Oxygen Carrier Aided Gasification (OCAG)

Abstract

Gasification is regarded as an effective clean utilization technology of solid fuel, which can convert the chemical energy of solid fuel into gaseous fuel. However, the primary gas products contain not only the essential gas products, but also an unacceptable amount of tars, which will cause operational problems such as blockage of downstream equipment during gasification. Catalysts are often used after the gasifier to catalyze tar in the pyrolysis product gas. However, the activity ofcatalysts generally declines over time, as they will be poisoned by prolonged exposure to an atmosphere containing elements such as sulfur, chlorine and alkali metals. In addition, under the condition of high tar content, carbon deposition may form on the surface of catalyst, which leads to the deactivation of catalyst. The oxygen carrier particles of natural ores not only can transport oxygen, but also contain various metal elements that can be used as catalysts for tar cracking. Introduce the OCs to replace inert bed materials may not only provide a cheap catalyst for the technology, but also complete the transfer of oxygen between the two reactors, this process is oxygen carrier aided gasification (OCAG).

Gasification is regarded as an effective clean utilization technology of solid fuel (especially for coal, biomass and solid waste), which can convert the chemical energy of solid fuel into gaseous fuel [1,2,3,4]. However, the primary gas products contain not only the essential gas products (such as CO, H₂, CO₂, CH₄ and light hydrocarbons), but also an unacceptable amount of condensable hydrocarbons, which are often referred to as tars. The tars will begin to condense at 350 °C [5], and will cause operational problems such as blockage of downstream equipment during gasification.

Tars are included in a comprehensive classification of complex mixtures of hydrocarbons from 1 to 5-ring aromatic compounds, oxygen and sulfur-containing hydrocarbons [1, 4]. The presence of tar components preclude raw gas from being used directly as fuel for gas turbines and internal combustion engines, or as a chemical feedstock. Consequently, the primary gas after gasification needs to be upgraded to gaseous products suitable for combustion and chemical applications. At present, a large number of research and review papers [3, 6,7,8,9,10] have been published in the field of tar removal and gas product regulation, among which the primary and secondary techniques associated with catalytic tar reforming are considered as one of the most promising hot gas cleaning methods. The primary tar cleaning has been included in the existing gasification process through the catalytic bed material to promote tars cracking, and the secondary tar cleaning needs to be carried out on the fixed bed or fluidized bed downstream of the gasifier. These can significantly reduce tar content and improve the flexibility of process optimization and gas regulation [1, 8, 10,11,12]. However, the activity of catalysts generally declines over time, as they will be poisoned by prolonged exposure to an atmosphere containing elements such as sulfur, chlorine and alkali metals (which are all present to varying degrees in the raw gas) [10]. In addition, under the condition of high tar content, carbon deposition may form on the surface of catalyst, which leads to the deactivation of catalyst [10, 13, 14].

5.1 The Proposed of Technical Routes

In 2011, Fredrik et al. [15, 16] proposed a new technique route for secondary catalytic tar cleaning (as shown in Fig. 5.1), which is performed in a dual fluidized bed reactor system that simultaneously removes carbon deposits on the catalyst surfaces, even at high tar contents. The new process can be applied to all types of biomass gasifiers, regardless of whether the gasifier technique includes primary measures for tar reduction or not.

Fig. 5.1

New concept for secondary upgrading of biomass producer gas (reprinted from [16] with permission of American Chemical Society)

The system in Fig. 5.1 consists of two reactors and two loop seals. The two reactor, one is regenerator reactor, also referred to as the air reactor (AR), the fluidizing agent is air; another is reformer reactor, also called the fuel reactor (FR), which is fluidized by the raw gas. The two loop seals are fluidized with inert gas or steam to ensure the circulation of the OC and prevent cross contamination of the gases in different reactor. In the FR, the metal oxide (Me_xO_y) as an oxygen supplier, catalyst and heat carrier for partial oxidation reaction and reforming reactions of the tar components. The part or all of the Me_xO_y is reduced to Me_xO_y-1 by hydrocarbons (C_iH_j), H₂, and CO [16]:

$$ \begin{aligned} {\text{C}}_{\text{i}} {\text{H}}_{\text{j}} + n{\text{H}}_{2} & + z{\text{CO }} + \left[ {2i + 0.5 \, \left( {j + z} \right) + n)} \right]{\text{Me}}_{\text{x}} {\text{O}}_{\text{y}} \\ & = \left( {i + z} \right) \, CO_2 + \, \left( {0.5j + n} \right) \, H_2 O \, + \, [2i \, + \, 0.5\left( {j + z} \right) \, + \, n]{\text{Me}}_{\text{x}} {\text{O}}_{{\text{y}} - {1}} \\ \end{aligned} $$

(5.1)

The Me_xO_y−1 in the bed acts as a catalyst for reforming the tars (C_nH_m) in the presence of reforming media (such as H₂O and CO₂ in the raw gas), the reactions can be simplified as [16]:

$$ {\text{C}}_{\text{n}} {\text{H}}_{\text{m}} + {\text{Me}}_{\text{x}} {\text{O}}_{{\text{y}} - {1}} + {\text{H}}_{2} {\text{O}} + {\text{CO}}_{{2} } = {\text{C}}_{\text{i}} {\text{H}}_{\text{j}} + {\text{H}}_{2} + {\text{CO}} + {\text{Me}}_{\text{x}} {\text{O}}_{{\text{y}} - {1}} + {\text{C}} $$

(5.2)

In addition to the desired tar-reforming reaction, the carbon generated by the additional carbonization reaction will be deposited on the surface of OC particles. While the Me_xO_y−1 moves into AR, Me_xO_y-1 will be oxidized to Me_xO_y, and the carbon on the surface of OC particles will be eliminated by oxidation. Both of these reactions are exothermic. At this point, OC particles as a heat carrier will bring heat to FR for the catalytic cracking of tar components. The regeneration of oxygen carriers and oxygen transport can effectively remove the tar, which also provides the possibility for the development of new catalytic bed materials. This technology can achieve many advantages: (1) even using natural oxygen carrier ore as the bed material can still significantly reduce the tar content in the product gas. (2) the deactivation of catalyst caused by carbon deposition is avoided and the reaction efficiency of the system is improved; (3) widened the selectivity of OC or catalyst, natural ore or synthetic catalyst can be selected according to product gas demand, providing the possibility to reduce catalyst cost; (4) more flexible product gas regulation capacity; (5) the tar cleaning system can be thermally integrated with the gasifier outlet temperature, which leads to minimal heat losses.

Vienna University of Technology [17,18,19,20,21] has proposed a dual fluidized bed (DFB) gasification technology (the basic principle is shown in Fig. 5.2) to reduce the tar in product gas, and designed and built a 100 kW_th pilot plant (the schematic diagram is shown in Fig. 5.3). Gasification reaction (in a bubbling bed) and combustion process (in fast fluidized bed) take place in separate reactors, which are thermally connected by a circulating bed material. The fuel is fed into the gasification reactor to react with the gasification agent to form product gas, and the residual char is transported to the combustion reactor along with the bottom bed material. The char is burned in an air reactor and releases heat to heat the bed materials, then hot bed material is captured by the separator and returned to the gasification reactor to provide heat for the gasification reaction (endothermic) [21]. The two reactors produce two different flows: product gas and conventional flue gas. In this technology route, the bed material is used to transfer heat, and when the catalyst is added to the bed material, it can significantly promote the cracking of tars while avoiding the carbon deposition problem of the catalyst.

Fig. 5.2

The schematic diagram of the DFB gasification technology (reprinted from [21] with permission of Elsevier)

Fig. 5.3

The schematic diagram of Vienna University of Technology’s 100 kW_th pilot plant (reprinted from [21] with permission of Elsevier)

The oxygen carrier particles of natural ores (such as ilmenite ore and manganese ore) not only can transport oxygen, but also contain various metal elements that can be used as catalysts for tar cracking. Therefore, introduce the OCs to complete or partial replace inert bed materials may not only provide a cheap catalyst for the technology, but also complete the transfer of oxygen between the two reactors. It should be noted that the activity of OC ore as a catalyst may be significantly lower than that of synthetic catalysts, but synthetic catalysts in operation are generally expensive and the proportion of catalyst added in the bed is generally low, while the addition of OC ore can be up to 100%. Increasing the concentration of OC ore may make up for the relatively poor catalytic activity. Therefore, the diversity of the selection of bed material combination provides enterprises with the possibility of efficient and low-cost operation, which greatly improve the flexibility of operation.

5.2 Gasification/Reforming Characteristics

5.2.1 OC Aided Tar Reforming

Lind et al. [15] used ilmenite (FeTiO₃) as the bed material to perform reforming experiments on the raw gas produced by the biomass gasifier, and found that oxygen can be continuously transported by the ilmenite in the system, and verified that the tar-cleaning concept can be achieved. The results show that when the tar content in the produced gas is about 30 gtar/Nm³, the gas residence time of 0.4–0.5 s reduces the total amount of tar by 35% at 800 °C (the tar compositions in raw gas and reformed gas downstream of the CLR-system are shown in Table 5.1). Branched tars and phenol were largely converted to pure aromatic compounds. Ilmenite shows high activity in the water–gas shift reaction (WGSR), the H₂/CO ratio increases from about 0.7 at the reactor inlet to about 3 at the reactor outlet, while a decrease in light hydrocarbons is observed, and the methane content downstream of the reactor system slightly increased (as shown in Fig. 5.4). In addition, carbon deposits on ilmenite were continuously removed by oxidation to CO₂. During the operation, inactivation of ilmenite or interference with oxygen transfer was not observed. After that, Lind et al. [16] compared the effects of two kinds of metal oxides, natural ore (ilmenite ore) and synthetic catalyst (NiO/AL₂O₃), on the upgrading of gasification product gas at operating temperature from 700 to 880 °C. The results showed that both materials have activity against tar decomposition and increase the yield of hydrogen, and the removal of tar and the generation of hydrogen were significantly enhanced with increasing temperature. Although the tar loading in the feed gas is as high as 30 gtar/Nm³, all phenolic compounds and most monocyclic branched tar are decomposed by two catalysts at 800 °C. Based on the calculation of tar to reformed gas, the tar removal efficiency with NiO/AL₂O₃ catalyst and ilmenite ore as bed material were 95% (880 °C) and 60% (850 °C), respectively (as shown in Fig. 5.5). In addition, the NiO/AL₂O₃ catalyst has rather high activity for tar removal already at 700 °C. For the ilmenite catalyst, compared to the increase in conversion when the temperature increased from 750 to 800° C, the total tar removal increased by 4 times as the temperature increased from 800 to 850 °C. In general, ilmenite ore have lower activity as tar-cleaning catalysts than NiO/AL₂O₃ catalysts. Analysis of the effluent stream from the regeneration reactor found that the carbon deposits had been removed from the catalyst, and no deactivation due to coke deposition was detected during the 8-h operation of the system. However, in the above studies, it was found that OC’s strong ability to transfer oxygen would lead to a decrease in cool gas yield, which could be adjusted by selecting the appropriate oxygen carrier and adding amount.

Table 5.1 Compositions in raw gas and reformed gas downstream of the CLR-system (adapted from [15])

Fig. 5.4

Changes in gas composition between reformed gas and raw gas (reprinted from [15] with permission of American Chemical Society)

Fig. 5.5

a Percentages of total tar removal in gas upgrading for NiO/Al₂O₃ and ilmenite (1.0% and 2.2% O₂), b Percentages of total tars decomposed by NiO/Al₂O₃ and ilmenite (1.0% and 2.2% O₂) (reprinted from [16] with permission of American Chemical Society)

Berguerand et al. [22] evaluated the upgrading effect of alkali-feldspar [(K, Na)AlSi₃O₈] ore as the bed material on gasification product gas in a single bubbling bed reactor. The results indicated that the catalytic cracking capacity of alkali-feldspar ore is higher than that of fresh olivine. Even at low temperature, alkali-feldspar ore can eliminate most of C₂H₂ and C₃H₆ (the most problematic compounds in fuel synthesis), and improve the water–gas shift reaction (as shown in Fig. 5.6). However, alkali-feldspar ore as bed material not only cannot reduce the methane concentration in the gas, but also can form methane. Alkali-feldspar ore exhibit a remarkable tar selectivity, which resulted in the reformed gas having exclusively pure ring-compounds. Moreover, the alkali-feldspar ore shows good mechanical properties and is suitable for fluidized bed, and displayed neither bed agglomeration nor loss of activity in a longer reducing periods, despite there was a carbon deposit (which can be easy to oxidized to CO₂). Furthermore, because (1) even high sulfur content (>100 ppm) in raw gas, the catalytic activity towards hydrocarbon reforming of alkali-feldspar ore is still unaffected; (2) alkali-feldspar ore has a really low oxygen capacity, it is a promising material and very suitable for the application of this technical.

Fig. 5.6

Molar yields of permanent gases for the dry raw and reformed gases (reprinted from [22] with permission of Elsevier)

5.2.2 OC Aided Gasification

Koppatz et al. [23] investigated the steam gasification behavior of biomass in a 100 kW_th dual fluidized bed (DFB) reactor system by using olivine and silica sand as bed material. The experimental results found that there is a significant difference in the activity of silica sand and olivine. The application of different bed materials will cause significant change in composition of the product gas, the relevant results as shown in Fig. 5.7. The volume percentages of H₂, CO, and CO₂ changed by about 5%, 10%, and 10%, respectively. Olivine is thought to increase the product gas yield and H₂ yield by promoting the CO shift reaction. In addition, it was also found that the application of olivine can change the yield and composition of tar. Compared with silica sand as bed material, the GC/MS detectable tar and grav. detectable tars yield of product gas in olivine case were reduced by about 35% and 60% respectively. By analyzing the condensable hydrocarbons (see Table 5.2), the result of tar groups distribution at 850 °C as shown in Table 5.3. It can be found that the relative fractions of primary, secondary and tertiary tars are not shifted, and 40–45% (olivine) and 31–33% (silica sand) of naphthalene was found in GC/MS tar complex. This is because under olivine condition, the higher hydrocarbons can be rapidly catalyzed to decompose and lead to the formation of Class IV tars, while silica sand has no catalytic effect on this process.

Fig. 5.7

Gas composition (main components) versus different solid inventories (reprinted from [23] with permission of Elsevier)

Table 5.2 Detected GC/MS species and grouping according to ECN and Milne et al. classification (adapted from [24,25,26])

Table 5.3 Distribution of tar groups at a temperature of 850 °C (adapted from [23])

Larsson et al. [26], Marinkovic et al. [27] and Vilches et al. [28] have used different kinds of bed material (ilmenite, olivine, bauxite, quartz sand) in the Chalmers University of Technology’s 2–4 MW_th dual fluidized bed biomass gasifier system to decrease the yield of tar. Larsson et al. [26] found that adding 12% ilmenite in the bed could reduce the tar yield by about 50% (mass), but the proportion of heavy tars in gas was increased, which may increase the dew point of the tar mixture. The transport of oxygen resulted in a decrease in the chemical efficiency of the gasifier and the calorific value of the product gas. Compared with the case of 100% silica sand as the bed material, the cold gas efficiency in ilmenite (12%) case decreased by about 10%, and the calorific value of the gas decreased from about 17 MJ/Nm³ (sand case) to 12.5 MJ/Nm³ (12%ilmeinte case). In addition, the effect of adding ilmenite is highly dependent on the gasifier’s operating conditions. With the increase of fluidizing velocity, the promotion of ilmenite on WGSR was enhanced due to the improvement of gas–solid mixing, while the tar-cracking reaction was weakened and the levels of heavy components was increased. Overall, ilmenite as a bed material seems promising to reduce the tar yield in gasifier, but oxygen delivery levels need to be limited. Marinkovic et al. [27] using olivine as a bed material to continuously run the gasifier for 9 days, the results showed that a gradual decrease in tar content was observed from the second day of operation (the bed material first showed signs of activation), and the tar yield on the fourth day was 30% lower than the first day. The activated olivine significantly increased the production of H₂ with a maximum H₂/CO ratio of 1.7. Obviously, in order to maximize the use of activated olivine as bed material to improve the gasification process, the activation process of olivine should be fully understood. Although the olivine has been widely used and studied, there is still uncertainty about the nature of its driving force for activation. In addition, the study also pointed out that the proportion of inorganics (from the ash of fuel) in the system also has a decisive influence on the final product gas composition and concentration, which will be discussed in detail in Sect. 5.4. Vilches et al. [28] evaluated the performance of four bed materials (olivine, bauxite, quartz sand and ilmenite) in the biomass gasifier, and found that all of the three activity bed material have tar cracking abilities. After one week of operation and exposure to biomass ash, the activities of bauxite and olivine to crack tars were further increased, both materials had catalytic effect on the WGSR, and bauxite shows a stronger ability to increase char conversion than olivine. Additionally, under the same operation, the oxygen carrying capacity of the four bed materials is as follows: quartz-sand < olivine < bauxite < ilmenite, as shown in Fig. 5.8. Interestingly, although the content of Fe in bauxite is lower than that in olivine, bauxite has higher oxygen transport capacity than olivine. This can be partially attributed to the ash load of bed material.

Fig. 5.8

Oxygen transport capacity of different bed materials after one week of operation, the steam-to-fuel ratios was 0.8 (reprinted from [28] with permission of American Chemical Society)

5.3 Screening of Bed Materials

Keller et al. [29] carried out screening study of up to 12 bed materials in a bubbling FB reactor with the aim of evaluating a serious of bed materials for tar-cracking process. The bed materials were based on the transition metals Mn, Fe, Cu, and Ni, and three of which were natural ores and other nine were synthetic materials. The bubbling bed was a small scale batch-feeding reactor, where allowed the bed materials can be exposed to reforming and regeneration conditions alternatingly. The conversion of ethylene from outlet of reactor was used as an indicator for the suitability of the bed materials for tar-cracking conversion. The results showed that the natural material bauxite and the synthetic materials NiO/α-Al₂O₃, CuO/MgAl₂O₄, and La_0.8Sr_0.2FeO₃/γ-Al₂O₃ all exhibit high ethylene conversion rates (as shown in Fig. 5.9), and these materials have a broad application prospect in OC assisted gasification/reforming technology. The above results only indicate the catalytic reactivity sequence of the materials, but do not mean that the materials with low reactivity cannot be used in fluidized bed gasification reactors. Because the choice of catalytic bed material not only needs to consider the reactivity of the bed material, but also needs to consider the mechanical properties (wear resistance), economy and whether will have other effects on the safe and efficient operation of the system.

Fig. 5.9

Catalytic ethylene conversion (γ_{C2H4,catalytic}) as a function of operating temperature for different kinds of bed materials (reprinted from [29] with permission of American Chemical Society)

5.4 Ash-Related Effects

Kirnbauer et al. [30] studied the long-term changes to the bed material during regular production in a commercial DFB gasification system, the results showed that the elemental composition of the used bed material had a significant increase in calcium and potassium concentration (as shown in Fig. 5.10), two calcium-rich layer formatted on the bed material (as shown in Fig. 5.11), the inner layer consists mainly of calcium silicates while the outer layer has a similar composition to fine ash (as shown in Fig. 5.12 and Table 5.4). The formation of calcium-rich layers is attributed to the high calcium content of wood ash and to the addition of calcium-rich additives to improve the catalytic properties of tar-cracking. Then, Kirnbauer et al. [31] investigated the difference between used olivine (from an industrial-scale plant) and fresh olivine on the gasification properties. The tests were carried out under same operations in a 100 kW_th pilot plant at the Vienna University of Technology. The results showed that the used bed material can increase the H₂ and CO₂ in the product gas, and the corresponding CO concentration was decreased. Moreover, the used bed material can also enhance the WGSR (Exothermic reaction), thereby reducing the energy requirement for gasification. During the test of the used olivine case, the tar content (detected by GC–MS) was reduced by about 80%, and the analysis of the tar showed fewer components. Furthermore, the results obtained from the used olivine as bed material in the pilot plant of 100 kW_th were very consistent with the results of the 8 MW_th industrial plant of Gussing, Austria, and confirming the good scale-up performance from 100 kW to industrial-scale plants.

Fig. 5.10

XRF analysis result of unused and used olivine (reprinted from [30] with permission of American Chemical Society)

Fig. 5.11

SEM images of used olivine (reprinted from [30] with permission of American Chemical Society)

Fig. 5.12

SEM of inner (1) and outer (2) layers of used olivine (reprinted from [30] with permission of American Chemical Society)

Table 5.4 The EDS results of unused and used olivine (adapted from [30])

The enrichment of inorganic materials in the fuel bed has an important effect on the activity of olivine and the entire reaction process. Marinkovic et al. [27] examine the effect of S and SiO₂ on the activity of olivine, S and SiO₂ sand were added to the fuel reactor, respectively, and it was found that the addition of S had a positive effect on the cracking of tars in the gasification product gas, while the addition of SiO₂ had a negative effect on the cracking of tars. This is because the K in the ash of biomass is present in the bed material in the form of leachable. Then the K salts react with S in the combustion reactor to form K₂SO₄ and migrate to the gasifier. In the reducing atmosphere of the gasifier, K₂SO₄ can converted into KOH or K₂CO₃, which have catalytic activity for gasification process. The addition of SiO₂ will form inactive potassium silicate with K, which will inhibit the catalytic effect of K on the gasification process, and cannot be attributed to the dilution effect of SiO₂ particles on the olivine bed material. In addition, the results also proved that the K is a vital active ingredient that promotes the gasification process. Vilches et al. [32] investigated the ability of ash-coated olivine to catalyze the gasification of biomass char in a lab-scale reactor, the olivine bed material has been pre-activated by the addition of S and K₂CO₃ in Chalmers DFB gasifier. The ash layer of olivine can catalyze the steam-char gasification by transferring catalytic potassium (presumably as gaseous KOH) to the char particles. The EDS line-scanning analysis (as shown in Fig. 5.13) of cross-sections of char sample can verify this transfer mechanism. When 40% of ash-coated olivine was used as bed material, higher concentration of K, Ca, S can be found in char (especially near the surface of particle).

Fig. 5.13

The EDS analysis of the concentration of K, Ca, and S in the cross-section of char, a 40% of ash-coated olivine as bed material; b 40% of untreated olivine as bed material (reprinted from [32] with permission of Elsevier)

In general, the alkali metal substances (especially K) in the biomass ash attached to the surface of the bed material particles have a promoting effect on biomass DFB gasification, including WGSR, char gasification and tar cracking processes. However, high alkali metal of ash-layer on the surface of bed material is often linked to bed agglomeration problems. Moreover, the high alkali metal content in fuels also can cause various ash-related problems during biomass combustion in FB boilers, such as fouling, corrosion and bed agglomeration [33]. When catalytic bed materials and catalytic additives are added to gasifier to reduce the tar yield, the ash characteristics and thermal characteristics may be different from those of combustion reactor, so the ash characteristics and its impact on downstream equipment need to be comprehensive considered.