## 6 Summary and Conclusions  
As mentioned before, the newly introduced E-STR gasifier [3] features low-temperature and high-pressure operating conditions, aiming to realize a higher H\({}_{2}\)/CO ratio of syngas for SNG production. However, information about the design characteristics or operation data has been barely available in public domain at present; hence, establishing a computational model for revealing the flow/thermal/chemical insights of the new gasifier can be a useful tool. Due to its high-pressure and low-temperature operation condition, tars will form inside the E-STR gasifier. Thus, the objective of this study is to modify the previously developed CFD model of E-GasTM gasifier, and implement additional mechanisms to systematically mimic related design characteristics, and further implement volatiles condensation and re-vaporization sub models into the modified CFD model to properly address the phenomena inside E-STR. For high operating pressure condition, different fuel and oxidant feeding distributions are compared. The results are summarized below. There are different flow and thermal patterns that are observed in the vicinity of the second-stage injector, due to unlike injection arrangements; however, with sufficiently large volume of the gasifiers and resident times, similar syngas compositions and outlet temperatures are achieved between E-Gas and E-STR without char recycling. Higher interior temperature is obtained in E-STR because all coal slurry is fed at the 2 nd stage rather than being separately injected as in an E-Gas gasifier. This also leads to longer particle residence time at the second stage for E-STR. Since detailed operation data have not yet been available, the recycling and reaction rates of char were investigated to search for reasonable operating characteristics, based on the chemical reactions built in the present CFD model. It seems that reasonable performance characteristics can be achieved in the case of 15% char recycle, which was then selected for subsequent studies. Incorporating the volatiles condensation-only sub-model results in the formation of about 6.47% liquid volatiles and an increase of outlet temperature about 135 K, due to the release of latent heat. When the re-vaporization model is incorporated together with the condensation model, under the test condition, the liquid volatiles re-vaporize with an amount depending on the set point of evaporation temperature. It is thus concluded that the volatiles condensation and re-vaporization sub-models have been successfully developed and implemented. These sub-models will be useful under the condition that the gasifier temperature is intentionally kept low, like in the E-STR gasifer. Attempts have been made to reduce the reactor temperature by changing the fuel feeding mass distribution between the first and second stages, as well as changing the oxygen/coal ratio. By introducing 20% coal-slurry at the 1 st stage and feeding the recycled char at the 2 nd stage, the temperature is reduced near the 1 st injector but is increased at the 2 nd stage. Under the studied condition, the present model can't predict reduced temperature inside the E-STR gasifier as claimed by the manufacturer [3]. By reducing the oxygen/coal ratio from 0.92 to 0.46 (i.e., with a 50% reduction), the temperature inside the gasifier increases near the 1 st stage but reduces to a lower temperature level for the rest of the gasifier, with a reduction of exit temperature from 1463 K to 1100 K.  
When the higher operational pressure is employed (70 atm), the temperature increases at the 2 nd stage. The mole fractions of both H\({}_{2}\) and CO in the exit syngas as well as HHV increase, but the outlet temperature maintains about the same. In summary, it is shown that a CFD model has been modified from its precedent and implemented with the volatiles' condensation and re-vaporization sub-models to study the gasification process in the newly developed E-STR gasifiers.