How do you reduce engine emissions?

Louise Smyth

Frank Griffin reports on using selective catalytic reduction to meet Tier 4 Final emissions regulations

The oil and gas industry has entered a phase where efficient, reliable and clean solutions for energy generation are more important than ever. In North America, US Environmental Protection Agency (EPA) 2015 Tier 4 Final non- road regulations govern every engine for mobile off-highway applications. Tier 4 Final regulations reduce particulate matter (PM) by 80% and oxides of nitrogen (NOx) by 45% vs Tier 2 regulations.

Cummins powered the first Tier 4 Final complete fracturing spread at a Pennsylvania airport project. As the first engine manufacturer to certify Tier 4 Final engines for the oil and gas market, the company is committed to providing its customers with the right solution.

There are various technology paths that can be used to meet Tier 4 Final emissions. Not all of these solutions help Cummins address the key concerns of the firm’s customers in the field. These concerns are: that technology must be simple to install; it must work for worldwide business; no major redesign of equipment should be necessary – maintain the same footprint, space claim and heat rejection as prior product; maintain or improve power and performance; improve fuel efficiency; and finally, maintain or improve durability and reliability of the solution.

To best address the core needs of its customers, Cummins has implemented the selective catalytic reduction (SCR) aftertreatment solution for its engines <751hp operating in the oil and gas field. Its broad experience with various aftermarket solutions has demonstrated that SCR aftertreatment is an ideal solution to meet the customer requirements for power, performance, reliability, durability, safety, ease of installation, and operational costs.

SCR is a scalable, modular and maintenance free aftertreatment that reduces NOx emissions present in engine exhaust. In step 1, engine exhaust containing NOx enters the SCR system. In step 2, the integrated ECM-controlled diesel exhaust fluid (DEF) doser injects the precise amount of DEF in a high efficiency spray pattern into the exhaust flowing into the decomposition chamber. In step 3, DEF atomises and quickly decomposes in the hot exhaust gas. The result is ammonia and water vapour. In step 4, ammonia and water vapour initiate a catalytic reaction, in which NOx is reduced to harmless inert gas. In step 5, clean exhaust of nitrogen gas, Co2 and water vapour is emitted.

Cummins engines >751 hp for oil and gas applications do not require a diesel particulate filter (DPF), diesel oxidation catalyst (DOC) or any form of regeneration with the Tier 4 Final SCR solution. On Cummins engines, improved combustion technology allows control of PM in the engine while SCR reduces NOx emissions in the exhaust. This delivers reductions in fuel consumption of 2-5% when compared with Tier 2 products while package size remains comparable. Additionally SCR is a ‘fit and forget’ system, designed to meet the engine’s life-to-overhaul with minimal maintenance requirements. This system enables heat rejection and peak cylinder temperatures to be kept at similar levels to Tier 2. A wastegated turbocharger arrangement enables more precise management of air flow, increasing efficiency and improving load acceptance.

DEF, the reactant used in SCR aftertreatment systems, is an aqueous solution of 32.5% industrial urea in 67.5% pure water. It is nonflammable and non-toxic. As a part of the SCR aftertreatment system, DEF is injected into the exhaust stream ahead of the catalyst. DEF freezes at 12 F (-11°C) and thaws at consistent concentration. DEF consumption varies by duty cycle and operating conditions, but is typically consumed at a rate of 3-5% of diesel fuel. Companies operating in the oil and gas field are becoming familiar with DEF and its use. In North America the trucks used to pull equipment in and out of the field have utilised DEF since 2010.

And many engines providing power to other off highway applications at well sites are also using SCR technology. DEF is readily available from the same suppliers that deliver bulk fuel as well as at well- established pump locations on all major interstate trucking routes.

Tier 4 Final emissions requirements and the strategies for meeting them are new to many in the oil and gas industry. With global on- and off-road engine experience, Cummins has clear emissions technology leadership and experience with the numerous technology paths available to meet increasingly stringent requirements. Its in-house experts evaluated all viable technology paths before identifying SCR as the ideal aftertreatment solution for engines above 751hp. Well over a million SCR systems are in operation today, delivering the performance, reliability and durability required by the oil and gas market.

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Frank Griffin is with Cummins.

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