Oct-2010

A step-by-step approach to managing emissions

A comprehensive review of combustion systems and controls underpins a strategy for emissions reduction

Barney Racine and Brendan Sheehan, Honeywell Process Solutions
William De Los Santos, Callidus Technologies
Jeffrey Rafter, Honeywell ECC Maxon

Article Summary

Nitrogen oxides, or NOx, result when the combustion of a mixture of air and fuel in a combustion engine or device produces temperatures high enough to drive endothermic reactions between atmospheric nitrogen and oxygen in the flame. In areas with large concentrations of industry or heavy traffic, the amount of NOx in the atmosphere can reach significant levels and negatively affect the environment and human health.

In light of these environmental and health hazards, the hydrocarbon processing and petrochemical industries face ever-increasing environmental regulations. Over the past two decades, a variety of measures aimed at mitigating the negative impact of emissions such as NOx have also emerged. These include a number of Federal NOx emission reduction programmes and the Clean Air Act Amendments of 1990, which aim to address these health concerns and ultimately reduce common air pollutants, including overall NOx.

To remain successful, companies must simultaneously meet the rising demands of these emissions regulations while keeping operating costs at a minimum. Striking this balance can be challenging. Various emissions requirements have made the need for timely and accurate emission data collection and reporting more important than ever. And companies must address ageing and inefficient technologies, which can become a drain on resources.

Still, it is possible for companies to address the balancing act of cutting emissions – specifically NOx – across the board while managing the cost of compliance. Choosing a comprehensive, single-source solution for reducing NOx emissions can help lower the total cost of ownership and achieve emission reduction goals. The ideal solution combines new and innovative process technologies that optimise the combustion process – from start to finish – while providing online control and monitoring in real time.

When applied to a process heater during the production of ammonia, for example, these types of solutions can effectively reduce NOx emissions while maintaining an optimal level of system performance. To successfully implement a NOx emissions-limiting solution, you must examine all points in the ammonia production process, including:
- Fuel composition and control of the gas train
- Burner design and management
- Flue gas recirculation
- Air and temperature control.

Beyond these points in the production process, a comprehensive emissions-reduction strategy can include post-combustion solutions, such as selective catalytic reduction (SCR), stack emissions monitoring, and regulatory compliance and reporting.

Fuel gas control
The first step in reducing any process heater’s NOx emissions begins by focusing on the combustion of fuel. As hydrocarbon fuels combust, the high-temperature reaction forms pollutants when the combustion conditions achieve peak flame temperatures or when fuel and air mixtures stray from optimal stoichiometry in any flame zone of a specific burner design. Although the flames are contained inside the heater, controlling the formation of pollutants, such as NOx, carbon monoxide (CO) and sulphur oxide (SOx), actually begins by looking outside the heater at the fuel gas control skid and air control devices.

Most installations consolidate fuel gas instruments and controls to a single location in a fuel gas skid or fuel train. The heater and skid can be fed with a variety of fuels, ranging from commercial-quality natural gas to refinery gas, to process off-gases. The mix of constituents in the fuel gas can dramatically affect heater emissions. Heavier hydrocarbons tend to be more difficult to mix with air and burn more slowly, and this decreased rate of reaction can increase the impact of a NOx formation pathway called prompt NOx. The presence of hydrogen in fuel gases can counter this by accelerating combustion rates and, with the proper burner design and air-to-fuel ratio control, can mitigate reactions that increase NOx.

To reduce emissions from hydrogen or heavier hydrocarbons, you must also understand how to detect the range of fuel composition changes. Active adjustment of fuel and air ratios can combat the formation of emissions by reducing variations in flame temperature and combustion conditions. A variety of methods can help measure fuel composition or quality, including complex gas calorimeters for widely varying fuels and tools to measure simple mixture density for slight changes in hydrocarbon content. Overall, diligent design and control system compensation can enable active air-to-fuel ratio adjustments to minimise peak flame temperatures and provide the correct feed to burners for pollutant control.

Once measured, changes in fuel gas can then lead to adjustments in air-to-fuel ratio with high-resolution control actuators in the fuel skid. Air registers located on the burner “windbox” assembly can enable combustion air control, while dampers in the stack or air piping can control available heater draught. By finely tuning air-to-fuel ratios, burners can operate at optimal conditions and reduce peak flame temperatures and burning speeds.

The ideal actuators combine high-resolution control architecture capable of 0.1-degree positioning accuracy with intelligent positioning feedback. This enables the integration of high-performance valves with advanced control algorithms. You can then apply actuators to fuel control valves, pressure-reducing valves, stack dampers, air control valves and process control valves. Today, many available valves operate on a 24 VDC power supply and a digital communication network. This architecture provides the advantage of distributed intelligence for increased reliability and improved safety through error reporting.