# A Detailed One-Dimensional Hydrodynamic and Kinetic Model for Sorption Enhanced Gasification
## 2 SEG Process Description and Model Development  
### Description of the SEG Process  
Based on availability of experimental data, atmospheric conditions were considered for sorption enhanced gasification (SEG). In this work, calcined limestone (CaO) was used as reactive bed material to capture CO\({}_{2}\) from the syngas through the carbonation reaction (CaO + CO\({}_{2}\)\(\rightarrow\) CaCO\({}_{3}\)). This shifts gasification and water-gas shift reactions towards hydrogen production. The carbonation reaction proceeds until the equilibrium condition is reached, where the equilibrium depends on the temperature and CO\({}_{2}\) partial pressure. Hence, the grade of CO\({}_{2}\) capturing can be set by different operating conditions. Adjustment of syngas composition by selecting gasification temperature is a great advantage of the SEG process. [PERSON] [19] reported that there is a relatively high CO\({}_{2}\) capture efficiency below 650 \({}^{\circ}\)C. For higher temperatures, the capture efficiency decreases rapidly. Based on the CO\({}_{2}\) volume concentration in the wet syngas, the CO\({}_{2}\) capture ceases at a temperature of 750-770 \({}^{\circ}\)C [19]. To a certain extent, operation above 750 \({}^{\circ}\)C and without CO\({}_{2}\) separation can also be achieved with a dual fluidized bed system by increasing the solids circulation rate. The concept of SEG, with its two fluidized bed reactors, is illustrated in Figure 1. The energy needed for the endothermic gasification reactions in gasifier is supplied by the highly exothermic CaO carbonation reaction and by the sensible heat of circulating solids flowing from the regenerator (combustion reactor). This means if the temperature in the gasifier exceeds the range for carbonation reaction, the process is only driven by the temperature difference between regenerator and gasifier. However, the temperature of the regenerator cannot be set arbitrarily high since otherwise sintering of CaO will increasingly occur, resulting in a loss of activity [20]. To avoid sintering significantly, the maximum regenerator temperature should be below 950 \({}^{\circ}\)C. Thus, in a limestone-based process, the maximum gasification temperature is fixed at about 850 \({}^{\circ}\)C; if gasification is to take place at higher temperatures, other processes must be considered. The temperatures in the regenerator of 850-950 \({}^{\circ}\)C are obtained by the combustion of unreacted char contained in the solids leaving the gasifier and flowing back to the regenerator. At these temperatures, the sorbent is regenerated through the endothermic calcination reaction (CaCO\({}_{3}\)\(\rightarrow\) CaO + CO\({}_{2}\)). If required, extra fuel can be added into the regenerator. The fluidized bed gasifier consists of a fluidized bed and a freeboard. Since main gasification reactions are located close proximity to solids, the gasification temperature is referred to as the average temperature in the fluidized bed. In this work, a gasifier temperature between 600-850 \({}^{\circ}\)C was investigated by means of simulations. The syngas produced from the SEG process is N\({}_{2}\)-free and lean in CO\({}_{2}\) and, therefore, has a high calorific value. Experimental results on SEG have shown that, at low gasification temperatures (around 600 \({}^{\circ}\)C), the CO\({}_{2}\) absorption is high and a H\({}_{2}\) fraction on the order of 70-75 vol\(\%\) (on a dry basis) can be reliably reached [11; 21; 22]. There is also an additional advantage associated with the use of CaO as a bed material: CaO-based bed materials are known to be catalytically active towards tar cracking. Therefore, despite the lower gasification temperatures used in the SEG process, it has been experimentally demonstrated that tar production can be up to 5 times lower than that in classical (steam) fluidized bed gasification processes without CaO [23; 24]. Apart from combustion with air, the regenerator can also be operated with pure oxygen and recirculated flue gas, known as the Oxy-SEG process.