Experimental studies on HCCI combustion of biomass syngas towards tar tolerant operation
(2015)
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ExperimentalstudiesonHCCIcombustionofBiomassSyngastowardsTarTolerantoperation.pdf
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- Authors
- Supervisors
- Jeanmart, Hervé
- Abstract
- Biomass syngas needs to be cooled before being usable in a conventional spark- ignition engine, in order to avoid knock. The tar impurities of raw biomass syngas condense due to this cooling process, which leads to the clogging and subsequent damage of critical process parts. Thus, in addition to cooling, the syngas needs to be purified of the tars, which adds to the cost and complexity of the gasification power plant. A novel technique is being investigated where the syngas temperature is kept above the tar dew point (about 250◦C) throughout the process, and thus, tar condensation and its related problems can potentially be avoided. A Homogeneous Charge Compression Ignition (HCCI) engine is suitable for such an application. The objective of this thesis is to experimentally study HCCI combustion with biomass syngas at intake temperatures above the tar dew point. Chapter 1, reviews the broad context of gasification and the associated tar impurities, where it argues that the SI engine, due to its limited intake temperature flexibility, significantly contributes in the making of tars into problematic compounds. The novel method of using the HCCI engine at intake temperatures above the tar dew point is introduced, along with listing the multiple challenges foreseen with such an approach. Of these challenges, two are identified which forms the main experimental works described in this thesis. The test bench, which includes a 435cc single-cylinder air cooled engine, and the associated instruments are discussed in Chapter 2. Chapter 3 discusses the various post processing methods used during this work which computes the rate of heat release from the raw in-cylinder pressure data, along with a host of other parameters. Thereafter, the main experimental works are described in Chapters 4, 5 and 6. In a real-world scenario, the syngas composition varies with time. To understand the HCCI combustion in response to such variations, studies with artificial syngas compositions were carried out, as described in Chapter 4. In these experiments, the time component was omitted and the focus was on determining the impacts of various possible syngas compositions as well as operating conditions on the HCCI combustion. It was found that increasing H2 content of syngas advanced the combustion which lead to the undesirable pressure ringing (equivalent to knock in SI engines). At an intake temperature of 250◦C and intake pressure of 1.2 bar, a maximum IMEP of 2.8 bar and an Indicated Efficiency (IE) of 35% could be attained with dry and pure syngas compositions. Since the raw syngas would also contain water and tars, the effects of these components were studied. Syngas moisture dampened the HCCI combustion, thus improving the IMEP potential. Two representative tar com- pounds (naphthalene and toluene) were used and it was observed that tars, due to their high heating values and higher heat capacities, had positive effects on HCCI combustion. Additionally, the effects of intake pressure and temperature were also studied with the conclusion that high intake pressures and low intake temperatures improved the engine performance. Since the HCCI engine lacks a direct control method, studies were carried out to explore the use of Exhaust Gas Recirculation (EGR) as a control method, as discussed in Chapter 5. EGR was found to successfully delay the combustion initiation and thus, in combination with the equivalence ratio which advanced the combustion, an effective HCCI control method for this context was demonstrated. An improvement in the maximum IMEP from 2.8 bar at EGR=0, to 3.3 bar at EGR=25% was achieved, a significant gain of about 25%. Studies with real syngas were carried out to investigate the effect of time variations in the syngas composition, as discussed in Chapter 6. Syngas from a two-stage downdraft gasifier was used in its cooled and purified form so as to reduce the complexity of the process and form a basis for future hot syngas experiments. Two experiments were carried out with time durations of 3 hours and 24 hours. For the 3 hours experiments, the gasifier was in a transient stage (warming up) and thus supplied syngas with a lower heating value. Decent HCCI combustion stability was observed which is interesting in the light of the highly sensitive nature of the HCCI combustion. For the 24 hours test, the gasifier was operating at its designed conditions, resulting in a higher heating value syngas. During the 24 hours test, the engine combustion was not stable and wide variations were observed. Additionally, high amounts of NOx emissions were recorded. The reasons for both the cyclic instabilities and high NOx could not be confirmed, however, some hypotheses are proposed. Finally, Chapter 7 concludes the findings of this thesis and reflects on the advantages and limitations of the concept as well as on the limitations of this thesis. Planned works following this thesis in the near and distant future are listed.
- Affiliations
Citations
Bhaduri, S. S. (2015). Experimental studies on HCCI combustion of biomass syngas towards tar tolerant operation. https://hdl.handle.net/2078.5/190047