NOx reduction: latest trends and thinking

There is a spectrum of conventional best available techniques and pioneering new technologies available to the power industry to address the important obligation of NOx reduction, writes Stephen Harrison


Measures to cut emissions are an intricate mix of limits, targets and caps

Credit: The Linde Group

Requirements for reducing air pollution emissions have been evolving over the past two decades and today are an intricate mix of limits, targets and caps.

In many parts of the world, industries emitting pollutants must not only comply with rigid emission limits, but also need to provide emissions data to numerous different agencies and bodies in order to comply with disparate legislative formats and reporting systems at the regional, national and international level.

The European Commission’s Industrial Emissions Directive (IED), which took effect over 2014 and 2015, is aimed at standardizing the maximum emission levels across a very broad range of industries, including the power industry, throughout the European Union.

The IED reorganized and coalesced seven existing and overlapping directives related to industrial emissions into a single, clear and coherent legislative instrument

One of the main reasons for the new directive was the inadequate and incoherent implementation of best available techniques (BAT) to optimize all-round environmental performance across the EU.

Additionally, the fact that previous provisions were spread across seven different legal instruments was deemed to place unnecessary administrative burdens on companies, particularly those with operations spanning several countries across the EU.

Many primary industrial sectors in the EU are already well regulated in terms of emissions, but the aim of the IED is to harmonize and standardize how they are regulated and how BAT is utilized across the entire region by setting minimum emissions benchmarks and improving the quality and consistency of implementation.

For companies already operating above and beyond this benchmark, there will be no change required to their operating protocols. For instance, in Sweden and Denmark, where a tax on nitrous oxide (NOx) and sulphur oxide (SOx) is in place, very little additional impact is likely.

The impact of the IED is therefore more likely to be felt in countries like France, the UK and certain member states in Eastern Europe, which have lagged behind more progressive environmental legislation.

The IED not only describes how measuring and monitoring of emissions should take place, but seeks to increase the use of best available techniques through its BAT Reference (BREF) documents.

The BAT approach is aimed at identifying and applying the best technology available worldwide and applying it as cost-effectively as possible on an industrial scale to reduce emissions and achieve a high level of environmental protection. The IED principally covers control of pollution to air and water and focuses on 13 specific pollutants and their compounds to air: NOx; sulphur dioxide (SO2); carbon monoxide (CO); volatile organic compounds (VOCs); metals; dust, including fine particulate matter; asbestos; chlorine (Cl); fluorine (F); arsenic (As); cyanides; substances and mixtures that have been proved to possess carcinogenic or mutagenic properties, or properties that may affect reproduction via the air and polychlorinated dibenzodioxins and polychlorinated dibenzofurans. For many industries, much of the impact for emissions to air will be focused on four pollutants – NOx, SO2, CO and VOCs.

NOx reduction technologies

The LCP directive, one of the seven existing EU directives that now forms part of the IED, requires member states to legislatively limit emissions from combustion plants with a thermal capacity of 50 MW or greater. The directive applies to large thermal plants, many of which are fossil-fuel power stations.

The link between legislation and technology is clear. The roll-out of regional directives like the IED which define BAT will serve to drive the development and raise the profile of new pollution control technologies around the world. The BREF documents which outline BAT will herald new lower Emissions Limit Values (ELV) that will necessitate investment in more advanced pollution control measures.

While some claim there is no cost-effective means to sustain the economic viability of removing NOx from their emissions, there is a spectrum of conventional BAT and also some newer, more pioneering technologies available to the power industry to address the important obligation of NOx reduction.

Some of these technologies include pollution control unit operations such as Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR), which are both outlined in the BREF documents, and innovative, newly introduced approaches such as low temperature oxidation NOx removal technology.

A common approach to controlling NOx emissions is to modify the basic combustion process within the furnace. By using oxygen instead of air in the production process, which removes the nitrogen ballast, energy efficiency is not only increased, but one of the most important benefits is the very significant reduction of both direct and indirect greenhouse gas emissions, including CO2 and NOx. CO2 emissions can be reduced by up to 50 per cent, and NOx emission levels of below 50 mg/MJ can be reached.

The LoTOx process is applied at a controlled temperature zone within the scrubbing system

Source: The Linde Group

However, since emissions vary widely according to changes in temperature and air/fuel mixing, modifications to the combustion process impact not only the emissions, but very frequently also the efficiency and operability of the furnace.

This renders NOx control a technically challenging undertaking that calls for an understanding of complex issues around combustion chemistry and plant operations, as well as the economic issues related to plant fuel consumption and maintenance. NOx reduction by combustion modification is limited, typically in the 30-50 per cent range, and must be implemented where it is effective and applicable without significant de-rating of the combustion furnace. Alternatively, replacement of the existing combustion equipment can be done, but this is obviously capital intensive.

NOx can also be treated post-combustion and the most commonly specified technique for the removal of high levels of NOx is SCR, a technology designed to facilitate NOx reduction reactions in an oxidizing atmosphere. It is called ‘selective’ because it reduces levels of NOx using ammonia as a reductant within a catalyst system.

The reducing agent reacts with NOx to convert the pollutants into nitrogen and water. SCR has been adopted effectively in lowering NOx emissions from gas-fired clean flue gas streams. However, in treating dirty gas streams from industrial processes involving kilns, furnaces and combusting coal or oil with SCR, there is a risk of the catalyst being compromised by chemical poisons in the flue gas, or blinded by the dust and particulate matter also resident in the flue gas.

SCR must be integrated into a high temperature region of the customer’s process, so if it is not included in the original design of the furnace, later installation will require a major rework of the process. The intermediate technology of selective non-catalytic reduction is also applicable in the high temperature regions impacting the client’s process. SNCR does not make use of a catalyst, but requires a highly defined temperature region to provide a reaction with ammonia. This technology is capable of achieving a 50-60 per cent NOx removal.

The effective temperature for reduction of NOx through a SCR catalyst is in the range of 200à‚°C to 400à‚°C – and for SNCR to be effective, the ammonia injection and reduction needs to be in the range of 900à‚°C to 1100à‚°C. Additionally, retrofitting NOx reduction solutions such as SCR or SNCR can often be disruptive of the industrial process and can have negative implications with respect to operations and costs.

At the pioneering spectrum of NOx removal solutions for the power industry is Linde’s LoTOx technology, which stands for low temperature oxidation and has been specifically developed for the control of NOx emissions. LoTOx, which works on ‘dirty’ exhaust gas streams to oxidize and then capture NOx, is a selective, low temperature oxidation technology that uses ozone to oxidize NOx to water soluble and very reactive nitric pentoxide (N2O5). The LoTOx process is applied at a controlled temperature zone within the scrubbing system.

LoTOx does not require additional scrubbers but can leverage those already installed to remove other criteria pollutants such as SOx. Dirty gas means gas with other criteria pollutants, typically particulate matters, SOx and other acid gases. Irrespective of NOx removal, for control of these pollutants, air pollution control devices such as wet and dry scrubbers are required to remove them. Integrating the LoTOx process within such air pollution control devices is relatively simple and truly results in a multi-pollutant removal system.

Inside a wet or dry scrubber, N2O5 forms nitric acid that is subsequently scrubbed by aqueous spray and neutralized by the alkali reagent. The conversion of higher oxides of nitrogen into the aqueous phase in the scrubber is rapid and irreversible, allowing an almost complete removal of NOx, in the region of 90-95 per cent – even as high as 98 per cent – from flue gases. The low operating temperature allows stable and consistent control, regardless of variation in flow, load or NOx content, and acid gases or particulates have no adverse effect on the performance of the LoTOx process.

LoTOx is highly versatile NOx removal process but ideally suited where the removal required is greater than 80 per cent or where stack emissions must be below 20 ppm. LoTOx process offers inherent flexibility in NOx removal. In most gases, simply increasing the amount of ozone injection may meet increasingly stringent regulations or limiting tiered NOx emissions.

Greater flexibility

The benefits of this technology include increased capacity, greater flexibility in the choice of feeds, increased conversion rates and reduced emissions. Since this technology is a post-combustion solution that treats the flue gas at the end of the customer’s process, it does not interfere with the process in any way. The system does not utilize a fixed catalyst bed and does not impact system hydraulics, making it robust and reliable, capable of operating without maintenance for periods of two to three years between refinery shutdowns. It is also able to manage unit upsets without impacting overall reliability and mechanical availability.

The ozone required is produced from oxygen on site in response to the amount of NOx present in the flue gas generated by the combustion process and the final NOx emission required.

As often happens when legislation is updated in a specific country or region, other countries outside its range adopt certain principles as a blueprint or starting point for their own local legislation. This is why many of the changes taking place in environmental legislation in the EU reflect developments in the US, where authorities like the Environmental Protection Agency (EPA) are also striving to level the playing fields across industries.

For example, a trend that is likely to be taken up in the EU in the future is the move towards speciation analysis in the VOC arena. Speciation analysis is defined as the separation and quantification of different chemical forms of a particular element. Until recently, determining total element concentrations was thought to be sufficient for environmental considerations, but now it has been recognized that it is important to understand the toxicological properties of the sample’s various components in order to manage environment risk more accurately.

This trend might lead to a closer focus on substances like benzene or toluene or oxides of nitrogen, since nitric oxide, nitrous oxide, and nitrogen dioxide each play a different role in the ambient air in terms of the pollution they cause.

A chemical additive like ammonia (NH3) is also very likely to come into the measurement species, because it is added to a lot of raw combustion gas processing operations which use SCR or SNCR technology to reduce NOx emissions. Some speculate that the focus in the EU may extend at some future date to regulate the emission of metals such as mercury, as is already the case in the US.

In line with the consistent trend towards lower emissions levels, it is clear that, as in the US, the scope of emissions legislation in Europe will extend ever wider to cover more factories that have not yet been greatly impacted by regulation. Ultimately, it is possible that within the next decade, the impact of EU emissions legislation will impact any operation burning material on an industrial scale.

Advancements in emissions legislation will therefore continue to challenge combustion-based companies as well as industrial gas companies like Linde to be able to supply products that underpin its requirements.

Stephen Harrison is Global Head of Specialty Gases and Specialty Equipment at Linde Group.

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