Meeting tighter NOx emissions rules

A low temperature oxidation technology uses ozone to remove very low levels of nitrogen oxide from refinery gases

Linde Gases

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Article Summary

Requirements for reducing air pollution emissions have been evolving over the past couple of 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 regional, national and international level – and legislation is going to get increasingly stringent. The global community is working to improve cooperation between emitting sources, monitoring systems – and the legislation they support – in order to reduce the number of serious pollutants being released into the air, soil and water to help mitigate the negative impacts on human health and adverse affects on the environment in coming years.

In 2007, the EU, acknowledging that existing legislation on industrial pollution was complex, sometimes inconsistent and not far reaching enough, adopted new legislation to strengthen the provisions already in force and reduce further industrial emissions. The new directive aims to improve the uptake and implementation of ‘Best Available Technologies’ (BAT), which maximise the use of technology in plant design, build and operation in order to drive down emissions. Critically, it also tightens current minimum emission limit values for large combustion plants and introduces minimum provisions on environmental inspections of installations and incentives for the development and employment of environmentally friendly technologies.

Nitrogen oxides, mainly consisting of nitric oxide (NO) and nitrogen dioxide (NO2), and commonly referred to as NOx, is among one of the major criteria pollutants listed under the Clean Air Act by the US Environmental Protection Agency and is also monitored by several other countries, notably China. NOx deriving from stationary combustion sources makes a major contribution to total emissions and proper control of NOx emissions could result in significant environmental benefits, especially when combusting oil and coal.

In most cases, NOx is a product of the combustion of fossil fuels or industrial processes and contributes to the formation of smog, acid rain and other health hazards. NOx undergoes chemical and photochemical reactions in the atmosphere and reacts with volatile organic compounds (VOCs) in the presence of sunlight to form smog and ground level ozone. The effect is very significant and harmful in the summer months to children and people with lung diseases such as asthma, causing damage to lung tissue and a reduction in lung function. NOx also reacts with ammonia, moisture, and other compounds to form small particles capable of penetrating deeply into sensitive parts of the lungs and causing or worsening respiratory diseases such as emphysema and bronchitis, also aggravating existing heart and lung disease. In addition, NOx interacts with oxygen in the atmosphere to produce the atmospheric pollutant low level (or tropospheric) ozone, which impacts on human health. NOx emissions also contribute to the formation of damaging acid rain, eutrophication and oxygen depletion degrading water quality and harming wildlife and plant life.

High levels of industrial activity and increasing vehicle emissions have elevated ambient NOx and ground level ozone levels in several critical geographies, particularly in the US, Europe and China. In response, environmental authorities are tightening their regulations governing NOx emission management and applying sector specific parameters. However, control of NOx from each source is a complex process affected by factors that include the amount and distribution of air in the combustion process, temperature, unit load and burner design.

The largest output of NOx emissions in stationary sources is from coal fired boilers, especially those in the power generation segment. Other major sources of NOx emissions include kilns and furnaces from the cement, lime, ferrous and non-ferrous metals industries. However, petrochemical processes also produce large amounts of NOx and other airborne pollutants, primarily originating from utility boilers, cogeneration units, process heaters, steam methane reformers, ethylene cracking furnaces and fluid catalytic cracking (FCC) regeneration units.

Consequently, the NOx emission levels specified for this sector are among the lowest in industry, highlighting the need for efficient NOx removal.

While some industry sectors claim there is no effective means to remove NOx from their emissions –or rather, no cost effective means to sustain the economic viability of such an operation – there is a spectrum of conventional and more recently introduced technologies available in the market to address this important obligation.

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% and, for NOx, emissions levels of below 50 mg/MJ can be reached.

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 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% to 50% 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 selective catalytic reduction (SCR), a technology designed to facilitate NOx reduction reactions in an oxidising 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 possess 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 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 selective non-catalytic reduction (SNCR) is also applicable in the high temperature regions impacting the 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% NOx removal.

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

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