Investigation of novel emission reduction technologies and use of liquid or gaseous fuels for curbing marine engines environmental impact

Abstract

The control of air pollution in all primary industries is the major technological challenge currently and for the upcoming decades. The effect of industrial carbon emissions on global temperature rise is now an undisputed fact, with all major institutions and governmental bodies examining pathways to reduce the amount of CO2 emitted in the atmosphere. Unless strict measures are implemented, the global average temperature increase is projected to reach levels that will have severe effects on population welfare and the environment. Beyond CO2, other emissions from fossil fuel powered infrastructure have been found to adversely affect human health and the environment. Some of the most harmful species emitted are NOx, a product of high temperature combustion and a hurdle of diesel engine design regardless of application. The marine sector accounts for roughly 3.3% of total anthropogenic CO2 emissions and is also one of the major sources of NOx emitted in the atmosphere, both on a global scale and near centres of high human population density. In this thesis solutions to limit the contribution of the marine sector on global emissions are investigated focusing on pollutants from marine internal combustion engines. Modern emission reduction technologies for the marine industry are examined and evaluated using measurement data collected from marine engines on-site, either on-board vessels or during factory acceptance tests of the engines. The direct measurements were complemented by data acquired using telemetry solutions, if required. The technologies investigated, aim to reduce CO2 and NOx emissions by direct or indirect methods. Both emissions are controlled by the International Maritime Organization (IMO) with limits that target total ship carbon and NO¬x emissions. The control of carbon emissions is enforceable since the start of 2023 via indexes that penalize high fuel consumption, hence CO2¬ emissions. In the case of NOx, limits have become considerably stricter in specific sailing areas since 2016. The CO2 reduction technologies examined are the use of natural gas in dual-fuel marine 2-stroke propulsion engines and the use of marine type biofuels. Dual-fuel engines operating on liquified natural gas (LNG) are, in their current form, a relatively new development and the total number of such engines in the field is still limited compared to diesel only designs. High interest for marine grade biofuels started roughly in 2018, with considerable number of tests conducted between 2020 and 2022 to verify compatibility with various types of marine engines and evaluate for potential effect on NOx emissions. The biofuel used for most of these tests and in the measurements conducted for this thesis contained 30% biodiesel sourced from waste oil feedstock. The NOx abatement solutions examined are exhaust gas recirculation (EGR) and selective catalytic reduction (SCR). Both technologies started their commercial use in maritime within 2016, with SCR being the solution mostly selected until recently. The thesis mainly concentrates on the 2-stroke engine which is the primary power source of commercial vessels and has the highest contribution to both CO2 and NOx emissions. The 2-stroke engine investigation involved two engines with the same characteristics but with different designs and settings regarding gas injection for the dual-fuel LNG high-pressure gas injection tests. For the biofuel trials five diesel engines of various types were tested. A diesel only engine and a one of the dual-fuel engines were considered for the EGR tests and four engines were tested using SCR. In addition, seven 4-stroke engines were measured during the biofuel tests. All 2-stroke engines tested were state-of-the-art including automatic control systems. For the above subjects limited information is available in existing literature, especially regarding data from full-scale applications and analysis of potential performance impact of the technologies based on experimental procedures and not mainly simulation results. The tests conducted included measurement of all major operational data from engines, with priority given in acquiring cylinder pressure traces and data from the engines’ control systems. Exhaust gas composition measurements were conducted for all engines to estimate their specific NOx emissions and acquire CO2 emission data via a streamlined procedure devised for fast and accurate on-board and on-site tests. The CO2 emissions were also calculated using fuel carbon content and fuel consumption. The cylinder pressure data were used to conduct heat release rate analysis and estimate the fuel combustion progress. A heat loss model was used to estimate gross heat release and allow calculation of fuel consumption where accurate measurement was not possible. The pressure traces were analysed to derive engine settings, specifically the timing of the exhaust valve opening and closing angle and the fuel injection angle. For the biofuels and EGR analysis, a multizone engine model was utilized to simulate engine performance and emissions. The model was based on pre-existing code that was modified accordingly. The validation of the model was performed using the data from factory acceptance tests (FAT), that are of high quality for engine operation at “conventional” conditions, with standard fuel and with/without EGR. The technologies found to have the highest impact on engine performance compared to a modern but conventional design were dual-fuel high-pressure LNG and EGR. Dual-fuel operation tests showed both similar and quite different performance compared to diesel only operation, depending on the engine version. For both LNG engines, high combustion “speed” and fuel energy content resulted in increased peak combustion rate and decreased total combustion duration, excluding some cases of high delay between the diesel pilot and main gas injection. Overall, the effect of the pilot injection strategy was found to considerably alter the combustion mechanism. For the EGR equipped engines the presence of recirculated gases resulted to slower combustion and peak combustion rate values were decreased. The combustion duration was increased. For the other two technologies tested, biofuels and SCR, effects on engine performance were rather limited. The use of biofuel did affect combustion to a degree but was in most cases not immediately discernible. The SCR system had minimal impact on engine performance and no effect on the combustion process as it is an exhaust gas aftertreatment device not imposing excessive backpressure. Analysis of engine tuning showed considerable differences in some approaches, with the highest degree of settings variation found for the EGR equipped engines and the updated version dual-fuel engine. For the dual-fuel engines, the previous generation engine showed a conservative approach regarding pilot fuel injection to limit peak combustion rate, pressure rise and maximum pressure values. The overall tuning resulted to nearly identical performance to diesel only operation. Different tuning approach was found for the newer engine that featured altered EVC timings, considerable pilot injection timing advance and use of a different injection profile for the diesel pilot. During EGR use both EVC and injection timing were considerably altered, as was the total mass flow in the engines. Other notable differences in engine settings were found for the low-pressure SCR system examined, in contrast with the high-pressure one, that barely affected engine operation. These measures were employed to increase exhaust gas temperature at the catalyst inlet to allow safer operation and improve NOx conversion efficiency especially at reduced load. The use of biofuels could have presented differences for fuel injection timing due to physical property differences, but only minimal effect was found. The only confirmed difference was earlier ignition due to elevated cetane number. The variations in tuning and the combustion process affected main operational parameters, specifically compression and peak pressure, pressure increase after fuel ignition and exhaust gas temperature. The total efficiency of the engines was impacted in most cases with increased fuel consumption compared to typical operation. The notable exceptions were both dual-fuel engines. The total fuel mass consumed, and total energy content supplied to the engine were either similar to or considerably lower than diesel-only operation. In the case of the updated version engine considerable improvement of efficiency was found due to advances in injector design and a different tuning approach. The highest fuel consumption penalty was measured during EGR use in the diesel and dual-fuel engines. The main reasons were the reduced rate of the combustion process, prolonged duration and the lower peak pressures achieved. SCR operation caused minimal fuel penalty due to the added backpressure either on the engine of after the turbine and some tuning requirements to increase exhaust gas temperature. The urea consumption, however, was substantial, and its cost may, depending on pricing trends, approach or surpass that of crude oil. This resulted to the SCR being the least desirable NOx reduction solution in terms of operating expenses, but also featuring the least number of considerations for potential issues on engine operation due to its relative simplicity and minimal effect on performance. The B30 biofuel did affect fuel consumption due to its lower calorific content and a slight combustion efficiency decrease compared to conventional fuels. The level of difference was evaluated as not concerning in terms of operating costs, but the issue of its higher price will remain until production capabilities increase. The CO2 emissions were affected by all the tested technological solutions due to the change in fuel consumption and/or the different carbon content of each fuel used. Biofuels showed only a small decrease of carbon tailpipe emissions compared to crude oil, while the lowest tailpipe CO2 emissions were found for MGO operation. Their benefit towards carbon emission reduction will only result in a well-to-wake basis. On the other hand, their application is simple and requires no engine modifications. LNG use in high-pressure injection dual-fuel engines confirmed its high potential for carbon reduction with both good fuel consumption values and low fuel carbon content. The other two studied technologies resulted in fuel consumption increase that raises CO2 emissions; despite the low consumption penalty for the SCR its impact was rated as considerable due to the urea production high energy requirements. The NOx emissions were not affected for the tested dual-fuel engines and were close to or below diesel-only operation. This is expected due to similar overall in-cylinder conditions and the use of the Diesel cycle principles. The increase in peak combustion rate, most prevalent in the updated dual-fuel engine, did not result in high change of NOx emissions or exhaust gas temperature. In the case of biofuels, NO¬x emissions were above conventional fuel operation in most cases. This was found for both the 2-stroke and 4-stroke engines tested. The level of increase was moderate, allowing for the NOx emissions standards of the tested engines to be retained. Due to almost identical engine performance compared to a conventional fuel, the NOx emission increase was attributed to the oxygen content of the biodiesel, increasing air-fuel ratio in the flame and reaction speed. The use of the multizone engine model confirmed the previous with good predictions of NO¬x emissions for biofuel and conventional fuels. Both the SCR and EGR systems were proven capable of substantial NOx reduction, with the maximum efficiency of the former being slightly higher. It is noted, however, that the SCR catalyst efficiency will decrease with use, and three-to-five-year interval renewals are required. For the SCR system the efficiency of catalysts operating on HFO was also confirmed, with some reservations regarding the system’s longevity. The EGR effect on NOx emissions was investigated in detail using the multizone engine model, with good results in replicating its effect on their formation. The required EGR percentage was also estimated and found on the high side for all loads. The use of the model allowed to determine the actual mechanism for NOx reduction and to examine of a solution for minimizing the fuel consumption penalty at all engine loads. Overall, the viability of all technical approaches tested was confirmed, most notably for the case of biofuels that were up to now unproven in the marine field. Tests with the dual-fuel LNG engines demonstrated high versatility on operation capabilities via engine tuning and it was established that high level of advances can be achieved between sequential engine generations. This is strong indication that, as both a short- and mid-term solution, LNG powered vessels will become the go-to approach for low-carbon fuel use. Between the two NOx abatement measures available and tested for large marine engines, both were found within expectations in real-world applications. Their comparison showed advantages and disadvantages for each system. The main concerns for the EGR system are high complexity, low number of operating hours in the field and high impact on engine operation. The SCR solution was superior in all of the previous terms, but its considerably higher operating costs are unfavourable in return of investment terms.

Σε αυτήν τη διατριβή μελετήθηκαν τεχνολογίες μείωσης εκπομπών για τον ναυτιλιακό τομέα, με στόχο τα CO2 και NOx παραγόμενα από κινητήρες που λειτουργούν με υγρά και αέρια καύσιμα. Η βάση για την έρευνα ήταν μετρήσεις σε ναυτικούς κινητήρες που πραγματοποιήθηκαν είτε σε πλοίο είτε κατά τη διάρκεια εργοστασιακών δοκιμών. Οι κινητήρες που μελετήθηκαν ήταν μεγάλοι 2-χρονοι κινητήρες πρόωσης και 4-χρονες γεννήτριες. Εξετάστηκαν δύο τεχνικές μείωσης εκπομπών για κάθε ρύπο. Για το CO2 βιοκαύσιμα και υγροποιημένο φυσικό αέριο (LNG), και για τον έλεγχο των NOx ανακύκλωση καυσαερίων (EGR) και εκλεκτική καταλυτική αναγωγή (SCR). Όλα τα προαναφερθέντα εισήχθησαν πρόσφατα στο ναυτιλιακό τομέα και θεωρούνται προηγμένες τεχνικές. Η χρήση βιοκαυσίμων εξετάστηκε μέσω δοκιμών σε πλοία σε 2-χρόνους και 4-χρόνους κινητήρες χρησιμοποιώντας μείγμα 30% βιοντίζελ και βαρύ πετρέλαιο, και με συνοδευτικές μετρήσεις με τυπικά ναυτικά καύσιμα. Η επίδραση στους ναυτικούς κινητήρες, ειδικά 2-χρονους, δεν είχε αναλυθεί εκτενώς μέχρι την παρούσα έρευνα. Ελάχιστη επίδραση βρέθηκε στην απόδοση του κινητήρα και στη διαδικασία καύσης, με μια μικρή αύξηση κατανάλωσης. Μετρήθηκε αύξηση στις εκπομπές NOx, που αποδόθηκε στο αυξημένο περιεχομένου του βιοκαυσίμου σε Ο2 βάσει ανάλυσης των μετρημένων δεδομένων και έρευνας με χρήση ενός πολυζωνικού μοντέλου. Ο αρνητικός αντίκτυπος στην κατανάλωση καυσίμου και τις εκπομπές ήταν μεγαλύτερος για τους 4-χρονους κινητήρες. Για όλους τους κινητήρες, η αύξηση ήταν μέτριου επιπέδου, επιτρέποντας τη λειτουργία εντός των θεσπισμένων ορίων εκπομπών. Μετρήθηκε ελάχιστη βελτίωση εκπομπών CO2, αλλά αναλύσεις κύκλου ζωής δείχνουν περίπου 25% βελτίωση στις συνολικές εκπομπές άνθρακα χρησιμοποιώντας βιοκαύσιμο. Η μελέτη χρήσης LNG πραγματοποιήθηκε σε κινητήρες διπλού καυσίμου με έγχυση του αερίου σε υψηλή πίεση. Οι μετρήσεις λειτουργίς και εκπομπών πραγματοποιήθηκαν σε 2-χρονους κινητήρων δύο συνεχόμενων γενεών κατά τη λειτουργία μόνο με πετρέλαιο και με χρήση πετρελαίου και αερίου. Η ανάλυση έδειξε ότι ο προηγούμενης γενεάς κινητήρας ήταν ικανός για σχεδόν ίδια απόδοση μεταξύ των δύο τρόπων λειτουργίας. Μία ελάχιστη μείωση αποδοτικότητας βρέθηκε κατά τη λειτουργία δύο καυσίμων. Η ποσότητα πιλοτικής έγχυσης πετρελαίου που απαιτείται για σταθερή λειτουργία διπλού καυσίμου ήταν χαμηλή. Η κυριότερη διαφορά για τη λειτουργία διπλού καυσίμου ήταν υψηλότερος αρχικός και μέγιστος ρυθμός καύσης. Γενικά η ρύθμιση του κινητήρα αυτού είχε ως στόχο ελαχιστοποιηθούν οι διαφορές μεταξύ λειτουργίας ενός και δύο καυσίμων. Στον κινητήρα νέας γενεάς βρέθηκε σημαντική βελτίωση όσον αφορά την απαιτούμενη ποσότητα πιλοτικής έγχυσης. Επίσης η οικονομία καυσίμου ήταν σημαντικά καλύτερη κατά τη χρήση φυσικού αερίου. Τα παραπάνω αποδόθηκαν σε βελτιώσεις στο σχεδιασμό του εγχυτήρα και σε διαφορετική ρύθμιση του κινητήρα. Η επίδραση των προηγουμένων στην καύση ήταν εμφανής, με πολύ υψηλότερο ρυθμό καύσης προανάμειξης. Η μέγιστη πίεση στους κυλίνδρους αυξήθηκε σε σχέση με τη λειτουργία με πετρέλαιο και με τον προηγούμενης γενεάς κινητήρα. Οι εκπομπές NOx ήταν σχετικά κοντά μεταξύ των δύο τρόπων λειτουργίας. Κατά τη λειτουργία διπλού καυσίμου οι εκπομπές επηρεάστηκαν από τη μορφή και χρονισμό της πιλοτικής έγχυσης, τις μέγιστες τιμές πίεσης και τη χαμηλότερη θερμοκρασία φλόγας του μεθανίου. Το οφέλη στις εκπομπές CO2 ήταν περίπου 23% και για τους δύο κινητήρες κατά τη λειτουργία με φυσικό αέριο. Η βελτίωση γενεάς προς γενεά ήταν περίπου 7% αναφορικά με τις εκπομπές άνθρακα. Το EGR δοκιμάστηκε σε έναν μεγάλο 2-χρονο κινητήρα πετρελαίου και έναν διπλού καυσίμου. Η τεχνική της ανακυκλοφορίας είχε μεγάλη επίδραση στο μηχανισμό καύσης. Ο ρυθμός καύσης ήταν πιο αργός και η διάρκειά της αυξήθηκε και στους δύο κινητήρες. Απαιτήθηκαν σημαντικές αλλαγές στη ρύθμιση του κινητήρα. Η επίδραση στο μηχανισμό καύσης οδήγησε σε αύξηση της κατανάλωσης, περίπου 5% για τον κινητήρα με πετρέλαιο και 4% για τον κινητήρα αερίου. Η συνολική αποδοτικότητα μείωσης NOx ήταν υψηλή, κοντά στο 80%. Πρακτικά δεν βρέθηκε διαφορά μεταξύ του κινητήρα αερίου και του κινητήρα πετρελαίου. Το ποσοστό EGR εκτιμήθηκε χρησιμοποιώντας μία προσεγγιστική μέθοδο με μετρημένα δεδομένα και ένα πολυζωνικό μοντέλο. Και οι δύο μέθοδοι έδειξαν υψηλές απαιτήσεις σε ποσοστό ανακυκλοφορίας, ενώ παράλληλα η έγχυση καυσίμου ξεκινούσε αργότερα προκαλώντας επιπλέον μείωση του σχηματισμού NOx. Το SCR δοκιμάστηκε σε δύο διαμορφώσεις πριν και μετά τον τουρμπίνα σε μεγάλους 2-χρονους κινητήρες. Η επίδραση στη λειτουργία του κινητήρα ήταν ελάχιστη. Βρέθηκε μόνο μια αύξηση της κατανάλωσης καυσίμου περίπου στο 1,5%. Επιβεβαιώθηκαν ορισμένες απαιτήσεις αλλαγής ρυθμίσεων για αύξηση της θερμοκρασίας εισαγωγής αερίου στον καταλύτη, οι οποίες συνέβαλαν στην αύξηση της κατανάλωσης. Η απόδοση της μείωσης των NOx ήταν καλή, 85% και 80% για τον καταλύτη πριν και μετά τον στρόβιλο αντίστοιχα. Το σύστημα απαιτούσε τη χρήση ουρίας και η κατανάλωση ήταν υψηλή, κατά μέσο όρο 18 g/kWh. Αυτό θα οδηγήσει σε υψηλό κόστος λειτουργίας του συστήματος. Μια σύγκριση μεταξύ EGR και SCR έδειξε σημαντικά υψηλότερο κόστος του συστήματος SCR με βάση τις σύγχρονες τάσεις των τιμών καυσίμων και ουρίας, και λαμβάνοντας επίσης υπόψη την απαίτηση αντικατάστασης καταλύτη κάθε λίγα χρόνια. Συνολικά, οι τεχνολογίες που διερευνήθηκαν διαπιστώθηκε ότι παρέχουν απτά οφέλη στη μείωση των εκπομπών, ενώ είναι αρκετά ώριμες για τρέχουσα χρήση ενώ ορισμένες έχουν επίσης σημαντικές προοπτικές για μελλοντική εξέλιξη.