Article pubs.acs.org/est
Selective Catalytic Reduction Operation with Heavy Fuel Oil: NOx, NH3, and Particle Emissions Kati Lehtoranta,* Hannu Vesala, Paï vi Koponen, and Satu Korhonen‡ †
VTT Technical Research Centre of Finland, Post Office Box 1000, FI-02044 VTT Espoo, Finland Wärtsilä Finland Oy, Post Office Box 196, FI-00531 Helsinki, Finland
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ABSTRACT: To meet stringent NOx emission limits, selective catalytic reduction (SCR) is increasingly utilized in ships, likely also in combination with low-priced higher sulfur level fuels. In this study, the performance of SCR was studied by utilizing NOx, NH3, and particle measurements. Urea decomposition was studied with ammonia and isocyanic acid measurements and was found to be more effective with heavy fuel oil (HFO) than with light fuel oil. This is suggested to be explained by the metals found in HFO contributing to metal oxide particles catalyzing the hydrolysis reaction prior to SCR. At the exhaust temperature of 340 °C NOx reduction was 85−90%, while at lower temperatures the efficiency decreased. By increasing the catalyst loading, the low temperature behavior of the SCR was enhanced. The drawback of this, however, was the tendency of particle emissions (sulfate) to increase at higher temperatures with higher loaded catalysts. The particle size distribution results showed high amounts of nanoparticles (in 25−30 nm size), the formation of which SCR either increased or decreased. The findings of this work provide a better understanding of the usage of SCR in combination with a higher sulfur level fuel and also of ship particle emissions, which are a growing concern.
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INTRODUCTION Ship engine emissions influence the chemical composition of the atmosphere and climate and air quality regionally and also have an impact on human health.1 Around 15% of global anthropogenic NOx and 5−8% of global SOx emissions are attributable to shipping.1−3 Since shipping routes near the coastlines, within 400 km of land, produce nearly 70% of ship emissions, shipping can significantly affect the air quality near large populations in coastal areas. Corbett et al.2 estimated that shipping-related particulate matter (PM) emissions are responsible for approximately 60 000 premature cardiopulmonary and lung cancer deaths annually on a global scale. And the relative impact of ship emissions is increasing since emissions from other sources are significantly decreasing due to the implementation of emission control techniques (e.g., in motor vehicles). These are all very good reasons to pay closer attention to shipping pollution and to continue to develop appropriate regulations. The IMO (International Maritime Organization) ship pollution rules limit the NOx and SOx emissions from ship exhaust. The PM is expected to decrease indirectly through the SOx limitations (by reduction of sulfate particle emissions), but at the moment, no direct PM limitations exist. Global limits are set for NOx and SOx, and in addition, stricter limits exist for special emission control areas. At the moment the sulfur content of fuel is limited to 3.5% globally and to 1.0% in SOx emission control areas (SECA). Further sulfur reductions will be implemented in 2015 for SECA reaching 0.1% fuel sulfur limitation and globally in 2020 (or 2025) reaching 0.5% © XXXX American Chemical Society
limitation. The sulfur decrease can be achieved either with decreasing the fuel sulfur level or by using exhaust aftertreatment (like scrubbers). The NOx limits are set for engines depending on the engine maximum operating speed. Limits are set globally (tiers I and II) and in addition for emission control areas (tier III). Tier III limits are on schedule for 2016, and this is expected to require the use of emission control technology. An effective NOx abatement technology is selective catalytic reduction (SCR), using a catalyst and ammonia as reducing agent. Several chemical reactions can occur in the SCR system, with the dominant one involving nitrogen monoxide, ammonia, and oxygen reacting to produce nitrogen and water.4 Due to the toxicity and handling problems associated with ammonia, water solution of urea is widely used as an ammonia source. The target is to have effective urea decomposition upstream from the SCR catalyst. Optimization of the urea feed plays a key role since all ammonia inserted into the SCR catalyst should be effectively utilized in NOx reduction and no ammonia should be found downstream of the catalyst. For example, in heavy-duty applications a hydrolysis catalyst (to boost the urea decomposition) upstream of the SCR catalyst5 and an NH3 oxidation catalyst (ammonia slip catalyst) downstream of the SCR catalyst can be utilized.6 However, Received: December 19, 2014 Revised: March 12, 2015 Accepted: March 17, 2015
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DOI: 10.1021/es506185x Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology these oxidation catalysts are usually not applicable to ships using fuels with higher sulfur levels. In practice, only vanadium-based catalysts are utilized in ship applications since V2O5 catalyst has been found to have high activity and sulfur tolerance (see, for example, refs 7−9). Roughly estimated, the operating temperature window for vanadium-based SCR catalyst is 300−400 °C.4 The SO3 found in the exhaust gas can react with the ammonia to form ammonium sulfates, which can deposit and foul the catalyst at low temperatures (below ∼300 °C).9,10 At high temperatures (above ∼400 °C) in addition to the SO3 found in the exhaust, SO3 can be formed due to oxidation of SO2 over the catalyst. This can result in an increase of particulate emissions. Obviously, the more sulfur that is available from the fuel, the more significant these effects can be. One performance study of SCR with HFO (heavy fuel oil with high sulfur level) as fuel was reported recently.11 The results yielded NOx reduction efficiencies from 75% to 99% with vanadium-based SCR catalysts. The result was also found to be in good agreement with other earlier published results from ship applications, presenting NOx efficiencies near 90% at exhaust temperatures above 300 °C.12−14 The hydrocarbon levels were also found to decrease while the amount of CO increased. The CO increase was suggested to be due to thermal decomposition of urea and/or a result of partial oxidation of HC compounds over the catalyst. At the same time, PM reductions from 14% to 45% were achieved, mainly due to oxidation of the organic carbon portion. These earlier published results, however, were mainly from rather optimum operation conditions for SCR; that is, the exhaust temperature was kept at 340 °C during the testing.11 Therefore, the sulfur-related challenges of possible ammonium sulfate formation at low temperatures and SO3 formation at higher temperatures were not notably observed. Variable SCR operation temperatures are, however, still relevant during shipping, for example, maneuvering in harbor areas. In this paper, SCR operation is examined with heavy fuel oil (with a sulfur level of 2.4%) as fuel at a wide temperature range from 260 to 450 °C. In addition to the NOx reduction efficiency studied in different conditions, attention is paid to particle emissions and how SCR affects those. Different catalyst loadings are utilized in the present study. In order to study the urea decomposition in a real exhaust gas application, ammonia and isocyanic acid measurements are utilized in selected conditions. Also, ammonia slip is continuously measured.
Table 1. Fuel Analysis Results for Sulfur and Major Metallic Elements Present in Fuel Oils sulfur (%) vanadium (mg/kg) sodium (mg/kg) nickel (mg/kg) iron (mg/kg) ash (%)
heavy fuel oil
light fuel oil
2.4 338 23 63 6 0.09
