Development of Pre-Turbo Catalyst for Natural Gas Engines

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Distributed power generation is an efficient method for reducing CO2 emissions through the elimination of transmission losses. Co-generation has similar benefits with higher thermal efficiency. Natural gas engines are very popular for these applications. Unfortunately, these engines emit significant levels of methane, which is a greenhouse gas.  Reduction of methane emissions would greatly improve the environment and provide greenhouse gas emissions credits.  The exhaust temperature downstream of the turbocharger in a natural gas engine is typically below 500°C. At these temperatures, methane is difficult to oxidize with current oxidation catalysts. It would be a much better option to install the oxidation catalyst before the turbocharger where temperatures are 100-150°C higher. Pressures upstream of the turbocharger are higher than downstream and also affect catalyst conversion efficiencies.

Misfiring events are common in natural gas engines. During misfiring events, the catalyst will see a sudden increase in hydrocarbon (methane). When this pulse of hydrocarbon hits the catalyst, it will be oxidized and generate a large exotherm which could lead to catalyst failure (mechanical and/or chemical). This issue is critical for a pre-turbo catalyst:

1)  Mechanical failure of the catalyst could lead to catastrophic turbocharger failure, a result of the turbine blades being damaged.

2)  Misfiring with catalyst installed before the turbocharger is more likely to ignite the methane pulse because of the higher temperatures in this location.  High exotherms from ignition could negatively affect catalyst performance.

Through careful catalyst design, one can minimize this risk and this paper will address these issues.

INTRODUCTION

Lean-burn natural gas engines are very popular for applications involving power generation and co-generation. On-site power generation reduces power transmission losses and in the case of co-generation also supplies heat. Unfortunately, due to incomplete combustion within these engines the exhaust contains trace amount of hydrocarbons with up to 1000 ppm of methane. Since methane has a greenhouse warming potential of about 20 to 50 times that of carbon dioxide, its emissions are becoming an area of concern. Although Denmark is the only jurisdiction with methane regulations today, it is likely that other jurisdictions will follow. Regulatory agencies are continually lowering emissions requirements on engines and methane emission standards are the next logical progression.  Reliable cost-effective technology to combat methane emissions is not yet available but is highly desirable.  The most common technology for exhaust gas cleanup is a catalytic converter. In most applications, the converter would be installed downstream of the turbocharger where temperatures are typically below 500°C. These temperatures are too low for achieving significant methane conversions with current oxidation catalyst technology. This paper will discuss a novel approach where the catalyst is installed upstream of the turbocharger where temperatures are 100 – 150°C higher.  However, installing a catalyst upstream of a turbo presents other complications that will be addressed in this paper.  The literature reports many catalyst formulations that can be used for methane reduction [1-16]. Unfortunately there are none that can maintain their performance for more than a few hours of operation. Palladium based catalysts have been shown to give modest conversions (~30%) in the temperature region of typical natural gas exhaust (~400°C) [1,2,6,12,14]. These catalysts show rapid performance deterioration caused by the small amount of sulphur present in the exhaust [1,2,6,7,12-15].

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