| dc.description.abstract | This thesis investigates lean premixed turbulent combustion technology using natural gas with hydrogen addition. Lean premixed combustion has been applied to the areas of gas turbines and automobile engines and its advantages of saving fuel, reducing [NOx] emission, and increasing thermal efficiency were proved. However, when this technology is applied, there are two major problems: (1) a substantial decrease in flame speed and (2) the tremendous increase of the misfire probability, leading to poor engine outputs and heavy emissions. In this work, hydrogen as an additive is used to solve these aforementioned two problems because hydrogen has very low flammability limit and very high laminar burning velocity (SL). With three different hydrogen additions (volume fraction 10%, 20%, and 30%) in lean CH4/air mixtures, a series experiments are conducted in the cruciform burner where an intense near-isotropic turbulence can be generated via a pair of counter-rotating fans and perforated plates. Using a pair of self-designed ion-probe sensors, turbulent burning velocities (ST) are measured quantitatively. In addition, emission concentrations of NOx and CO are measured for the first time to understand combustion characteristics of lean natural gas with hydrogen addition.
The results indicate that a small hydrogen addition not only can slightly expand the lean flammability of CH4/air mixtures, but also can increase largely values of ST. At fixed values of (=0.6 or 0.7) and u’’/SL, values of ST increase with the amounts of hydrogen addition. However, turbulence cannot increase values of ST continuously. The slope of ST/SL vs. u’’/SL is found to bend towards the horizontal when values of u’’/SL are greater than about 4. This is the so-called bending effect, as also observed in burning CH4/air mixtures with hydrogen addition. All the present data can be correlated with an empirical relation with the form of (ST-SL)/u’’=C1Da^C2 , where C1, and C2 are experimental constant and Da is the Damköhler number. This empirical correlation is first proposed by Shy et al. (2000c) using the same methodology without hydrogen addition. Emission concentrations of [NOx]/[CO] increase/decrease with the amounts of hydrogen addition, respectively. For examples, at u’’/SL=2.0 and =0.7, values of [NOx] are respectively 98, 136, 158, and 188 ppm for 0%, 10%, 20%, and 30% hydrogen addition. This indicates that hydrogen combustion has a serious problem of NOx formation due to its very high flame temperature. Values of [CO] vary from 40 ppm to 55 ppm for any values of u’’/SL ranging from 0~7 and at =0.7. These experiments suggest that the hydrogen should not exceed 20% for premixed CH4/air turbulent combustion and values of u’’/SL should be controlled within the range of 4~10, such that lower [NOx] emissions and higher values of ST/SL range from 6.5 to 12 can be achieved. These results are relevant to the combustion performance of natural gas fire power plants. | en_US |