The whole world is fighting against global warming so as to keep the atmosphere’s CO
2 concentration well below the critical value of 450 ppm [
1]. To meet the requirement of the Paris Agreement [
2], it has been reported that carbon-negative power generation technologies should be developed and deployed [
3]. Bioenergy with carbon capture provides such a transformative route, not only because biomass is carbon-neutral during the thermal conversion process, but also because biomass is the fourth largest fuel source following coal, oil, and gas on the earth [
4]. However, low calorific value is one essential drawback that hinders the direct utilization of biomass [
5]. Converting biomass into high-quality syngas via gasification can well solve this issue. The syngas from biomass can then be used for power generation or for utilization in the steel industry [
6,
7]. Because biomass is hard to grind into fine powder but relatively easy to compress into pellets, a fluidized bed is then the proper reactor for biomass gasification. A dual fluidized bed gasifier (DFBG), which consists of a bubbling fluidized bed gasifier and a fast fluidized bed combustor, can generate high-quality syngas by avoiding direct contact between air and syngas [
8]. However, DFBG cannot capture CO
2 in situ, so carbon-negative gasification cannot be realized. Based on DFBG, a quadruple fluidized bed gasifier (QFBG) is conceptually designed by integrating the chemical looping oxygen generation technology and DFBG [
9]. The chemical looping oxygen generation process is also realized in a dual fluidized bed with Mn
2O
3 and Mn
3O
4 as bed material. Thereby, there are four fluidized bed reactors including a bubbling fluidized bed gasifier for biomass gasification and CaO carbonation, a fast fluidized bed oxyfuel combustor for residual char combustion and CaCO
3 calcination, a bubbling fluidized bed oxidation reactor for Mn
3O
4 oxidation, and a fast fluidized bed reduction for Mn
2O
3 reduction. With the four integrated fluidized bed reactors, biomass can be converted into H
2-rich syngas. Since the bed materials are circulated inside the reactors, the heat absorbed and the heat released can be theoretically balanced. However, due to the formidable complexity of QFBG, related experiments or numerical simulations are seldom reported.
Liu [
10] did the thermodynamic modeling of biomass sorption-enhanced chemical looping gasification, and the energy efficiency of this technology was found to be 64.6%. Pröll [
11] studied the H
2-rich syngas production by selective CO
2 removal during biomass gasification in a 100 kW DFGB using the thermodynamic equilibrium approach and found that H
2 volume fraction in the dry syngas could reach 65–75%, and CO
2 volume fraction was in the range of 6–13%. Koppatz [
12] experimentally studied the calcium-enhanced biomass gasification process in an 8-MW DFBG and found that the H
2 volume fraction in the syngas was about 50%, and the CO
2 volume fraction was about 12.3%. The low H
2 fraction was probably caused by the low temperature (about 650 °C), and high steam partial pressure. Hejazi [
13] once modeled biomass steam gasification in a DFBG with lime-based CO
2 capture using the stoichiometric equilibrium method. It was found that the CO
2 capture rate could be over 70%. Cormos [
14] studied the chemical looping air separation cycle for decarbonized power generation based on oxyfuel combustion in terms of energy and cost. It was found that the manganese looping cycle could be more efficient than the cryogenic process, and could improve the net efficiency by 2–3.5 percentage points. The CO
2 capture penalty was reduced to 5–7 net energy efficiency points. Mei [
15] studied the reactivity and lifetime of an oxygen releasable manganese ore and found that repeatable O
2 gas release was available, and the reactivity and lifetime of Mn ore were better than the often-used ilmenite. Yan [
9] studied the biomass/coal co-gasification properties in a DFBG reactor using a one-dimensional model with the assistance of the commercial software, Aspen Plus. The cold gas efficiency was up to 78.9% under the proposed optimum condition. This model coupled the fluidized bed hydrodynamics and reaction kinetics. However, the model is one-dimensional and the research object is DFBG. Yan [
16] studied the property of QFBG using the commercial software, Aspen Plus. It was found that the H
2 mole fraction in the dry syngas is higher than 70%, the CO
2 mole fraction in the dry flue gas is around 97%, and the net carbon discharge can be negative when the biomass blending ratio is over 0.5. The QFGB property was preliminarily studied. However the research simplified QFGB as one-dimensional, so the predictions cannot reflect the practical situation. Yan [
17] studied the biomass steam gasification process in a DFBG reactor based on the granular kinetic theory (GKT) with the assistance of the commercial software, Fluent. The H
2 mole fraction was predicted to be 46.62%, and the cold gas efficiency was 82.9%. Although this is a three-dimensional simulation, the object is DFBG, which is quite simple compared with QFBG. Moreover, GKT treated the bed material and solid fuel as a fluid phase, which requires that the particle dimension should be uniform rather than dispersed. Yan [
18] also studied the property of the DFBG reactor based on the multiphase particle in cell (MPPIC) method with the assistance of the open-source software, OpenFOAM. The model predictions were compared with experimental data, and the operating characteristics of the DFBG were predicted. The MPPIC method considered all particles as computation parcels and track each parcel in Lagrangian coordinates, so it is more advanced than GKT for fluidized bed simulation. However, the research object was still DFBG rather than QFBG. Pissot [
19] compared four DFBG configurations including heat supply by air combustion, oxyfuel combustion, chemical looping gasification, and electrical thermal, and found that the oxyfuel and the chemical looping gasification scheme exhibited the lowest energy demand for CO
2 separation. This is also the reason why QFBG integrates chemical looping technology. Wang [
20] predicted hydrogen production via chemical looping reforming in a DFBG reactor based on the granular kinetic theory. A bubble-structure-dependent drag coefficient model was proposed and the model predictions were validated against experimental data.
From a literature review, it can be concluded that calcium-enhanced biomass gasification can increase the H2 fraction and decrease the CO2 fraction in dry syngas. However, the stripped carbon cannot be captured if air is used as an oxidizer for the calcination process. To gasify biomass whilst capturing CO2, oxyfuel combustion should be introduced. According to literature reports, the more efficient manganese-based chemical looping air separation approach is chosen and is coupled with the calcium-based DFBG to form the QFBG in this work. Although the QFBG property has been studied with the Aspen Plus platform, the three-dimensional simulation of QFBG has not been carried out. This work aims to further study the physicochemical processes of QFBG with the computational fluid dynamics approach so that the QFBG characteristics can be known better. The processes in QFBG include the fluidization of different particles like limestone, manganese ore, and biomass, heterogeneous reactions like biomass pyrolysis and char gasification, homogeneous reactions, and the coupling of hydrodynamics with chemical reactions. There is currently no solver that can simulate such a complex process, so a new solver based on OpenFOAM is proposed and validated in this work.