CO₂ gasification of biogenic fuels in a dual fluidized bed reactor system

In November 2016, the European Union published a recast of the Renewable Energy Directive–Recast to 2030 (RED II) to strengthen the awareness of the countries regarding the mitigation of the effects of the climate change. RED II targets the renewable energy consumption in Europe up to 32% by 2030. Additionally, fuel supply companies have to replace a minimum of 14% of the energy consumed in road and rail transport by renewable energy carriers until 2030. Fuels used in the aviation and maritime sectors can contribute to the 14% transport target but are not obliged [1]. Based on early estimates carried out by Eurostat, the global CO₂ emissions increased by 1.4% to 32.5 Gt in 2017 [2]. Due to the fact that CO₂ emissions account for about 80% of the overall greenhouse gas emissions [3], negative values of CO₂ emissions are required to mitigate the climate change effects. This was also reported by IPCC in 2014 [4]. For this purpose, the development of efficient technologies with the aim of recycling CO₂ within the processes is necessary. In this way, it is possible to influence the closing of the overall carbon cycle in a positive way and reduce CO₂ emissions at the same time. This idea of a closed carbon cycle with CO₂ reuse within the process can be traced back to the 1970s. At that time Steinberg et al. already investigated the use of nuclear power-generated electrolytic hydrogen and oxygen to convert CO and CO₂ recovered from industrial processes to synthetic methanol [5]. Nowadays, the thermochemical process “biomass gasification” could be a renewable option for realizing the idea mentioned above. Through the gasification of biomass and the recycling of produced CO₂ within the process, a high-valuable product gas can be generated, which could be further upgraded to synthetic fuels. This work focuses on the dual fluidized bed (DFB) CO₂/steam biomass gasification (see Fig. 1), which has a great potential in this field and is therefore a main research topic at TU Wien since many years. So far, only steam was used as gasification agent in the DFB pilot plant at TU Wien. Therefore, the use of CO₂ instead of steam presents a novel possibility of reducing CO₂ emissions within the DFB process and producing a CO-rich product gas. The following points make the use of CO₂ as gasification agent in the DFB biomass gasification process attractive:

Besides the opportunity to recycle CO₂ within the DFB gasification process, it is also possible to convert alternative, low-grade fuels in the DFB pilot plant. In this way, the conventional biomass gasification of wood can be shifted to a sustainable gasification process and contribute to a sustainable energy engineering. By the use of alternative, low-grade fuels, the life cycle of these materials can be increased and the CO₂ emissions reduced. Therefore, this research focuses on the reuse of alternative, residual materials and the conversion to high-valuable products by reusing CO₂ within the process. Extensive investigations dealing with fuel variations in the DFB gasification system using steam as gasification agent were already carried out successfully at TU Wien [8‐10]. However, the gasification of biogenic, low-grade fuels with the use of CO₂/steam mixtures as gasification agent has not been investigated so far in the DFB reactor system. Therefore, investigations dealing with the influence of CO₂/steam gasification of bark, rapeseed cake and lignin on the product gas (PG) composition are presented within this paper. Softwood was used as reference fuel. In Figs. 1 and 2, the DFB gasification system, which was used for the test campaigns, is shown. It consists of a gasification reactor (GR; blue rectangle) and a combustion reactor (CR; red rectangle). The two reactors are connected by loop seals (horizontal arrows). A bed material circulates between the GR and the CR across the loop seals. On the one hand, the bed material serves as a heat transport agent, which transports the produced heat by burning residual char from the CR to the GR. In this way, the endothermic gasification reactions can take place. On the other hand, it acts as a catalytically active material to enhance the ongoing chemical reactions. The GR is visually divided in (i) a lower part, where devolatilization and gasification takes place and (ii) an upper part, where gasification and reforming reactions occur. In the GR, the solid fuel and the gasification agents such as CO₂ and steam are introduced. In the CR, air and optional additional fuel are introduced. The optional additional fuel compensates the heat losses in the pilot plant and enables temperature control during the experiments. The main product of the GR named product gas is composed of carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), ethylene (C₂H₄), steam (H₂O) and other minor components. The flue gas of the CR contains mainly CO₂, H₂O, N₂ and O₂. The product gas of the GR can further be used in different syntheses to produce fuels such as FT fuels [12, 13], chemicals or mixed alcohols [14].

On the left side of Fig. 1, relevant chemical reactions during CO₂/steam gasification are displayed. The heterogeneous gas-solid reactions (Eqs. 5–7) are mainly dominant in the lower gasification reactor and the homogeneous gas-gas reactions (Eqs. 1–4) mainly prevail in the upper GR. Eq. 1 displays the water gas shift (WGS) reaction, which is considered as the most essential homogeneous gas-gas reaction in gasification processes. In Eq. 2, the deviation from WGS equilibrium pδ_eq,WGS is presented. The equilibrium constant K_p,WGS(T) was calculated using the software tool HSC [15]. A value of zero of pδ_{eq, WGS} would mean that the equilibrium state of the WGS reaction is reached, whereas a deviation from zero implies a deviation from the equilibrium state. A negative algebraic sign indicates that the actual state is on the side of the reactants. This means that a further reaction is thermodynamically possible. However, if the algebraic sign is positive, the actual state is on the side of the products. This state can thermodynamically not be reached via the WGS reaction alone, but by, e.g., intermediate products of the devolatilization of higher hydrocarbons. Kuba et al. found out that the WGS reaction is a follow-up reaction of the steam reforming reaction, where part of the produced CO during steam reforming is converted into H₂ and CO₂ in the presence of H₂O. [16] Eq. 3 describes the endothermic steam reforming of hydrocarbons and is especially important for steam gasification. In contrast, Eq. 4 presents the endothermic dry reforming reaction, which decomposes hydrocarbons with CO₂ to CO and H₂. When CO₂ is used as gasification agent, a second reaction, the endothermic Boudouard (BOU) reaction, plays a crucial role. This was also stated in literature [17]. The Boudouard reaction displayed in Eq. 5 is a heterogeneous solid-gas reaction and aims the production of CO via the reaction of char from pyrolysis with CO₂. In Eq. 6, the logarithmic deviation from the Boudouard (BOU) reaction pδ_{eq, BOU} is shown. The equilibrium constant K_p,BOU(T) was calculated by the use of the software tool HSC [15] as well. When pδ_{eq, BOU} is 0, the Boudouard reaction is in equilibrium. When pδ_eq,BOU > 0, the state of equilibrium lies on the product side, whereas when pδ_eq,BOU < 0, the equilibrium is located on the reactants side. The heterogeneous water gas reaction is shown in Eq. 7 and describes the production of CO and H₂

The fact that there is limited or nearly no literature dealing with CO₂/steam gasification of different biogenic residues makes the investigations presented in this publication unique. Most of the existing literature covers the reaction of char with CO₂ and steam through thermogravimetric analyses. For example, Butterman et al. found out that the carbon conversion was complete, when a mixture of 25/75 vol% of CO₂/steam was used compared to a conversion of 90% when pure steam was introduced in a temperature range of 800 – 1000 °C. They assumed that CO₂ enhanced the pore structure, particularly the micropores of the residual carbon skeleton after drying and devolatilization. This facilitated the efficient CO₂ gasification of the solid [6]. Huang et al., Mitsuoka et al. and Habibi et al. reported that alkali metals in the biomass ash enhanced the reactivity of char for CO₂ gasification catalytically [18‐20]. Zhang et al. also revealed that the critical factor for an improved char reactivity under mixed CO₂/H₂O atmosphere was the catalytic activity of alkali metals like Na [21]. Cheng et al. discovered that the CO₂ amount in the gasification agent, the CO₂ to biomass ratio, the size of the woodchips as well as the moisture content of the woodchips had a great impact on the gasification behaviour. The cold gas efficiency as well as the CO₂ conversion rate were both decreased with increasing moisture content and particle size. [22] Another researcher, Ren et al., considered the gasification temperature as the most significant parameter which affects the gasification reactivity of char and controls the gasification rate [23]. These findings and hypothesis were used for comparison with the generated results within this publication.

Table 3 presents the main operational parameters for the steady state operation of the presented test campaigns 1 to 5. SW was gasified with pure olivine as well as a mixture of olivine and limestone to have a reference case for the gasification of the biogenic fuels and to be able to compare the results to pure steam gasification tests. Olivine forms Ca-rich layers, which improve the catalytic activity, via the interaction of the bed material particles with the woody biomass ash on the surface of the particles after days of operation [43, 44]. Additionally, Wagner et al. [45] and Kuba et al. [16] found out that also the fuel ash has an impact on the product gas composition due to an increased catalytic activity caused by layer formation. However, for the presented test campaigns, it was not possible to run the pilot plant continuously for a few days to simulate the operation of an industrial-sized gasification plant. Therefore, limestone was added to olivine as bed material to simulate the bed material after days of operation for the biogenic fuels SLI and BA with low ash contents. For RSC, which showed a higher ash content, only olivine was used as bed material.

The fuel power introduced into the GR (P_GR) for all test campaigns was in a relatively similar range of 81 to 93 kW. The amount of additional fuel, which was introduced into the CR (P_CR) to control the gasification temperature and to compensate the relatively high heat losses of the pilot plant was about 55 kW with a maximum for test campaign 3 of 71 kW. To enable a comparison of these test campaigns with test campaigns, where pure steam was used as gasification agent, a ratio between P_CR and P_GR was calculated. For pure steam gasification test campaigns, a P_CR/P_GR ratio of around 0.5 is a typical value, but it depends on the type of fuel introduced into the GR as well as the operating parameters [8, 28]. For the presented test campaigns, where a mixture of CO₂ and steam was used as gasification agent, increased P_CR/P_GR ratios were calculated. This implies that the CO₂ gasification requires more heat (see Eq. 14) than pure steam gasification (see, Eq. 15). The CO₂/H₂O ratios of the gasification agent introduced into the GR were kept quite similar for all test campaigns. The temperatures in the lower GR, “T GR_lower^a”, the temperatures in the upper GR, “T GR_upper”, and the temperatures at the outlet of the CR, “T CR_outlet”, are displayed in Table 3 for all test campaigns. The measurement positions of the temperatures are shown in Fig. 2. The CO₂ to fuel ratios φ_CO2F as well as the CO₂ to carbon ratios φ_CO2C were in a range of 1.1 to 1.3 kg_CO2/kg_fuel and 1.8 – 2.1 kg_CO2/kg_C. The steam to fuel ratios φ_SF were between 0.3 and 0.5 kg_H2O/kg_fuel and the steam to carbon ratios φ_SC ranged between 0.6 and 0.9 kg_H2O/kg_C. These values account for the fluidization of the loop seals with steam. The H/C ratios were 0.09 and 0.10 for the fuels BA and SLI due to their high amount of C. For SW and RSC, a similar H/C ratio of 0.12 was calculated. The O/C ratios were in the same range for SLI, BA and RSC, but higher for SW, because SW exhibited the highest share of O.

The CO₂/steam gasification of biogenic fuels was conducted successfully in the 100 kW_th pilot plant at TU Wien. Five experimental campaigns were conducted using bark, lignin and rapeseed cake as fuel. Softwood was used as reference fuel. Pure olivine and mixtures (90/10 wt%) of olivine and limestone were used as bed material. The following results could be generated:

To sum up, for the formation of a high content of CO in the product gas, fuels with a high amount of C and increased contents of alkali metals in the biomass ash are recommended. Additionally, higher temperatures enhance the conversion efficiency. A possible application for a CO-enriched product gas could be the dimethyl ether (DME) synthesis to produce fuels and chemicals. Thereby, a H₂/CO ratio of ~ 1 is necessary [51]. However, the synthesis gas can be used in other upgrading processes, where higher or lower H₂/CO ratios are required, as well. The huge advantage of the DFB gasification reactor system is the possibility to adjust the H₂/CO ratio in a broad range, which enables to use the synthesis gas in a lot different of applications. To investigate the CO₂ gasification in the DFB reactor system in more detail, future research should focus on the gasification using pure CO₂ as gasification agent and the influence of gasification temperature.