Paper Offer # 05P-660 Asssment of Corrosivity Associated with Exhaust Gas
Recirculation in a Heavy-Duty Diel Engine Michael D. Kass, John F. Thomas, Dane Wilson, and Samuel A. Lewis, Sr.
Oak Ridge National Laboratory
Andy Sarles
Virginia Polytechnic Institute and State University Copyright © 2004 SAE International
ABSTRACT
A high-resolution corrosion probe was placed within the airhorn ction of the exhaust gas recirculation (EGR) loop of a heavy-duty diel engine. The corrosion rate of the mild-steel probe elements was evaluated as a function of fuel sulfur level, EGR fraction, dewpoint margin, and humidity. No significant corrosion was obrved while running the engine using a No. 2 grade, < 15ppm sulfur diel fuel; however, high corrosion rates were obrved on the probe elements when operating the engine using a standard grade No. 2 diel fuel (~350 ppm sulfur) while condensing water in the EGR loop. The rate of corrosion on the mild steel elements was found to increa with increasing levels of sulfate in th
e condensate. However, the engine conditions influencing the sulfate level were not clearly identified in this study.
INTRODUCTION
BACKGROUND
Exhaust gas recirculation (EGR) is being ud as a means of lowering NOx emissions from heavy duty diel engines. During this process, a portion of the exhaust is recirculated back to the cylinder (via the intake manifold) where the exhaust gas acts as a diluent. This lowers combustion temperature which reduces the formation of nitrogen oxides (NOx). EGR is currently ud to meet on-highway NOx emissions for Tier 3 emission levels for heavy-duty diel engines. Very high levels of EGR have been shown to push combustion to low temperature regimes where both NOx and PM levels are low. This is currently an area of inten study.
During the combustion of diel fuel, corrosive gas containing sulfur and nitrogen are produced. With EGR the corrosive gas are returned to the intake manifold where ambient conditions (such as temperature and humidity) and coolant conditions are believed to play a critical role in the formation of highly corrosive acidic compounds, especially sulfuric acid (1,2). Development of an in-s
itu measurement system would significantly advance the understanding of the corrosion potential associated with EGR, thereby enabling engine manufacturers to establish boundary conditions on engine operation to avoid high levels of corrosion. SULFURIC ACID FORMATION
During the combustion of sulfur-bearing fuel with excess air, most of the sulfur is converted into gaous SO2 or absorbed into the particulate matter (PM) emissions. A small fraction is also converted into gaous SO3 (1-4). Sulfuric acid is primarily formed in diel exhaust by a
two-step process. In the first step gaous SO2 reacts with oxygen in the exhaust to form SO3 which is described as follows:
2SO2 + O2J 2SO3
The SO3 subquently reacts with moisture in the exhaust to form sulfuric acid according to the following reaction:
2SO3 + H2O J H2SO4
Under typical exhaust conditions sulfuric acid will conden near temperatures approaching 150o C while water condensation will occur at temperatures clo to
25-30o C (1-4).
PROBE DESCRIPTION
The corrosion probe ud in this study is an electrical resistance (ER) bad probe manufactured by Cormon Ltd. The probe us their proprietary CEION technology to enable high resolution measurement (< 1micron) at relatively high sample rates (up to 0.25 Hz). A photograph showing the probe tip is shown in Fig. 1. The overall diameter of the probe is 2.54 cm and
contains two, esntially identical spiral elements; one (which is the shiny element in Fig. 1) contains the actual expod corroding surface, while the other (dark element in Fig. 1) is protectively coated to inhibit corrosion. This cond element simultaneously measures the gas temperature and provides for temperature compensation (since temperature had a pronounced effect on electrical resistance). The signal processing unit has up to 4 channel capability. This system was originally developed to monitor oil pipeline wear and for suba applications (6). We believe this paper reprents the first published report describing the application of this technology to monitor corrosion within engine exhaust.
2.54cm
Corroding Element
Insulated
Element
Fig. 1. Photograph of the corrosion probe tip showing the location and geometry of the corroding and insulated elements
The standard method for elucidating the corrosion behavior within engine exhaust systems is to place specialized coupons in an exhaust stream and expo them for long time periods. Accurate measurement of corrosion rate normally necessitates running an engine in excess of 100 hours for a given test condition, which is time consuming and costly. The coupons must be replaced before each test condition and also require careful preparation prior to analysis. A quicker, less-intrusive technique is to utilize a highly nsitive ER corrosion probe. This method enables corrosion rate determination within 1 hour and does not require removal between each t point. Prior evaluations using the Cormon probe have shown that an accurate measurement could be made within 30 minutes for each operating tpoint and the measured corrosion rates were determined to be highly repeatable (typically within 5%).
The purpo of this investigation was to evaluate the performance of the Cormon probe and to asss the relative corrosivity associated with exhaust gas recirculation in a heavy-duty diel engine. In particular, we were interested in evaluating the corrosion behavior as a function of condensation (sulfuric acid and moisture) and fuel sulfur level.
The subject of corrosion induced by EGR has proven to be a nsitive topic for engine companies. Therefore, we have not included any engine related specifics in this paper. The EGR system ud in this study is not ud in a commercial configuration, and the engine model is veral years old. We do not believe that the results we prent are specific to any particular make, model or class of engine. It is also important to note that condensing and/or corrosive conditions in engines with EGR may be relatively uncommon for many applications, and could be minimized through engine control, especially in cold weather. The reader should also bear in mind that the all corrosion-related data pertains to mild steel only. It is well understood that the corrosion behavior is a direct function of the material being corroded and, as such, corrosion rate results obtained on mild steel cannot be predicted or extrapolated to another material type. This is especially true for diel engines, which do not typically contain mild steel components. However, the corrosion results obtained using the mild steel probe elements can be ud to evaluate the relative corrosivity between operating conditions and the corrosion potential of the exhaust gas environment.
EXPERIMENTAL
ENGINE SETUP
A heavy-duty diel engine was equipped with a high-pressure EGR system containing an EGR valve that enabled manual control. The engine was coupled to a General Electric direct current motoring dynamometer capable of absorbing 224 kW (300 hp). A schematic showing a top-view engine layout is shown in Fig. 2. The recirculated exhaust gas fraction was injected downstream of the intercooler as depicted. The probe was mounted vertically on top of the airhorn as shown and the probe elements protruded approximately 1 to 2 cm into the intake gas stream. The length of recirculated exhaust and inlet air mixing was approximately 1.2 meters from the inlet air/recirculated exhaust junction to the intake manifold. This was a modification to ensure the exhaust and fresh compresd air charge were completely mixed in the intake. In addition a water injection system was installed to add moisture to the (after-turbo, compresd) supply air if necessary.
Fig. 2. Schematic of engine layout including EGR loop. The condensation sampling line was located in the bottom of the airhorn almost directly underneath the probe as shown in Fig. 3. This location allowed condensate to accumulate via gravity in the sampling line during a test run. The sampling line had valves at both ends to facilitate condensate collection and removal without having to stop the engine.
Fig. 3. Side view schematic of modified airhorn showing the locations of the corrosion probe and the condensate sampling line. TEST PROTOCOL
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An important part of this study was to confirm that fuel sulfur concentration is, in fact, the major contributor to the corrosivity associated with recirculated exhaust. In this study two fuel types were evaluated; an ultra-low sulfur-bearing fuel containing less than 15 ppm S (BP-15) and standard No. 2 diel fuel containing approximately 350 ppm sulfur. (Note that the No. 2 diel fuel meets the current EPA guidelines, while the BP-15 meets the 2007 on-highway diel requirements.) The engine was run at speeds between 1200 and 1550 rpm and torque ttings ranging from 454 to 542 Nm
(80-95% of full load). The exhaust pressure at the conditions was high enough to overcome the b
oost pressure, thereby introducing significant levels of exhaust into the intake. The level of exhaust to be recirculated was manually controlled using a prototype EGR valve (included in Fig. 2) and control system. By manipulating the valve position, we were able to t and maintain the EGR fraction for a given test condition. During this investigation the EGR level was t at values which ranged from 5 to ~16 %, depending on the test condition. A standard analytical bench was ud to measure the NOx, CO, CO 2, HC and oxygen concentrations in the exhaust and in the intake manifold. The humidity, and therefore dewpoint temperature, was maintained by a conditioned combustion air handing system. A water injection system was also installed on the air supply line to provide additional humidity to the EGR loop, if needed.
Prior experience using the corrosion probe to measure exhaust gas corrosion indicated that a minimum sampling time of 30 minutes was necessary to obtain reliable corrosion rate values; therefore, corrosion monitoring times in this study were typically longer than 45 minutes. During the cour of the investigation, the airhorn as measured by the probe For each test condition where condensation was obrved to occur, the condensate sampling line was thoroughly purged prior to condensate collection by opening the fill and purge valves (e Fig. 3). When the sample line contained appreciable levels of condensate, the condensate was emptied into a Teflon vial by first cl
osing the fill valve and opening the purge valve to relea the condensate into the vial. The condensate samples were subquently analyzed for acetate, formate, sulfate, nitrate, and nitrite content via ion chromatography.
RESULTS AND DISCUSSION
During this investigation the supply air humidity and temperature were maintained near 56 percent and 24o C, respectively. The water dewpoint temperature inside the airhorn was estimated according to the following general scheme:
1) The flowrate, temperature and relative humidity of intake air are measured values in the plenum leading to the engine. Water partial pressure in the air was estimated by programming standard atmospheric psychometric data (atmospheric pressure is a good approximation at the measurement point) into the data acquisition system as fitted polynomial equations. 2) The flowrate of diel fuel is measured and the composition is approximated as CH 1.85, (1.85 H atoms for every carbon atom). All hydrogen is assumed to be burned to form water.
我是一颗小小的石头3) The exhaust mass flowrate is known from the air and fuel flowrates into the engine. The amount of water in the exhaust is known from steps 1 and 2.
4) The intake manifold gas is compod of air and a fraction of exhaust. This fractional amount of EGR is calculated by measuring NOx levels in the exhaust stream and in the intake manifold air/exhaust mixture.
5) From proper accounting in the previous steps, the mole fraction of water in the intake manifold is known. The manifold pressure is measured, allowing the partial pressure of water to be calculated and from that value, the dew point. A thermocouple near the corrosion probe provides the temperature value to compare with the dew- point, giving the dew-point margin.
Condensation of moisture could be clearly obrved in the condensate sample line (Fig 3). The ont of moisture condensation was obrved to correlate very well when the manifold temperature dropped below the calculated dewpoint value. During this investigation, the temperature in the airhorn ction fluctuated between 29o C and 35o C depending on the test condition. This temperature fluctuation corresponded to the ri and fall of the supply water temperature to the intercooler. Typically, during conditions of condensation, the temperature inside the airhorn could be maintained between 1 and 2 degrees Celsius lower than the dewpoint value. However, becau the intercooler water temperature oscillated 5 degrees it was difficult to stabilize the temperature inside the airhorn for long time periods.
CORROSION PROBE RESULTS
Influence of Condensation
The measured effect of condensation and EGR fraction on the corrosion rate for 15 ppm fuel sulfur and 350 ppm fuel sulfur is plotted in Figs. 4 and 5 respectively. The results obtained while operating the engine using ultra-low sulfur (15 ppm) fuel are shown in Fig. 4. Here the corrosion rate was less than 1 mmpY. Except for the highest and lowest data points, the remainder of the data resided near a value of 0.4 mmpY. As shown by the spread of the data points, there was no obrvable difference between the values reprenting the condensing and noncondensing conditions. In addition the EGR fraction does not appear to have any discernable effect on the corrosion rate. The median value of 0.4 mmpY is considered to be a measurable but relatively insignificant corrosion rate.
In contrast the corrosion rate determined for the 350 ppm fuel sulfur level was greatly affected by the ont of condensation as shown in Fig. 5. The rates of corrosion obtained during condensation for this fuel type were all greater than 4 mmpY, which is considered significant, while the data point reprenting the noncondensing condition was 0. This point is significant becau a number of investigations have demonstrated that the ont of condensation for sulfuric acid can be expected to
occur near temperatures approaching 150o C for combustion exhaust systems (1,2). The temperature inside the intake manifold during this investigation ranged from 25 to 34o C, which is considerably below the sulfuric acid dewpoint temperature; however, no obrvable corrosion was measured for this condition. The results indicate that enhanced corrosion occurs not with the ont of sulfuric acid condensate but with the ont of moisture condensation.
The data in Fig 5 also do not show any clear relationship between the rate of corrosion and the fraction of EGR for the fuel containing 350 ppm sulfur. However, a more elaborate investigation with better humidity and temperature control is needed to determine whether this is true or not. A direct comparison of the 15 ppm and 350 ppm sulfur fuel types is shown in Fig. 6 which more clearly shows that in spite of the large scatter, the corrosion rate is substantially higher when running the engine on 350 ppm fuel sulfur versus operating the engine using ultra-low sulfur fuel (BP-15). This result indicates that potential corrosion within a heavy-duty diel intake manifold, due to EGR, can be mitigated by running the engine on a low sulfur fuel.
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Fig. 4. Corrosion rate within the airhorn as a function of EGR fraction and condensation for low sulfur (15 ppm)
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Fig. 5. Corrosion rate within the airhorn as a function of EGR fraction and condensation for high sulfur (`350 ppm) fuel.
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Fig 6. Corrosion rate determined as a function of fuel sulfur level during conditions of water condensation. In summary the two factors that most influence the corrosion rate appear to be the fuel sulfur level and the ont of water condensation.
Influence of Humidity and Dewpoint Margin
Since significant corrosion rate values were obtained when operating the engine using 350 ppm sulfur-bearing fuel, the influence of humidity and dewpoint margin were examined for this condition. The humidity was controlled by manipulating the supply air handling system and by injecting a water spray into the supply air downstream of the compressor. The range of relative humidity varied between 53 to 58 % as shown in Fig. 7. The resulting data do not appear to show any correlation of the rate of corrosion with humidity while operating the engine using
350 ppm sulfur fuel in a condensing situation.
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C o r r o s i o n R a t e , m m p Y
Fig 7. Measured corrosion rate as a function of humidity while operating using 350 ppm sulfur fuel and undergoing condensation.
Another variable that may be important is the dewpoint margin which is defined as the difference between the actual temperature and the dewpoint temperature during condensation. If the sulfate fl
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ux in the exhaust is constant for a given condition, then the concentration of
sulfuric acid dissolved in the condensing water should be inverly proportional to the condend water being formed. The data shown in Fig. 8 support the supposition of increasing corrosivity with decreasing dewpoint margin for tho points where the dewpoint margin was greater than 1.5o C. However, the two corrosion rate values corresponding to the two lowest margin ttings do not. This is another variable that
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warrants further investigation.
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Fig. 8. Measured corrosion rates for different dewpoint margins while operating using 350 ppm sulfur-bearing fuel (No. 2 diel).
Ion Chromatography Results
Ion chromatography was performed on the condensate specimens to measure the dissolved levels of acetate, formate, nitrite, nitrate and sulfate. Analysis of the data revealed that negligible amounts of acetate, formate, and nitrate were measured in the condensate samples. However, appreciable levels of nitrate and sulfate were detected in the collected condensate samples. The nitrate and sulfate concentrations found in the condensate samples taken from all of the operating conditions using 15 ppm fuel sulfur are shown 9. The results show that even though the fuel sulfur level was low, measurable amounts (5 to 110 mg/L) of sulfate and nitrate were detected. Note that it is very likely that a portion of this sulfate originated in the lubricant. The data also show that the concentrations of the nitrates and sulfates are on the same order and similar for veral samples. The low concentrations of nitrate and sulfate in the condensate apparently do not create conditions of high corrosivity within the condensate.
