and species of the fuel and the prevailing combustion conditions. The other mechanism is “thermal NOx” and refers to the chemical formation of NO and/or NO2 from N2 and O2, which occurs at temperature exceeding 1400℃.

For European plants, the Emission Limit Value range for NOx is 50 to 150 mg/Nm³ depending on thermal rating. In urban areas, due to the cumulative effect of NOx from vehicle traffic, lower ELVs are often imposed.

Pre-control measures for NOx

NOx emission come from fuel NOx and thermal NOx formation. Fuel NOx emission originate from some of the nitrogen species in the fuel while thermal NOx will generate when flame temperature goes above 1400℃. The controlling NOx emission technique can focus on either fuel nitrogen concentration or flame temperature or both.

In Circulating Fluidized Bed (CFB) combustion the high solids concentration together with external and internal recirculation of solids ensures a vigorous mixing of the fuel with the combustion air, which leads to a very quick dissipation of energy and thus a very homogenous temperature profile and moderate temperatures throughout the reactor. Therefore, the CFB furnace can inherently control thermal NOx generation. Choosing natural gas with low nitrogen content as fuel for the CFB furnace can significantly reduce the fuel NOx emission further.  Some thermal NOx can be generated when a continuously operated pre-heat and pilot burner is used.

Post-control measures for NOx

Selective and non-selective catalytic reduction are technically feasible options to reduce NOx emissions. However, such options should be considered with some caution as the temperature profile in the preheating stages would be affected by injecting NOx reducing agents such as ammonia and/or urea, which may have other detrimental effects for efficiency or product quality.

Another option for post-control is the use of a dry sorbent injected upstream of a filter.  For example, sodium-based products (such as thermally activated sodium bi-carbonate) readily react with acid gases and can therefore reduce the NOx concentration. However, in the presence of SOx there may be competing mechanisms, so a careful dosing of sorbent becomes critical.

Sulfur Oxides (SOx)

Sulfur oxides, primarily SO2 and sulfur trioxide (SO3), are formed whenever any material that contains sulfur is burned. From 95 to 100% of the total sulfur oxides emissions are in the form of SO2.

SOx emissions limits in the EU are 5-400 mg/Nm³, depending on fuel type and thermal rating. SOx emissions are not only environmentally harmful but can also result in costly corrosion problems in the plant. The risk of SO3 dew point related corrosion can to some extent be mitigated by designing the plant for a higher shell temperature at the affected areas and by operating at higher air/fuel ratios to increase the ESP inlet and stack off gas temperature. Both of these options do however significantly impact on the specific fuel energy consumption, thereby driving up the CO2 emissions as well as incurring higher operating costs.

Pre-control measures for SOx

Sulfur dioxide is formed whenever any material that contains sulfur is burned under oxidizing conditions. Therefore, choosing a lower sulfur content fuel is a fundamental way to reduce SOx emissions. As sulfur in natural gas can be removed easily and economically and the elemental sulfur recovered as a by-product can be sold as a raw material, natural gas contains only trace amount of sulfur. Therefore, the best available technique to control SOx emissions is utilizing low sulfur content fuel, natural gas, which directly reduces the SO2 generation without installation and maintenance, and waste disposal costs. However, in some cases the infrastructure may be lacking for using low S natural gas as fuel.

Post-control measures for SOx

There are effective existing technologies for add-on emissions control for SOx removal, such as dry sorbet injection or various scrubbing technologies. For example, injection of activated sodium bicarbonate in the right dosing ratio can reduce SO2/SO3 emissions by up to 98% and above.

Installing a dry sorbent-based system in a calciner would require installation of an additional system downstream of the existing ESP or baghouse to prevent the recirculation of the spent sorbent within the calcination process, which would obviously contaminate the product alumina.  Similarly, the use of a (dry or wet) scrubber would also require installation of a system downstream of the calciner ESP to remove the SOx emissions by spraying water into the gas which causes the SOx to react forming sulfuric acid.

The pro with either of these methods is that they can be highly effective in reducing SOx emissions, but as a downside they do come with a significant investment cost and also require a continuous supply of dosing chemicals and/or disposal of residues.

Particulate Matter (PM)

The characteristics of the particulate exhaust stream affect the choice of the control device. These characteristics include the range of particle sizes, exhaust flow rate, temperature, moisture content, and various chemical properties such as explosiveness, acidity, alkalinity, and flammability. The most commonly used devices for controlling particulate emissions in alumina calciner are electrostatic precipitators (ESPs) or fabric filters (bag houses)

In the EU the particulate matter emission limit ranges from 5 to 30 mg/Nm³ depending on thermal rating and other factors.

Pre-control measures for PM

Pre-control measures to reduce dust emissions are mainly targeted at reducing the number of fine particles entering the final gas cleaning stage (typically and ESP or baghouse) either by improving the feed quality and/or reducing the generation of fines within the calciner.  It is well known that the quality of the hydrate impacts on the resulting particle breakage in the calcination process. There are several chemical and physical contributors, only some of which have been quantified, which can be broadly classified as follows: hydrate morphology, hydrate shrinkage, hydrate size distribution, hydrate strength and impurities.  It is highly likely that many of these parameters are interlinked, for example the impurities are expected to influence the morphology and hence also the particle size distribution. There is also a correlation with operational parameters (such as calcination temperature, feed rate, airflows, etc.) on the observed particle breakage.

Mechanical / design factors also play a major role on the particle breakage in the calciner. Any high velocity impact / collision by particles on other particles or by particles onto internal vessel surfaces increases the probability for fragmentation/breakage.  The impact of mechanical / design factors on particle breakage is outside the scope of this report and is typically evaluated in the design phase and considered in the plant and equipment design.

Operational measures in the calciner that can be taken to reduce particle breakage include: stabilization of process parameters, minimizing velocities by reducing excess air, reducing the calcination temperature, reducing feed rate if feasible, installing or increasing usage of hydrate bypass.

Furthermore, upgradation of internals (such as fluidizing nozzles, gas nozzles or cyclones) can help to reduce the particle breakage and therefore also the PM emissions.

Post-control measures for PM

With few exceptions, almost all of Outotec’s CFB calciners employ ESP (Electrostatic Precipitators) as post control measure for PM capture. ESPs induce an electrostatic charge across parallel plates/electrodes to charge and then remove particulates from a gas stream. Once the particles are collected on the collector plates they must be removed from the plates without re-entraining them into the gas stream. This is usually accomplished by knocking them loose from the plates and forcing the collected layer of particles to slide down into a hopper. An efficiency above 99% is commonplace with modern ESP technology. ESPs work best at higher particulate concentrations (1 to 10 g/m3) and are cost effective for large gas volumes compared for example to bag houses.

Additional benefits with ESPs over baghouses are:

• better collection efficiencies for very small sized particles
• lower pressure drop, and thereby lower specific electrical energy consumption
• lower operating costs, as frequent replacement of bags can be avoided
• less susceptible to fire
• no risk of clogging filter bags

Carbon Monoxide (CO)

Carbon monoxide (CO) gas forms primarily when carbon fuels are not burned completely. The combustion mechanisms resulting in CO emissions are influenced by residence time, temperature and turbulence/mixing as well as the gas composition.  As such, incomplete combustion, and resulting formation of CO, in CFB alumina calciner can therefore be an effect of for example:

• Too low combustion temperature or local cold spots
• Too high combustion temperature (dissociation of CO2), mainly applicable for the preheat burner
• Too low excess air levels (air fuel ratio too low)
• Incomplete mixing of fuel, combustion air and solids or bypasses of “pockets” of not fully combusted fuel
• Incomplete residence time for combustion (particularly in cases where the plant load has been increased above the original design capacity)

In addition, some CO may also form prior to the calcination furnace due to the breakdown / incomplete combustion of organic compounds (such as dewatering chemicals / filtration aids) in the second preheating stage.

CO is highly toxic even at relatively low concentrations and typically regulated (e.g. in the EU) at max 100 mg/Nm³ for industrial plants.  Generally, CO emissions are quite low in CFB alumina calcination plants, due to the over-stoichiometric combustion conditions and relatively low combustion temperatures.

Pre-control measures for CO

The pre-control measures for CO emissions can be divided into: 1) operation and control measures, and 2) maintenance and equipment measures.

Operation and control measures can include:

• Air fuel ratio and/or oxygen level control (both in terms of operating at the optimum level and in terms of operational stability)
• Ratio of primary air to secondary air
• Sufficiently high combustion temperature (but avoid too high temperatures or temperature excursions)
• Furnace dP and temperature distribution (top to bottom and around circumference) to avoid cold spots
• Furnace sealpot fluidization air for optimum solids recirculation
• Stability of operation in general, and in particular feed rate and temperature control. Blowers and air fans also contribute to the stability of the plant.  Can be achieved also in old plants with implementation of advanced process control systems, such as the Outotec Pretium Calciner Optimizer

Maintenance and equipment specific measures:

• Maintenance of oil lances for proper spraying and fuel distribution
• Maintenance of gas lances
• Maintenance and/or replacement of furnace nozzles for optimum primary air distribution
• Maintenance of secondary air nozzles
• Uniform distribution of air at secondary air nozzles
• Tuning and maintenance of preheat burner
• Recycle cyclone efficiency (maintenance and/or upgradation of internals)

Post-control measures for CO

Add-on techniques used in other industries (heat and power generation or incineration plants) such as recirculation of flue gases are not directly feasible for alumina calciners, as the flue gas is part of the heat recovery scheme in the preheating stages.  In CFB alumina calciners the flue gases are directly contacted with the relatively cold solids feed stream in a counter current scheme in order to recover energy by preheating the feed (with the heat carried with the flue gases). Therefore, to consider flue gas recirculation a completely different flow sheet arrangement would be required where at least some of the flue gases are not used for preheating of the feed but instead recirculated back to the furnace.  This would also have far reaching impacts on process control and safety systems design.

Alternatively, either catalytic or thermal after-burners/incinerator (as is known from the waste incineration industries) could be considered to reduce the CO emissions in-situ, but this would off course result in increased specific fuel energy consumption, incur additional pressure drop in the plant etc.

In this context, it should also be noted that several of the more common NOx abatement techniques result in increased CO emissions.

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In the table below, you will find a summary of the pre- and post-treatment concepts applicable to CFB alumina calciners that have been discussed in this article.

It’s important to remember that the stability of plant operation, both in terms of avoiding unforeseen shutdowns / trips, avoiding or reducing excursions of process parameters, and in terms of process control, plays an important role in reducing the emissions. And here, digitalization and advanced process control can help to stabilize operations and can be also used for predictive maintenance. Moreover, digitalization can also provide required benchmarks, indicators and incentives for emissions management beyond just keeping the emissions below the prescribed limit. As an example, Outotec’s Perficiency approach (see the referenced TMS paper mentioned below) expands from the traditional Total Effective Equipment Performance (TEEP) and Overall Equipment Effectiveness (OEE) metrics to evaluate equipment/plant performance by introducing other parameters in the evaluation including environmental aspects. This can provide not only information of a plant’s direct emissions, but also a comparison or benchmark to the plant or technology reference. Thereby these system increases awareness of environmental performance and can motivate the plant operator to improve operation beyond just meeting the standard.

Metso Outotec is a world leader in alumina refinery technologies, with a particular focus on calcination and hydrate filtration, tube digestion (together with Hatch), settler washers, red mud filtration and security filtration.

Sources:

• Control Techniques for Carbon Monoxide Emissions, EPA-450/379-006
• How Digitalization can Further Improve Plant Performance and Product Quality – Outotec Pretium Advisory Tool for Alumina Calcination, Presented at the TMS 2017 Conference and Exhibition
• https://www.solvay.com/en/brands/solvair-solutions