4.1. Vapor effects on flame stability
In anode exhaust gas, moisture level could be extremely high which makes it difficult to gain a steady flame in the combustor. It is mainly because some radical decomposed from H2O suppresses burning reactions of syngas. Therefore, in IGFC systems, anode exhaust gas should be condensed before oxy-combustor to remove some water. In that case, steam partial pressure at entrance of combustor is determined by the saturated vapor pressure. A single PSR model is used to study the relationship between the condensation temperature and flame temperature, which is of great importance to flame stability. Ignite temperature is set to 1100K. Resident time is 1 s. Results are shown in Fig. 6.
It is shown in Fig. 6 that the equivalent adiabatic flame temperature Tad decreases with combustor inlet temperature increasing. It is because moisture content is lower under lower condensation temperature. If the condensation temperature is higher than 350K, flame temperature is lower than ignition temperature which means excess steam existing and failing to ignite. In practical systems, steam could not be wiped out completely which make Tad always lower than it without vapor. For cycling1 case, Tad under dry condition is around 1700K. In view of heat loss during burning process, it might be proper to have a Tad 500K higher than ignition temperature to gain a steady flame. Based on above, 315K might be an ideal condensation temperature for system process design where Tad is about 1680K.
4.2. CO conversion
For the exhaust gas treating device aiming at CO2 capture, the most important indicator is the processing capacity. For oxy-combustor, it could be represented by the dry CO2 mole fraction in exhaust gas after burning. CO conversion after combustor could be determined by the flame temperature and oxidant quantity. Beside equivalent ratio, heat loss affects flame temperature directly. As mentioned, forced convection is set outside flame chamber to control the liner temperature which would certainly bring heat releasing problem. CFD method is used to calculate the CO2 mole fraction after treatment under different equivalent radio and liner convective coefficient conditions. Simulation results are shown in Fig. 7. Besides, data of experiment above is shown in Table 2.
It shows in Fig. 7 that CO2 mole fraction after burning under dry condition is higher than 0.958 under every calculating cases. It indicates that the existing combustor performs well in tail gas treating. Convective coefficient seems to show slight effects on CO conversion, which may due to tiny influence to flame temperature caused by liner heat loss. CO mole fraction varies obviously with excess O2 quantity relatively. It should be noticed that CO2 content decreases when excess O2 quantity rises. This confirms that CO could be almost fully converted under flame burning condition (Ilbas et al. 2019). Unreactive O2 decreases the CO2 mole fraction. For practical device, excess O2 helps CO being burned out which reduces CO emission in consideration of unexpected unmixedness at jet outlets. Therefore, 5% excess O2 could be recommended since a acceptable CO2 capture rate is gained. The liner convection could be enhanced as far as possible for it shows little influence on CO conversion. Table 2 shows that CO2 concentration of dry flue gas after burning is over 0.958, which could be considered as an ideal rate for capture. Single cases have a slightly higher concentration percent due to more H2 and H2O content. They would be dried out before capture process. It is experimentally proved that CO can be converted to CO2 efficiently by fully oxy-combustion.
Table 2
Experiment data of CO2 mole fraction in dry exhaust gas after burning.
Case | CO2 mole fraction |
Single1 | 0.962 |
Single2 | 0.962 |
Cycling1 | 0.958 |
4.3. Liner temperature
Liner temperature of combustor should be paid attention to since high temperature leads to creep deformation which may damage the combustor especially for long term running systems. That might be another key indicator for oxy-combustor evaluating. Mostly, the liner temperature is determined by flame temperature inside. However, for an oxy-combustor aiming at CO2 capturing, equivalent ratio might varies in a narrow range. It makes total flame temperature range relatively small especially for diffusion flame. In that case, liner temperature is influenced by heat transfer inside and outside. Heat transfers in from high temperature burning gas and then out to cooling air outside chamber. For anode exhaust gas characterized by extremely low calorific value, strong swirling might be bring in to the flow field organizing inside to stabilize the flame. Tangential angle of fuel jet determines centrifugal force of flow which would affect convective heat transfer inside liner for fuel flow rate is times to oxygen flow rate. Moreover, tangential angle of jet influences mixedness of fuel and oxygen which may affect local flame temperature. Maximum liner temperature is calculated by CFD model under different outside convective efficient and fuel jet tangential angle. Results are shown in Fig. 8.
Max liner temperature decreases with the outside convective coefficient, which proves that force convection outside flame chamber is an effective way to protect the combustor. It can be noticed that max liner temperature has an optimal value at tangential angle around 25°. Main reason for this might be as following. Stronger swirling makes a better mixing condition. When jet tangential angle is around 20°, mixing between fuel and oxygen is not ideal. High level of unmixedness leads flame to burning under low equivalent ratio, which causes high local temperature. Liner temperature increases when local temperature is high. When jet tangential angle is around 30°, larger centrifugal force makes high temperature gas scour liner more fiercely which may cause high liner temperature, although mixing level is more ideal. It can be inferred that an optimal tangential angle, which could vary with dimensions of combustor, exists for liner temperature controlling.
4.4. Pollutant emissions
Pollutant emission is always an important indicator for different kinds of power plants. For SOFC, electrochemical reactions take place inside fuel cells. Differing from traditional plants, no chemical energy converts to heat which might cut pollutant formation process. However, oxy-combustor adds burning step into systems which may cause some emission problem. In practical IGFC system, syngas contains amount N2 even O2 gasification technology is involved. So combustor pollutant is mainly consisting of CO and NOx. CRN model is used to compute pollutant emissions of the oxy-combustor. Cycling1 case with a total fuel utilization rate of 86% is computed compared with Single2 case whose total fuel utilization rate is 80%. As shown above, this leads to a difference between components of exhaust gas. In this period, exhaust gas contains 3% N2 by volume in simulation case. Ratio of other component is kept as before. Emissions under different excess O2 quantity are computed. Results are shown in Fig. 9 and Fig. 10.
It is shown that both CO and NOx emissions decrease with rising of excess O2 percent. It is known that NOx formation is directly related to flame temperature. When excess O2 percent is low, local flame temperature could be higher which leads to higher NOx emissions. On the other side, more excess O2 supply makes CO burnt more completely which means lower CO emissions. Therefore, excess O2 supply is in favor of pollutant reducing. Combined with mentioned, if CO2 concentration could meet the requirement of CO2 capturing, O2 should be supplied as more as possible. Barely influence of excess O2 percent is shown on combustor exit temperature since O2 flow rate varies slightly. Single2 case releases more CO and NOx than Cycling1. It is because that Single2 case has a lower total utilization rate of 80% in fuel cells which means exhaust gas contains more combustible component. Flame temperature of Single2 case is about 2000K whereas it is 1650K for Cycling1 case. High temperature leads to high NOx emission. Besides, Single2 case contains more H2 so that H2O mole fraction is high after burning. Water vapor prevents CO converting to CO2 especially under high temperature, which leads to high CO emission level (Wang et al. 2016). The experiment data above also shows some agreement to that. At this perspective, anode cycling design seems to have a better performance on emission reducing in theory compared with single pass design for no extra steam addition and less H2, though more experiments are needed further. As known, NOx formation increases extremely when temperature exceeds 1800K. It could be concluded that for system design, total fuel utilization percent had better be high enough to make oxygen flame temperature of anode exhaust gas lower than 1800K, which make systems environment friendly.