**1. Introduction**

Global greenhouse gas (GHG) emissions have been increasing exponentially for the past 60 years, mostly due to the use of oil, coal, and natural gas [1,2]. Hereby, CO2 is the major GHG, accounting for 65% of the total amount. Replacing the usage of fossil fuels in combustion or gasification processes with biomass enables the reduction of CO2 emissions.

Sorption enhanced gasification (SEG) has been considered a promising technology for tailored syngas production with in situ CO2 capture. The plant configuration of a SEG process is based on the conventional steam gasification process carried out in indirect heated (or allothermal) dual fluidized bed systems. This conventional steam gasification process uses an inert bed material as a heat carrier to deliver the heat for the endothermic gasification, enabling N2-free syngas with typical hydrogen content of 20–40 vol% (on a dry basis). The syngas of this process can be used for power and heat generation, and the technology has already been applied in a number of industrial-scale facilities with thermal power ranging from 2–20 MW [3–5]. Accordingly, a number of different models describing the steam gasification of biomass have been developed and published [6–8].

Gordillo and Belghit [9] developed a two-phase model for steam gasification of biomass char which considers hydrodynamic phenomena; however, pyrolysis was not included, which is an important step in the biomass gasification process. Agu et al. [10] proposed a detailed one-dimensional model for steam gasification in a bubbling fluidized bed, which considers a Lagrangian approach for solid fuel particles extending the often used assumption of a uniform distribution of fuel particles. In contrast to conventional steam gasification, the SEG uses a reactive bed material (e.g., limestone) as the heat carrier, which enables the in situ capture of CO2 in the gasifier. This CO2 capture a ffects gasification reactions and shifts hydrogen concentrations in the syngas up to 75 vol% (on a dry basis) [11]. Thus, SEG has been considered to be a suitable process for the synthesis of hydrogen, transport fuels, and chemicals [12,13]. Hereby, models are important in finding the best operation strategies for the gasifier, especially if the gasifier is part of a complex production path, including mass and energy integration. However, few models exist which have considered the CO2 sorption characteristics in fluidized bed reactors. Inayat et al. [14] and Sreejith et al. [15] used a detailed approach based on reaction kinetics, but hydrodynamics were not in focus. In the work of Hejazi et al. [16,17], a model to describe sorption enhanced gasification in a bubbling fluidized bed was presented. This model considers hydrodynamics in the fluidized bed, but evaluations of process parameter over reactor height were not included due to the assumption of a uniform temperature throughout the dense bed. Another comprehensive SEG model has been published by Pitkäoja et al. [18]. This model considers a transport disengaging height to describe the amount of reacting particles in the freeboard, which is important for the overall syngas composition. However, minor focus is placed on the bed activity, which depends (in a dual fluidized system) on the mean particle residence time, as well as the carbonation/calcination cycles. Considering the issues discussed above, we developed a detailed SEG model focusing on various key aspects, including (i) hydrodynamic features of the fluidized bed reactor and evaluation of the process parameters over reactor height; (ii) kinetic aspects of the steam gasification process; and (iii) reaction kinetics of CO2 capture through carbonation in the fluidized bed, as well as by elutriated particles in the freeboard. As deactivation of the sorbent is an important issue in a SEG process, a deactivation model is included in this work.

#### **2. SEG Process Description and Model Development**

#### *2.1. 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 CO2 from the syngas through the carbonation reaction (CaO + CO2 → CaCO3). 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 CO2 partial pressure. Hence, the grade of CO2 capturing can be set by di fferent operating conditions. Adjustment of syngas composition by selecting gasification temperature is a grea<sup>t</sup> advantage of the SEG process. Poboss [19] reported that there is a relatively high CO2 capture e fficiency below 650 ◦C. For higher temperatures, the capture e fficiency decreases rapidly. Based on the CO2 volume concentration in the wet syngas, the CO2 capture ceases at a temperature of 750–770 ◦C [19]. To a certain extent, operation above 750 ◦C and without CO2 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 di fference 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 ◦C. Thus, in a limestone-based process, the maximum gasification temperature is fixed at about 850 ◦C; if gasification is to take place at higher temperatures, other processes must be considered. The temperatures in the regenerator of 850–950 ◦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 (CaCO3 → CaO + CO2). If required, extra fuel can be added into the regenerator.

**Figure 1.** Schematic diagram of sorption enhanced gasification (SEG) process (up to 750 ◦C) and extended steam gasification mode (up to 850 ◦C); option of oxy-fuel operation.

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 ◦C was investigated by means of simulations.

The syngas produced from the SEG process is N2-free and lean in CO2 and, therefore, has a high calorific value. Experimental results on SEG have shown that, at low gasification temperatures (around 600 ◦C), the CO2 absorption is high and a H2 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. In this case, the flue gas contains no nitrogen but mostly consists of CO2, which can be stored or utilized [25].

#### *2.2. Development of a Dual Fluidized Bed Gasifier Model*

In Figure 2, the SEG modeling concept considered in this work is illustrated. The proposed model is based on details of the 200 kW dual fluidized bed (DFB) facility at the IFK. It consists of a bubbling fluidized bed (BFB) gasification reactor and a circulating fluidized bed (CFB) regenerator reactor. The gasifier model was discretized along the reactor length, in order to calculate all relevant transport values and for model validation purposes, to compare with the underlying experimental data. The regenerator is modeled in a simplified way, considering mass and energy balances as well as combustion and calcination reactions, in order to enable the coupling with the gasifier through the circulating solid.

**Figure 2.** Modeling concept of SEG in a dual fluidized bed system.
