Controlling cogeneration facility NOx emissions
The price of natural gas makes it an attractive fuel source for electricity production. While natural gas turbines have often been used as the prime mover, natural gas reciprocating engines are becoming increasingly popular because of their relatively rapid startup times and their relatively high simple-cycle efficiencies. Before pursuing this alternative, emission requirements must be fully understood.
Emission control requirements
The U.S. Environmental Protection Agency (EPA) divides the country into a set of areas, each of which is typically a county or major urban area. Through its National Ambient Air Quality Standards, the EPA looks at six criteria pollutants to ensure they are in compliance with emission levels that have been determined through a balanced stakeholder process. These criteria pollutants are found throughout the country and are distinguished from hazardous air pollutants, which are usually associated with a unique type of processing plant. The criteria pollutants that are associated with natural gas engines are typically ozone, carbon monoxide (CO), and nitrogen dioxide (NO2). NO2 is a component of NOX, which is an important precursor for ozone. Therefore, NOX is often a key parameter to be measured.
Each criteria pollutant has a major source threshold (MST), which is the number of tons per year that can be emitted by a facility if it wishes to avoid the potentially complex Title V process. Title V requires a public consultation process for the project and has a number of procedural and reporting obligations that many projects wish to avoid. It is important to note that many areas have local requirements that may be more severe than the national EPA standards.
The MST for a project is site dependent. If the project site is in an attainment area (i.e., the area currently meets the EPA-required emissions levels for a given criteria pollutant), the MST is higher than if a site is in an extreme nonattainment area. Examples for NOX MST are shown in Table 1.
Emission control constraints, issues
For many sites, the constraining pollutants will be the MSTs for NOX and CO. Typically, natural gas engines have relatively low allowable pressure drops in their exhaust systems. This, coupled with the installation of heat recovery devices in the exhaust stream and the need to silence the exhaust noise, does not leave much available pressure drop for the emission control system. There is an increasing trend to combine the silencer with the emission control device. This combination allows the benefits of exhaust silencing by the catalysts to be taken into account when doing the overall system design from an acoustics standpoint. Another important constraint for many sites is the physical space required for the emission control system. Space allocation for any piece of equipment is expensive.
An ideal emission control system:
- Achieves the regulatory requirements, which for most sites means meeting the NOX and/or CO requirements
- Has low pressure drop, allowing the placement of heat recovery devices in the exhaust stream
- Has low temperature drop so that downstream heat recovery devices still have high levels of heat extraction potential
- Requires relatively little space by integrating silencing and providing flexible installation options
- Is highly cost-effective.
Markham District Energy Inc. (MDE) is a thermal energy utility owned by the City of Markham. The City of Markham is a rapidly growing community on the northeast border of Toronto with a population of approximately 300,000. MDE supplies thermal and electrical energy to a broad range of customers with global operations including Honeywell, Motorola, IBM, a regional hospital, and a number of significant residential and commercial properties. MDE is a recognized innovator and received the International District Energy Association’s highest honor, the 2013 System of the Year Award.
This article focuses on two natural gas cogeneration projects commissioned in 2013 and early 2014. Both projects were implemented under the overall project management of Peter Ronson, vice president at MDE. The natural gas engines for both facilities were supplied by Toromont CAT Power Systems, Concord, Ont. Safety Power Inc. supplied the selective catalytic reduction (SCR) systems as a subcontractor to Toromont for the project. Bur Oak Energy Centre is the larger of the two facilities (see Figure 1). The facility produces hot water, chilled water, steam, and electricity for a major hospital, community center, library, and other buildings located in close proximity. As part of its energy portfolio, the site includes a 4-MW natural gas engine (see Figure 2). The site had to meet stringent NOX limits through an SCR emission control system (see Figure 3). Space was a major constraint, which required the SCR to be mounted vertically. As a result, the SCR for this 4-MW engine occupied less than 50 sq ft of floor space. Because of extensive downstream heat recovery equipment in the exhaust, the SCR had to have a low pressure drop and low heat loss. "We had a complex set of constraints that needed to be met by our emission control system, particularly at the Bur Oak site," said Ronson. "We are pleased with the overall performance of the system."
The second project is the Birchmount Energy Centre (see Figure 4). This site includes a natural gas engine and 3-MW generator. The site also had to meet stringent NOX limits through an SCR emission control system.While this plant has downstream heat recovery, unlike Bur Oak, there is no downstream steam generation, making the pressure drop requirements somewhat less onerous. However, as with Bur Oak, space was at a premium and the SCR system was mounted vertically.
The generators for the two plants provide a combined 7 MW of electricity and 7 MW of thermal energy to the Markham District Energy systems. As previously mentioned, the SCR reactors for both projects were mounted vertically to save space inside the facility. The vertical orientation required some structural modifications and computational fluid dynamics (CFD) modeling to ensure the emission reductions could be obtained within the relatively small available footprint.
The Birchmount system was commissioned in December 2013, and the Bur Oak system was commissioned in January 2014. All of the urea injection equipment and associated controls are housed in a single, relatively small control panel. Safety Power uses a model-based control algorithm that measures engine NOX output and has an exhaust mass flow sensor to measure total moles of NOX that must be eliminated. This is combined with a downstream NOX sensor that corrects for any model inaccuracies. The combination of CFD analysis to make reactor size as small as possible and advanced control technology ensures customer requirements are met.
Bob Stelzer is the chief technical officer for Safety Power Inc., Mississauga, Ont. He leads the engineering team that developed the company’s ecoCUBE family of products, which has been configured for more than 50 engine types from most of the world’s major engine manufacturers. He is a mechanical engineer with a master’s degree in engineering.