Low-Temperature Gasification of Biomass and Lignite: Consideration

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Low-Temperature Gasification of Biomass and Lignite: Consideration of Key Thermochemical Phenomena, Rearrangement of Reactions, and Reactor Configuration Jun-ichiro Hayashi,*,† Shinji Kudo,† Hyun-Seok Kim,† Koyo Norinaga,† Koichi Matsuoka,‡ and Sou Hosokai‡ †

Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan

‡

ABSTRACT: This paper discusses gasification of solid fuels, such as biomass and lignite, at temperatures well below 1000 °C, which potentially realizes a loss of chemical energy (LCE) smaller than 10% but encounters difficulty in fast and/or complete solid-to-gas conversion in conventional reactor systems. First, key thermochemical and catalytic phenomena are extracted from complex reactions involved in the gasification. These are interactions between intermediates (i.e., volatiles and char), catalysis of inherent and extraneous metallic species, and very fast steam gasification of nascent char. Second, some ways to control the key phenomena are proposed conceptually together with those to rearrange homogeneous/heterogeneous reactions in series/ parallel. Third, implementation of the proposed concepts is discussed assuming different types of gasifiers consisting of a singlefluidized bed, dual-fluidized bed, triple-bed circulating fluidized bed, and/or fixed (moving) bed. The triple-fluidized bed can attain gasification with a LCE as small as 10% by introducing enhancement and/or elimination of the key phenomena and another way to recuperate heat from gas turbine and/or fuel cells (i.e., power generators in gasification combined cycles) into chemical energy of fuel gas. A particular type of fixed-bed gasifier is proposed, which is separated from a pyrolyzer to realize not only control of the key phenomena but also temporal/spatial rearrangement of exothermic and endothermic reactions. This type of gasifier can make a LCE smaller than 4%. Even a conventional single-fluidized bed provides simple and effective gasification, when tar-free/reactive char is used as the fuel instead of parent the one and contributes to a novel integrated gasification fuel cell combined cycles with a theoretical electrical efficiency over 80%.

1. INTRODUCTION Gasification is a thermochemical way or process to convert carbonaceous solid fuel into gases, such as H2, CO, and CH4, for power generation or chemical production. In the authors’ view, as shown in Figure 1, significance of the gasification is its

Many state-of-the-art technologies for the gasification, in particular, that of coal, are based on partial combustion at temperatures well above 1000 °C. Coal gasification in entrained flow gasifiers has already been applied to integrated coal gasification combined cycles, which are called IGCCs.1−3 Some have been operated commercially, while others are in demonstration stages.1,3 It is also expected that gasification of biomass will be implemented in the near future for power generation. Novel fixed-bed, dual-fluidized-bed (DFB), and pressurized fluidized-bed gasifiers have been developed toward commercial production of power.4 It appears that many technologies for gasifiers and upstream/ downstream processes have already been near commercialization or demonstration. However, studies on the gasification and related technologies should be continued if there is room for further improvement of technical/economical performances of gasification. This paper makes a strong focus on the energy efficiency of gasification and considers potentiality of that at temperatures of 300−900 °C. Current gasification largely relies on partial combustion at elevated temperatures, which allows for very fast thermochemical reactions to convert solid fuel into

Figure 1. Scheme of integration of chemical and thermal energies.

function of integrating different energy resources (chemical energies) into a simplified set of gaseous products as above, i.e., syngas, which are common among energy and chemical industries. The gasification also enables the integration of thermal and chemical energies into chemical energy of syngas, recuperating the former energy in terms of exergy rate. © XXXX American Chemical Society

Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 12, 2013 Revised: October 30, 2013

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dx.doi.org/10.1021/ef401617k | Energy Fuels XXXX, XXX, XXX−XXX

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syngas but, at its expense, causes a large loss of chemical energy (LCE) of the fuel. The chemical energy of a given fuel is defined as its heat of combustion on either a lower heating value (LHV) or higher heating value (HHV) basis. The LCE is given by the following equation: loss of chemical energy (LCE) chemical energy of syngas =1− chemical energy of original fuel

Exergy analysis of an IGCC,5 which is based on oxygen-blown coal gasification with electrical efficiency of 48%, shows that the exergy loss occurring at the gasification (22%) is greater than any other major parts of the IGCC: combustion of fuel gas (19%), gas/steam turbines (7%), and oxygen production (4%). Improvement of energy/exergy efficiency of the gasification is thus important. The exergy loss in the gasification is attributed to exothermic reactions that are associated with the consumption of oxygen. It is therefore needed to reduce this consumption and, instead, increase that of steam or CO2, which causes endothermic reactions. The progress of endothermic reactions means integration of chemical and thermal energies to another form of chemical energy (i.e., that of syngas) and results in improvement of exergy rate of the original thermal energy. Figure 2 illustrates a result from a simple thermodynamic analysis of gasification of lignite and woody biomass with oxygen and steam or, otherwise, steam alone with heat supply, assuming an adiabatic gasifier. For the steam−oxygen gasification, the LCE of the original fuel is shown as a function of the temperature of syngas at the reactor exit, which is defined as Texit. Although not shown here, the consumption of oxygen increases with Texit. It is not necessarily the temperature for the gasification. Regardless of the reactor configuration and number of conversion stages, Texit determines the LCE, in other words, the recovery of chemical energy. Figure 2 indicates five different types of gasification, the potentiality or validity of which is discussed in this review based on knowledge from existing studies. Different types of gasification (types I−V) have different Texit values and corresponding loss/gain of chemical energy. These types of gasification are introduced here briefly. Type I gasification represents oxygen or air-blown gasification at temperatures well above 1000 °C, and it is the most popular type employed in large-scale coal gasification.1−3 Chemical energy loss as much as 20% is inevitable. Type II gasification is defined as that with Texit ranging from ca. 700 to 900 °C. Fluidized-bed gasification is a typical example of this type of gasification, and it can potentially reduce the chemical energy loss to ca. 10%. Type III gasification is supposed to be operated in a fluidized bed at temperatures in a range similar to that for type II gasification. The gasifier accepts an external supply of heat from hightemperature power devices, such as solid oxide fuel cells (SOFCs) but no oxygen. Because of a fully endothermic nature, type III gasification gains the chemical energy by more than 20% when the char from biomass or lignite is used as the fuel instead of their parents. Type IV gasification has Texit as low as 300 °C, which is too low to drive gasification, but a special arrangement of parallel/consecutive reactions and reactor configuration enables the decrease of Texit to such a level and minimizes the chemical energy loss. The pyrolysis and the following reactions, such as reforming of volatiles and gasification of char, are physically separated from each other.

Figure 2. LCE of original solid fuel in the gasification that is operated in an adiabatic reactor as a function of the temperature of syngas at its exit temperature, Texit. Assumptions for the gasification of types I, II, and IV are as follows: (1) oxidizing agents, pure O2 and steam at 25 and 200 °C at the gasifier inlet, respectively; (2) elemental compositions of fuels, biomass (a Japanese cedar), C100H145.4O69.8; lignite (a Victorian lignite), C100H80.8O27.5; (3) calorific value of fuels, biomass, 18.5 MJ LHV kg−1, daf; lignite, 26.0 MJ LHV kg−1, daf; (4) steam/carbon molar ratio (S/C), 0.60; (5) product gases, H2, CO, CO2, and CH4; (6) H2O, H2, CO, and CO2 are chemically equilibrated at Texit; and (7) CH4 yield is fixed at 4.0% on carbon basis. For type III gasification, the following assumptions were made: (1) A lignite char from pyrolysis at 600 °C (C100H35.0O12.0, 29.2 MJ LHV kg−1, daf) is fully converted with steam to H2 (2.01 mol mol−1 of C), CH4 (0.04 mol mol−1 of C), and CO2 (0.96 mol mol−1 of C). (2) Neither oxygen nor air is employed. For the type V gasification, it is assumed that the lignite (C100H145.4O69.8) is fully gasified with subcritical water with a stoichiometry: C100H145.4O69.8 + 80H2O = 27.9H2 + 46.3CH4 + 53.7CO2.

More details of this type of gasification will be mentioned later. Type V gasification is often called subcritical water gasification or hydrothermal gasification, and it employs hot compressed water, instead of steam. The chemical energy loss in this type of gasification is theoretically around zero because of the balance between the endothermic formation of CO and H2 and the exothermic formation of CH4. This paper reviews recent advances in the understanding of thermochemical and catalytic reactions and reactor systems relevant to type II−V gasification, aiming to (1) extract key reactions to be enhanced or suppressed to realize fast progress of solid-to-gas conversion, (2) draw concepts effective for realizing control of such key reactions, and (3) examine the applicability of existing and recently proposed reactor systems to low-temperature gasification. For all of the gasification processes ranging from types II−V, technology development is challenging, although the degrees of technical maturation are different from one another. In the design of a novel gasification B


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process, its performance should be examined in all aspects ranging from scientific principles to economic/social impacts. This paper does not cover such a huge range but discusses type II−V gasification from viewpoints of thermodynamics, mechanism/kinetics of thermochemical/catalytic reactions, and reactor configuration toward minimization of the LCE (maximization of chemical energy recovery). This paper also focuses on the gasification with steam, steam/oxygen, and steam/air in considering type II−IV gasification. CO2 is a gasifying agent more important than steam in oxygen-blown gasification, which is common for type I gasification.1−3 CO2 is also generated in situ in type II−IV gasification and involved in thermochemical and catalytic reactions to a more or less extent, but intentional feeding of CO2 into the gasifier together with steam or instead of steam has not necessarily been general.

2. KEY CHEMICAL PHENOMENA IN LOW-TEMPERATURE GASIFICATION 2.1. Overview of Thermochemical Reactions in Gasification. This section discusses the characteristics and mechanism of thermochemical reactions that are involved in the gasification at temperatures below 1000 °C, considering mainly the chemical interaction between the solid and gaseous pyrolysis products (i.e., char and volatiles), behavior of metallic species as catalysts, and fast steam gasification of nascent char from the pyrolysis and then draws some novel concepts that can be applied to type II−IV gasification. Difficulty of low-temperature gasification arises mainly from that of complete conversion of the pyrolysis products within time of their residence allowed in a practical size of reactor. Figure 3 illustrates the major reaction pathways in the gasification.6,7 The pyrolysis is the primary step of gasification, and it produces solid and gaseous products, i.e., char and volatiles, respectively. The volatiles consist mainly of light molecular weight gases (H2, CO, and CO2), light hydrocarbon gases (normally, C1−C5), and organic compounds having six or more carbon atoms per molecule, which are generally termed tar. The char and volatiles undergo reactions with oxidizing agents, which are termed char gasification and reforming, respectively. The char and volatiles also experience thermal cracking that eliminates functional groups from tar and causes aromatization. 2.2. Difficulty of Homogeneous Reforming of Tar and Steam Gasification of Char. Extensive reforming of lower hydrocarbons is not necessary unless the syngas is sent to a downstream process for producing liquid fuels/chemicals from CO and H2 or producing pure H2, but that of tar is mandatory for both power generation and liquid production.3,8,9 Thermal cracking of tar takes place regardless of the presence/absence of an oxidizing agent, and it rapidly enriches the tar with MAHs and PAHs, ultimately forming a carbonaceous solid, termed soot. It is very difficult to quickly reform MAHs and PAHs with steam in the gas phase at temperatures below 1000 °C.6,7,10,11 Even in the presence of oxygen, it is hard to oxidize the aromatics effectively, because oxygen reacts with light gases, such as H2 and CO, preferentially to hydrocarbons.11,12 These characteristics consequently lead to an idea to apply a catalyst to the steam reforming of tar. Steam gasification of char has long been studied, mainly because it is the rate-determining step in the overall gasification process13 and knowledge of kinetics and mechanisms of both non-catalytic and catalytic reactions has been accumulated.

Figure 3. Thermochemical reactions and intermediates involved in gasification. Primary volatiles are nascent ones produced by the “intraparticle” pyrolysis. These consist of inorganic gases (H2, H2O, CO, CO2, and minor species, such as NH3, HCN, and H2S), light hydrocarbons (C1−C6), and tar (≥C6). The primary volatiles undergo thermal cracking in the gas phase and are converted to the secondary volatiles mainly consisting of inorganic gases, and light hydrocarbons, and secondary tar, of which major components are monoaromatic hydrocarbons (MAHs) and polyaromatic hydrocarbons (PAHs). The thermal cracking is significant at temperatures over ca. 500 or 600 °C for biomass and lignite, respectively. The thermal cracking of char involves thermochemical reactions after completion of tar evolution. Functional groups remaining in the char are converted to light gases, such as inorganic gases and C1−C2 hydrocarbons. Condensation among aromatic ring systems forms H2, causing their growth in size. The reforming is defined as oxidation of the primary/secondary tars into CO and H2 in the gas phase or over catalyst/catalyst-like solid. The gasification is defined as the oxidation of char to form CO and H2.

Inhibition of steam gasification of char by H2 is conventional knowledge.14−17 The inhibition is caused by associative or dissociative chemisorption of H2 onto the char surface, forming CH or CH2.15,17 Partial pressure of H2 in a practical gasifier depends upon the consumption of steam and is often equivalent to that of steam. In such an atmosphere, the rate of steam gasification is lower by an order of magnitude or even more than that in the absence of H2.17−19 2.3. Chemical Interaction between Volatiles and Char. Recent studies18,20 showed that the pyrolysis-derived volatiles are much stronger inhibitors than H2 for the steam gasification of char. Volatiles are chemisorbed onto the char surface and decomposed generating hydrogen radicals and depositing carbon, termed coke. Bazardorj et al.18 investigated continuous steam gasification of a lignite at 850−950 °C in an atmospheric bubbling fluidized bed. They found that the inhibition by the volatiles, in particular, tar in the vicinity of char, even prevented it from undergoing gasification beyond a certain level of conversion, which was 25−70% depending upon the temperature. Figure 4 compares the amount of in-bed char during the continuous feeding of the lignite to that predicted by a kinetic model and suggests nearly complete inhibition of non-catalytic gasification in the bed. They also investigated continuous steam gasification of an acid-washed lignite (free from inherent catalysts) in the same fluidized bed and found that the net char conversion was even negative because of carbon deposition from the tar faster than the steam gasification. The deposition C


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Figure 5. Relationship between the initial rate of steam gasification of char from coal/lignite or that from the demineralized one (adapted with permission from ref 35). Figure 4. Change in the amount of char in a fluidized bed with a cumulative amount of Loy Yang lignite fed into the bed at 900 °C.18 Plot, measured amount of char; line, calculated amount of char. Case 1 assumes that the steam gasification of char takes place obeying a Langmuir−Hinshelwood-type kinetics, identical to that in the absence of volatiles. Case 2 assumes that the rate of non-catalytic gasification is lower than case 1 by a factor of 30 because of inhibition by the volatiles. Case 3 assumes that the non-catalytic gasification does not occur at all, while only the gasification catalyzed by inherent metallic species is allowed. In addition to the assumption of case 3, cases 4 and 5 assume that the loss of catalyst occurs at rates 1.5 and 3 times greater than cases 1−3, respectively (adapted with permission from ref 18).

well-developed porous structure of char that induces more active carbon sites at higher concentrations.35 2.5. Volatilization of AAEM Species and Its Enhancement by the Volatile−Char Interaction. Volatilization of AAEM species from the pyrolyzing lignite and biomass occurs even at temperatures lower than 600 °C,41,42,44 and these species undergo repeated volatilization from and readsorption (or condensation) onto the char,43 unless they react irreversibly with mineral matter, such as silica, alumina, and aluminosilicates, or converted into salts, such as chlorides and sulfide. Recent progress in understanding of the behavior of AAEM species in atmospheres relevant to steam gasification, which was comprehensively reviewed by Li,20 shows that the volatilization of AAEM species, particularly that of Na and K, is promoted by exposure of char to the volatiles.59−65 It is believed that hydrogen radicals supplied from the volatiles reduce char-bound AAEM species to metals and that their volatilization is promoted consequently.20,42,62,65 Vapor pressures of K, Na, and Ca in the form of metal are much higher than those of corresponding hydroxides, carbonates, and oxides. Thus, the chemical interaction between the volatiles and char promotes the release of AAEM species from the char, in other words, the loss of the catalysts for the steam gasification. A sufficiently high concentration of char, an effective adsorbent of vaporous AAEM species, in the gasifier may help them be retained in/on the char matrix. In this sense, maintaining a fixed or moving bed of char in the gasifier is a way to maximize the catalysis of AAEM species under volatile−char interaction. On the other hand, fluidized beds have disadvantages in holding the AAEM species unless the char concentration in the bed is so high as to prevent them from reacting with the fluidizing medium, such as silica sand. Kajita et al.19 reported that Na and K were easily transferred from char to mineral particles of alumina when they were in contact with each other at 800 °C, even in the absence of either volatiles or steam. The irreversible reactions between AAEM species and fluidizing medium not only diminish the inherent catalysis but also bring about problems in operating the fluidized bed, such as defluidization, because of the formation of Na/K silicates with a melting point temperature below 1000 °C. 2.6. Promotion of Decomposition of Tar by the Volatile−Char Interaction. As mentioned above, the chemical interaction between the volatiles and char, which is herewith referred to as VCI, inhibits the steam gasification of char, decreasing the intrinsic reactivity of char and promoting the loss of inherent catalysts. These have negative impacts on

of tar onto char is unavoidable even for the pyrolysis in a droptube reactor, in which either gas or particle residence time is within seconds.22−24 Exposure of char to volatiles also enhances structural evolution of char, diminishing its reactivity with steam.20,25−29 The volatiles supply hydrogen radicals inducing rearrangement of aromatic ring systems (ARSs) and causing conversion of smaller ARSs to greater ARSs that have lower reactivity toward steam. Recent studies on FT−Raman spectroscopy of char clarified such structural changes of char, resulting in the decrease in the char reactivity.25,26,29 2.4. Catalysis of Inherent Metallic Species in Steam Gasification of Char. The presence or abundance of alkali and alkaline earth metallic (AAEM) species is a particular feature of biomass and lignite.30−32 The AAEM species are present in the original solid fuel in a form of organically bound cations (e.g., −COO−Na+) or, otherwise, inorganic salts, such as chlorides and carbonates. The pyrolysis transforms more or less portions of the AAEM species into those highly dispersed in/on the carbonaceous matrix of char, which catalyzes the steam gasification of char.33−40 Many reports and reviews are available on behaviors of AAEM species and their influences on the pyrolysis,41−45 gasification, and combustion of char46−58 of biomass and lignite. For the gasification of chars from biomass and lignite, catalytic gasification often contributes to the overall gasification much more extensively than non-catalytic gasification. Figure 5 shows combined effects of coal rank and acid washing (removal of minerals and metallic cations prior to the pyrolysis) on the initial rate of steam gasification of char.35 The reactivities of chars from lignites with carbon contents of ca. ≤70% are clearly higher than those of chars from subbituminous and bituminous coals, and this is explained mainly by the catalysis of inherent AAEM species while partly by the D


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Figure 6. Characteristics of in situ reforming of tar from rapid pyrolysis of lignite in a fixed bed of char from the same lignite.24 R750, R800, R850, and R900 indicate the steam reforming at a fixed-bed temperature of 750, 800, 850, and 900 °C, respectively. The individual graphs show timedependent changes in the yields of typical aromatic compounds found in the tar. At 750−800 °C, the initial high activity of char is lost within 10−20 min. However, at 900 °C, activity high enough to decompose the di-, tri-, and tetra-aromatics completely is maintained during the time elapsed, only allowing for a small amount of benzene to survive in the bed.

conversion of tar by a sequence of carbon deposition onto the char surface and subsequent or simultaneous steam gasification of carbon under catalysis of AAEM species, as mentioned above. The char is not a catalyst but a promoter of decomposition of tar. The char is automatically and continuously formed in the gasifier, and this is an important advantage of employing char for tar elimination over applying catalysts, such as Ni−Al2O3 and natural minerals. 2.7. Pyrolysis-Driven Fast Steam Gasification of Char. Results of some recent studies on the rapid pyrolysis of lignites in a steam or CO2-containing atmosphere suggest very fast steam gasification of nascent char. Iwatsuki et al.22,23 pyrolyzed pulverized lignites in an atmospheric drop-tube reactor, where the particle residence time was 4.3 s. They found that the char yield from the pyrolysis in the presence of steam (40 vol %) was only 72−81% of that in its absence. The product analysis showed that such a low char yield was due to steam gasification of the nascent char with an estimated rate of 4−6 × 10−2 s−1. This rate of reaction was higher by 1−2 orders of magnitude than that of conventional ex situ steam gasification of chars from slow pyrolysis of the lignites. Thus, the nascent char reacted with steam at such a high rate, despite the presence of VCI. Jamil et al.85 investigated rapid pyrolysis of a lignite in atmospheric CO2 in a wire-mesh reactor with a heating rate of 103 °C s−1 and a peak temperature of 900 °C. They found that the char underwent very fast CO2 gasification while heated from 600 °C (just after completion of tar evolution) to 900 °C, in other words, in a period as short as 0.3 s. The average rate of the CO2 gasification was in the order of 10−1 s−1. Such a very fast gasification in the heating period was evidenced even for a char from the acid-washed lignite that was nearly free from AAEM species. Nascent char from the rapid pyrolysis is still rich in functional groups (precursors of light gases, such as H2, CO, H2O, and

the kinetics of char gasification. However, from a different viewpoint, VCI is a favorable chemical event that greatly promotes the decomposition of tar. There have been a number of reports that claim high activity of both biomass and lignite chars to decompose tar and aromatics at temperatures higher than 800 °C.66−78 It is believed that the decomposition takes place in micro- and nanopores of char depositing carbon onto the pore surfaces.71 If the char retains catalytic species, such as AAEM species, the deposited carbon is gasified with steam at a rate equivalent to that of carbon deposition and pores are regenerated.71,72 The tar can thus be reformed with steam in a sequence of carbon deposition onto the char surface and steam gasification of the deposit.71,72 A unique type of tar decomposition promoter was also reported. Carbon deposit inside the nanoporous system of γ-alumina particles, which had been formed from tar from the pyrolysis of biomass, had a high activity toward decomposition of tar following an autocatalytic mechanism.79−82 Rapid pyrolysis of pulverized lignites was investigated in steam-containing atmospheres in a drop-tube reactor,22,23,77 and it was demonstrated that more than 99% of the primary tar was in-situ-converted into gas by steam reforming over coexisting char particles as well as thermal cracking in the gas phase. Zhang et al.83,84 reported that loading of Na/Ca onto the lignite and an increase in the char concentration in the reactor were both effective in promoting the conversion of tar. In situ steam reforming of tar within a fixed bed of char24 was even more rapid and extensive, and it converted PAHs completely at 900 °C, as shown in Figure 6. Temperatures as high as 900 °C allow for simultaneous progress of char gasification with the tar reforming while requesting retention of AAEM species on the char and the presence of steam at a certain level of concentration.24 It is believed that the reforming of biomass tar and lignite tar obey the same mechanisms, i.e., the E


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positive impacts on the gasification of biomass and lignite. Recognition of VCI leads to two different concepts for lowtemperature gasification. One is elimination of the interaction for avoiding inhibition of the char gasification, while the other is intensification for promoting the reforming of tar. These two concepts appear to contradict each other, but it is possible to realize both concepts simultaneously by a particular reactor configuration, which will be explained in more detail, showing an example of type II gasification. Under appropriate conditions, steam reforming of tar over char is completed within a contact time shorter than 1 s. On the other hand, time constants for the VCI-promoted volatilization of AAEM species and rearrangement of the carbonaceous structure of char are in an order of 1 min or even longer.20 A sequence of intensified VCI within seconds (for reforming of tar), isolation of char from resulting gas, and steam gasification of char without VCI is a promising way to realize both concepts simultaneously. AAEM species can catalyze not only the char gasification but also the in situ steam reforming of volatiles over the char surface in a sequence of deposition of carbon and its steam gasification. The char plays a role of support of AAEM species, which is formed automatically and in situ in the reactor. Loading AAEM species onto biomass/lignite or char can intensify the steam reforming of the volatiles and also the pyrolysis-driven steam gasification of char. A high volatile nature of Na and K may extend their lifetimes by transfer among char particles via the vapor phase in the absence of minerals that chemically capture and deactivate the alkalis.

CH4), of which the content is 15−20% on a mass basis just after completion of the tar evolution. The decomposition of such functional groups may leave active sites on the char surface. Matching in the rate between the pyrolysis of char and steam or CO2 supply seems to be one of the necessary conditions for the above-mentioned fast gasification22,23,85 that is driven by the pyrolysis of functional groups. Loading of AAEM species on the lignite contributes to further acceleration of the pyrolysis-driven gasification of char. Masek et al.86 reported that rapid pyrolysis of Na- and Ca-loaded lignites in an atmosphere with a steam concentration of 50 vol % converted them at carbon conversions of 88 and 67%, respectively, with an particle residence time of 2.8 s, which corresponded to 82 and 47% conversions of the nascent chars, respectively. Similar results were reported later by Zhang et al.83 Bazardorj et al.21 and Kitsuka et al.87 examined the kinetics of in situ steam gasification of chars from the rapid pyrolysis of lignites (Figure 7). They employed a unique drop-tube/fixed-

3. APPLICATION OF A SINGLE-/DUAL-FLUIDIZED BED AND A TRIPLE-COMBINED BED CIRCULATING FLUIDIZED BED TO TYPE II GASIFICATION Type II gasification is operated at temperatures at 700−1000 °C according to existing studies on gasification in a fluidized bed or that combined with another reactor, such as a catalytic reformer, downstream.88−96 Figure 8 shows the theoretical performance of gasification with steam and air or steam and oxygen (O2), which is predicted by a thermodynamic analysis with assumptions as described in the caption. In theory, type II gasification can recover 85−90% of the chemical energy of the biomass or lignite at Texit lower than 800 or 900 °C, respectively. Texit for a single-fluidized-bed (SFB) gasifier is theoretically the same or nearly the same as the bed temperature, in other words, that for the pyrolysis, steam reforming, and char gasification. The recovery of chemical energy, hereafter referred to as CER, for the lignite is higher by 2−3% than that for the biomass, and this is due to the difference between their thermodynamic properties and elemental compositions. Use of oxygen instead of air increases CER by 6−8%, but as it is known well, the oxygen production is associated with a relatively large energy penalty. The power consumed for the oxygen production (ca. 0.3 kWh/kg of O2) corresponds to LCE of the lignite by about 5% LHV when the following conditions are considered: O2/C = 0.3 mol/mol and electrical efficiency of IGCC = 48%. The use of oxygen is virtually impossible for smaller scale gasifiers but necessary for gas-to-liquid processes. Decreasing S/C by 0.2 seems to lead to increase CER by ca. 2%. According to the thermodynamic analysis, the gasification of lignite requires higher steam conversion in the gasifier. Although not shown in Figure 8, the steam conversion for the oxygen−steam gasification of the lignite is 59% at Texit = 800 °C, while as low as 15% for the biomass. The steam conversion

Figure 7. Char yields from the rapid pyrolysis of lignites prepared from Loy Yang lignite.86 H form, acid-washed (AAEM-free) lignite; Ca form, Ca-ion-exchanged lignite (Ca content, 3.2 wt % dry); Na form, Na-ion-exchanged lignite (Na content, 2.8 wt % dry); DFR, a droptube/fixed-bed reactor for determining the char yield from the rapid pyrolysis (primary pyrolysis) with minimized VCI as well as steam gasification of char;21 DTR, a drop-tube reactor in which VCI and in situ steam gasification of char took place following the primary pyrolysis within a particle residence time of ca. 2.8 s.22 The difference in the char yield between DFR and DTR is due to the net progress of the in situ steam gasification of char (in the case of DFR > DTR) or, otherwise, that of VCI-induced deposition of tar onto the char (DFR < DTR).

bed reactor. A small amount of pulverized lignite was injected into the tubular reactor, of which the bottom was closed by a quartz filter as the gas−solid separator. The lignite was heated at a rate around 104 °C s−1. The nascent char formed by the pyrolysis was immediately isolated from the volatiles and exposed to a flow of steam (40 vol %) that was forced to pass through the filter. The initial rate of steam gasification was, however, lower than expected from the report by Iwatsuki et al.22 It was suggested that a too high of a heating rate of the lignite and char prevented steam from attacking active sites that was formed from the pyrolysis of functional groups of the char during the heating period. There may thus be an optimum range of the rate of the pyrolysis of the functional groups of char. The mechanism of the pyrolysis-driven fast gasification of char is not understood well, and therefore, more detailed studies are needed for its full use in a practical gasifier. 2.8. Concepts in the Design of the Reactor for LowTemperature Gasification. As reviewed in the above, recent progress in understanding of VCI has clarified its negative and F


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Figure 8. Chemical energy recovery and O2 consumption as a function of Texit for single-stage gasification of biomass and that of lignite with air and steam (air−steam) or oxygen and steam (oxygen−steam). The assumptions for the calculation are the same as those for Figure 2, unless otherwise indicated below. For panels a and b, S/C = 0.8 mol/mol. The sensible heat of syngas is used for generation of steam at the temperature (Ts) in an ideal boiler (heat exchanger) with ΔT = 50 °C. Ts is 150 °C or higher, and the highest temperature is given as far as ΔT = 50 °C is satisfied. A lower limit of Texit, 769 °C, is indicated for the oxygen−steam gasification of lignite. This is due to the fact that the sensible heat of the syngas at lower Texit is not enough to generate steam and heat it to 150 °C. Panel c, oxygen−steam gasification of biomass; panel d, oxygen−steam gasification of lignite. In panels c and d, the lower limit temperatures are indicated in the same ways as those in panels a and b.

and this suggests how difficult the complete elimination of tar is. Devi et al.,97 Anis and Zainal,99 and Srinivas et al.101 comprehensively reviewed previous studies on the elimination of tar from syngas by chemical ways, such as catalytic reforming inside the gasifier or downstream (primary methods97), and physical ways, such as dry/wet scrubbing, filtration, and absorption (secondary methods97). Here, a focus is made on the catalytic steam reforming of tar, following a series of studies by Aznar et al.,88,93 Navaéz et al.,90,92 Gil et al.,91,95 and Caballero et al.,94,97 who studied on bubbling fluidized-bed gasification of woody biomass at 780− 860 °C employing S/C at 0.4−1.1 and O2/C at 0.20−0.44, many of which were within the ranges shown in Figure 8. As mentioned previously, consumption of O2 selective to tar as well as char is very difficult in a gasifier, such as a SFB, and therefore, the reforming with steam is important. Woody biomass normally contains AAEM species, and it is believed that these are involved in the reforming to a more or less

involves that by the water−gas-shift reaction, but its contribution is relatively minor. The gasification of lignite thus needs a more active catalyst for the steam reforming and gasification of volatiles and char, respectively, than the biomass gasification. The pyrolysis of lignite forms more char and less tar than that of biomass.6,97,99 Thus, the catalytic roles of AAEM species or other metallic species in the char gasification are important. The biomass gasification, instead, requires active and durable catalysts for the steam reforming of tar and conditions minimizing catalyst deactivation. 3.1. Reforming of Tar in a Fluidized Bed. Elimination of tar has been a most important subject in the development of biomass gasification, regardless of the way to use the syngas.97,99,101 It is generally required that the concentration of tar in the syngas is lower than 100 and 5 mg/Nm3 dry for application to an internal combustion engine and gas turbine, respectively.100 Roughly saying, a tar concentration at 100 mg/ Nm3 corresponds to its yield in an order of 0.01 wt % dry feed, G


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Figure 9. (a) SFB gasifier and (b) that combined with the catalytic steam reformer.

extent. Their overall “inherent” catalysis is, however, far below the level needed to eliminate the tar over the char surface. Fluidized-bed gasification without applying an “extraneous” catalyst often allows for a tar concentration in the syngas as high as 4000−20 000 mg/Nm3 dry.90,91 The addition of a natural mineral, dolomite, to the bed material (e.g., silica sand) and also a fixed bed of this natural catalyst is effective to reduce the tar concentration to a level of 2000 mg/Nm3 dry or lower. Such a concentration of tar is still higher by 1−2 orders of magnitude than that required for direct application of the syngas, but dolomite plays an important role as a guard catalyst when a highly active catalyst, e.g., Ni/Al2O3, is applied downstream. Combinations of dolomite (in the fluidized bed) and commercially available Ni-based catalyst (fixed bed at 775− 860 °C) successfully reduced the tar concentration to 2−25 mg/Nm3 dry downstream of the catalytic steam reformer.92−94,96 Dolomite was in fact effective for avoiding quick deactivation of the Ni-based catalyst.88 The combination of a fluidized bed containing a moderate and cheap catalyst (such as dolomite and olivine102) and a catalytic steam reformer seems to be a potential gasifier that enables complete or nearly complete elimination of tar (Figure 9). However, there remain some important subjects toward industrial implementation. First, tolerance for minor compounds containing Cl, S, and also volatilized metallic species is an important performance of the catalyst. Second, a fixed-bed (packed-bed) reformer may not be durable unless fines of char and ash are completely removed between the gasifier and reformer. Corella et al.103 introduced a monolith-type catalyst that allowed fines to pass through the reformer, but the performance of the catalysts was not significant as those reported previously.92−94,96 The third subject is a minimization of the loss of char from the gasifier because of its entrainment.90,95,103,104 Chars from the biomass gasification normally have low densities. Then, recycling char particles to the bed is not necessarily effective. For example, a loss of char by 5% on a carbon basis leads directly to about 5% reduction of CER. It is equivalent to the necessity of reduction of Texit by 100−200 °C, according to Figure 8. 3.2. Steam Gasification of Lignite in a Fluidized Bed. In many previous studies on the gasification of coal or lignite in a fluidized bed, either catalytic or non-catalytic reforming of tar was not necessarily a major focal point. It might be recognized that the steam gasification of char was much more important than the reforming of tar. Much fewer studies on the catalytic reforming of tar from coal/lignite might also be arisen from more contents of sulfur in these solid fuels, which poisons transition-metal-based catalysts for steam reforming. The char yields from the pyrolysis of coal/lignite are often more than

60% on a carbon basis, and this makes complete gasification difficult. Bazardorj et al.18 investigated continuous steam gasification of a lignite in a bubbling fluidized bed without allowing for entrainment or discharge of char particles. Nonetheless, they found limited conversion of char at 850−900 °C with its steady accumulation in the bed. Such suppressed char gasification with steam was caused primarily by VCI, as previously shown in Figure 4. It is not believed that increasing the partial pressure of steam greatly improves the rate of char gasification if a Langmuir−Hinshelwood mechanism governs the kinetics of steam gasification. It was also suggested by Bazardorj et al.18 that the char gasification was catalyzed by inherent AAEM species even under VCI. Thus, rapid progress of char gasification requires a catalyst in a SFB at temperatures below 900 °C. A catalyst or its precursor should be loaded directly onto the lignite, unless it contains AAEM species and/or transition metals at significant concentrations. However, loading alkalis is a very challenging and risky task because it is difficult to avoid reactions between alkali and bed material (silica or alumina) that result in the formation of alkali silicates or aluminate with low fusion/melting temperatures. Catalytic gasification of char is also discussed in the following sections in which type III and IV gasification are considered. Both types of gasification require catalysts for the steam reforming of tar and gasification of char. A special example of fluidized-bed gasification is a potassiumcatalyzed steam gasification of coal, which was developed by Exxon Research and Engineering Co. in a period around 1980.105,106 K2CO3 was loaded to a bituminous coal from an aqueous solution with a content of 15 wt % of the coal. The catalyst-loaded coal was then fed into a bubbling fluidized bed at 3.5 MPa and 700 °C. The developer claimed that such a high pressure and low temperature enhanced the formation of CH4, diminished the endothermic nature of the overall reaction, and consequently eliminated the necessity of feeding oxygen to the gasifier. The operating conditions employed were largely different from those assumed in Figure 8. The conversions of coal carbon and steam were 85−90 and 35%, respectively.6,7 The K2CO3 catalyst was recovered by washing the char with water. It was estimated by researchers that CER of this gasification was 60−70% under practical conditions.6,7 This Exxon process is a novel one even nowadays, because very few processes of catalytic coal/lignite gasification reached demonstration stages in the past. The present authors examined the performance of this synthetic natural gas (SNG) production by thermodynamic analysis assuming the following stoichiometry and complete conversion of the same lignite as considered in the previous analysis. The conversion of steam was assumed to be 35%.6,7 H


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obtained under the following conditions: Texit (gasifier) = 700− 850 °C, Texit (combustor) = 950 °C, and S/C = 0.7−1.9. The simulation predicted CER around 0.80 at S/C = 0.8−0.9 and Texit (gasifier) = 800−830 °C, where the total amount of char and coke (over the alumina) to be burned in the combustor was in agreement with that minimum required by the endothermic reactions in the gasifier. The maximum CER of 0.80 is equivalent to that for SFB gasification with air−steam at equivalent S/C and Texit (see Figure 8), although these CERs should not be compared strictly. In principle, if given the same set of S/C and Texit (gasifier), DFB exhibits a CER slightly lower than SFB because the flue gas carries a certain quantity of heat at temperatures higher than Texit (gasifier) away from the gasifier. Among the advantages of DFB over SFB, as briefly mentioned above, use of oxygen (in air) exclusively to “residual” char from the steam gasification is deemed to be very important. In both DFB and SFB gasifiers, the char has to react with steam under VCI that strongly suppresses the reaction. However, in the former gasifier, there is no need of complete gasification of char with steam. Even if there is an upper limit of the char conversion under VCI,18 this may not be a problem, unless the residual char (to be burned in the combustor) is much more than required to supply heat to the gasifier. When biomass is the fuel for the DFB gasification, because of the small char yield from the fuel, it is often needed to feed additional fuel or a portion of product gas to the combustor.79,107 The use of a fluidizing medium that catalyzes carbon deposition from the tar79,80 is even effective in the control of the amount of carbon burned in the combustor. The necessity of a shorter residence time of char in the gasifier is another advantage of DFB. A longer residence time of char in a bubbling fluidized bed tends to cause more entrainment, unless the char conversion is as sufficiently fast as combustion. DFBs have been applied to gasification of biomass, lignite, and their mixture, as recently reviewed by Kern et al.107 They also performed air−steam gasification of a lignite in a circulating fluidized bed that is a major type of DFB at Texit (gasifier), Texit (combustor), and S/C of 850 °C, 920 °C, and 0.87, respectively. According to their data, the carbon conversion in the gasifier and combustor were 65 and 35%, respectively. They estimated a cold gas efficiency of 0.773 with consideration of heat loss for an industrial-scale plant. Our thermodynamic analysis of air−steam gasification (SFB), which assuming the same property of lignite, Texit (gasifier), and S/C as reported by Kern et al.,107 gave CER of 0.79. A comparison this CER to the above cold gas efficiency shows that the DFB gasifier is equivalent to the SFB gasifier in terms of CER under the assumption of full conversion of lignite. 3.4. Gasification in a Triple-Combined Circulating Fluidized Bed. A novel type of DFB (circulating fluidized bed) was proposed by Guan et al.113 as a candidate of a gasifier for advanced IGCC (A-IGCC) and advanced integrated coal gasification fuel cell combined cycle (A-IGFC),98,114−116 in which heat from the gas turbine and/or fuel cells is transformed to hot steam and then supplied to the gasifier. Theoretical power generation efficiencies of A-IGCC and A-IGFC are as high as 57−59 and 65−70%, respectively. The proposed type of DFB, hereafter referred to as triple-bed combined circulating fluidized bed (TB-CFB), involves three reactors: a downer for the pyrolysis and reforming of volatiles, a bubbling fluidized bed for char gasification, and a riser for the full or partial

C100H80.8O27.5 (lignite; 25°C) + 1.89H 2O (water, 25°C) = 53.2CH4 (Texit = 700°C) + 46.8CO2 (700°C) + 1.23H 2O (steam; 700°C)

The CER following the above stoichiometry is 97.6% LHV. It was found that the overall reaction was largely endothermic, with a necessity of heat supply of 138 kJ/mol C lignite (31.5% LHV of lignite). Even with the ideal use of the sensible heat of hot syngas (including steam) to generate 200 °C steam (with ΔT = 50 °C), the process needs more heat equivalent to 11.0% LHV lignite. Direct production of CH4 by low-temperature gasification is attractive. However, there remain technical challenges, such as the reduction of the S/C ratio for decreasing the heat demand for generating steam, maintenance of fluidization with a heavy loading of the K catalyst, and its recovery/reuse. 3.3. Gasification in a DFB. Figure 10 shows a schematic of a DFB, which is another candidate of a type II gasifier.98 This

Figure 10. Schematic diagram of the DFB gasifier. FM = fluidizing medium.

type of gasifier consists of two fluidized beds: gasifier and combustor, into which steam and air are fed, respectively. In the gasifier, the pyrolysis of solid fuel and subsequent steam reforming/gasification of volatiles/char occurs in a flow of steam. The unconverted fraction of the char is sent to the combustor together with the fluidizing medium (bed material) and burned completely there. A large portion of the heat of combustion is transferred to the fluidizing medium as the heat carrier, which is returned to the gasifier for driving the endothermic reactions. The DFB has some advantages over the SFB as found in the literature.97,98,107−112 In the DFB, oxygen is consumed exclusively for combustion of char that survived in the gasifier. Then, steam is allowed to react with the volatiles and the most active portion of the char in the absence of oxygen in the gasifier. The syngas is not diluted by N2 even if air is used for the char combustion. In other words, there is no need of using oxygen instead of air. Hosokai et al.79 investigated steam gasification of woody biomass and sugar cane bagasse in an atmospheric fluidized bed of nanoporous and active alumina. The alumina successfully decomposed tar into coke, while the coke further promoted the decomposition by an autocatalytic mechanism.80 The use of the alumina resulted in complete inbed elimination of heavy tar (tetra-aromatics and greater aromatics), while the total concentration of di- and triaromatics in the syngas was about 2000 mg/Nm3, suggesting the necessity of further steam reforming. Hosokai et al.79 also performed a numerical simulation of gasification in the DFB based on their experimental data I


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Figure 11. TB-CFB gasifier and temperature history and fate of solid fuel.

with the volume fraction of the nylon shots, even up to 50 vol %. It is expected that a high concentration of char in the circulating solid contributes greatly to enhancement of decomposition of tar over the recycled char. Partial oxidation of char increases its surface area, and this is favored to faster decomposition of the tar. Matsuoka et al.119−121 experimentally simulated the gasification of lignite in a TB-CFB using γ-alumina79,80 as the bed material. Temperatures of the riser, downer, and fluidized bed were maintained at 950, 900, and 800−850 °C, respectively. The yield of aromatic compounds (phenols, diaromatics, triaromatics, and tetra-aromatics) were below 0.01% on a lignite carbon basis, and this was attributed to very fast decomposition of tar over the fresh/recycled chars and coke deposited on the alumina. It was found that the total gas yield from the downer and fluidized bed was higher by a factor of 1.45 than that for a conventional DFB. These results clearly show effectiveness of well-timed intensification and subsequent elimination of VCI.119 Matsuoka et al.119 also carried out a simulation of a TB-CFB assuming an option that the gasifier (i.e., the fluidized bed) accepts heat from a gas turbine or fuel cell in the form of steam at 700 °C. This option gave CER as high as 0.9 at S/C = 1.0, which was higher by ca. 0.1 than that with a stand-alone option. This result is just a reasonable one but interesting if the inlet temperature of steam is taken into consideration. A simulation was performed assuming a simplified single reactor, as shown in Figure 12. The gasifier accepts steam at a

combustion of char. A schematic diagram of TB-CFB is illustrated in Figure 11. The fuel is fed to the downer instead of the bubbling fluidized bed and mixed with the bed material and char circulating through the system and steam. The volatiles formed by the rapid pyrolysis are in-situ-decomposed over newly formed and recycled chars, while the resulting carbon deposit (coke) is gasified with steam. Thus, in the downer, VCI is intensified within a period of up to several tens of seconds. The gas is separated from the char at the gas−solid separator between the downer and fluidized bed. The char is sent to the fluidized bed and gasified with steam but without oxygen/air. VCI is eliminated in the fluidized bed, although inhibition of the steam gasification by H2 is unavoidable. The unconverted char is sent to the riser and gasified with oxygen or air or burned. Controlling the char conversion in the riser enables the recycling of the char to the downer, the increase of its concentration in the circulating solid, and thereby enhancement of the reforming of volatiles. The temperature history and fate of solid fuel in TB-CFB is illustrated in Figure 11. The gasification in TB-CFB is operated with three concepts: intensification of VCI, elimination of VCI, and pyrolysis-driven fast char gasification. Fushimi et al.117 demonstrated stable circulation of bed material at a high density and velocity in a large-scale cold model of TB-CFB.118 They simulated fast circulation of silica sand and char using mixtures of glass beads and nylon shots and successfully performed fast and high-density circulation J


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the steam gasification of char because its yield is low enough to be used as the secondary fuel to be burned in the combustor. Employment of a catalytic reformer at the gasifier downstream (see Figure 9b) is an idea because the endothermic nature of the steam reforming can reduce the syngas temperature while eliminating the residual tar. Assuming that the tar reforming in the catalytic reformer is complete, the degree of the reforming in the fluidized bed would be optimized within a certain range. However, the catalyst in the catalytic reformer must be highly active (even at 700 °C or lower temperature) and durable in an atmosphere containing sulfur, chlorine, and vaporized AAEM species. Removal of particulate matter, such as fines of char and ash, between the gasifier and catalytic reformer is a challenging task.103

4. APPLICATION OF A SFB TO TYPE III GASIFICATION 4.1. Super IGFC. The steam gasification, if a sufficient quantity of heat at a high temperature is given, can convert the solid fuel completely without the aid of oxygen or air. The endothermic nature of the steam gasification theoretically enables the integration of the chemical energy of fuel and thermal energy into chemical energy of syngas. Recently, a novel concept of IGFC122 was proposed. A schematic diagram of the system is illustrated in Figure 13. Fuel cells, such as solid

Figure 12. Effect of the increasing inlet temperature of steam on the increase in chemical energy of resulting syngas for oxygen−steam gasification of lignite.

temperature (Tsteam) within a range of 200−800 °C and S/C = 0.8 together with the lignite (at 25 °C) and oxygen (at 25 °C) and converts the lignite. Texit is fixed at 800 °C, and therefore, increasing Tsteam results in the decrease in the oxygen consumption. The other assumptions are the same as those presented previously. Figure 12 presents the relationship between the increase in the enthalpy of steam by increasing Tsteam and the resulting increase in chemical energy of syngas. It is noted that the increment of CER is even more than that of thermal energy of steam. In other words, thermal energy (for increasing Tsteam) is fully recuperated to chemical energy of the syngas. An increase in CER in such a way is due to the decrease in the oxygen consumption. In practice, it is needed to increase the steam consumption (conversion) to compensate for the decrease in the oxygen consumption to realize the concept of chemical recuperation of thermal energy. Both A-IGCC and A-IGFC involve recycling of thermal energy from the gas turbine or fuel cells to the gasifier. This aims at the above-described chemical recuperation of thermal energy and reduction of exergy loss at the gasification. The concept of heat-to-chemical energy recuperation can be applied to any type of gasifier, but in practice, DFBs are the likeliest candidates. 3.5. Future Challenges in Development of Type II Gasification. A limitation of single-, dual-, and triple-combined fluidized-bed gasifiers is that Texit is determined by the temperature for the gasification or, otherwise, partially by that for the char combustion in the dual- and triple-combined fluidized beds. This limitation is originally due to a feature of the fluidized bed that homogenizes the reaction zone in terms of the concentration and temperature. It is therefore essential to reduce the temperature for the steam reforming of volatiles and steam gasification of char. For the biomass gasification in a dual- or triple-combined fluidized bed, the steam reforming of tar is more important than

Figure 13. Schematic diagram of super IGFC.122

oxide fuel cells (SOFCs), normally work at temperatures as high as 900−1000 °C with a very high electrical efficiency while forming heat at such high temperatures. In this novel IGFC, which is named super IGFC, high-temperature heat from the fuel cells is transferred to endothermic gasification directly. The gasifier works by playing a role of the thermal-to-chemical energy recuperator. A theoretical electrical efficiency of the proposed cycle is well above 80% when a steam turbine is combined (but without gas turbine). The fluidized bed would be the best candidate of the gasifier to be combined with the fuel cells because it can maximize the rate of heat transfer. As seen in Figure 13, pure or nearly pure H2 is used as the fuel for the fuel cells. Then, the syngas from the gasifier has to be deeply treated for removal of minor impurities, CO2, and tar (aromatic compounds). Escape of aromatic hydrocarbons, even that of benzene and toluene, from the gas-cleaning/purification system should be avoided in the proposed IGFC system as well as the general IGFC.123,124 Some recent studies claim that SOFCs can accept aromatic hydrocarbons, such as toluene and also tar, from gasifiers;125−128 however, feeding steam together with the syngas seems to be mandatory for minimizing carbon deposition and accumulation on the anode material, and this will cause reduction in the power generation efficiency. Thus, for application of biomass or lignite to such an advanced power K


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Figure 14. Yields of individual aromatic hydrocarbon homologues released from char during its rapid heating to 920 °C.131 “db” indicates the number of double bonds per molecule.131 Typical compounds (non-substituted): 3 db, benzene; 4 db, indene; 5 db, naphthalene; 6 db, acenaphthylene and biphenyl; 7 db, phenanthrene and anthracene; 8 db, pyrene and fluoranthene; 9 db, chrysene; 10 db, perylene and benzo[e]pyrene; 11 db, benzo[ghi]perylene; and 12 db, coronene.

gasification at 800 °C or lower temperatures. It may thus be needed to load a catalyst to char or the parent fuel prior to the pyrolysis. Provided that the char is gasified with steam in a fluidized-bed gasifier, application of an alkali metal catalyst would cause problems, such as defluidization, as mentioned previously. Loading transition metals, such as Fe and Ni, by impregnation of precursor salt or ion exchange is not encouraged because both normally need water-soluble salts containing Cl, S, or N. On the other hand, loading Ca, employing CaO, Ca(OH)2, or CaCO3, seems to be reasonable. Their solubilities in water are much less compared to chlorides and nitrates but high enough for Ca loading to lignite by ion exchange.140−142 In the fluidized-bed gasification, Ca may react with the bed material, such as silica sand, but the melting point of the resulting Ca silicate is over 1500 °C. The risk of defluidization is thus minimized. The catalytic activity of Ca species highly dispersed in the char matrix has been demonstrated.138,139 Characteristics of steam or CO2 gasification of Ca-loaded chars were rigorously investigated from the 1980s to 1990s.143−151,151−156 The present authors tried to predict the performance of steam gasification of Ca-loaded lignite char but found that there was no model considering the combined effects of the char conversion (entire range), Ca concentration, partial pressures of H2O, CO2, and H2, and temperature on the kinetics of gasification. Unfortunately, in most previous studies on the Cacatalyzed char gasification, kinetics and mechanism were analyzed only at specific char conversions or in a limited range of the conversion. Thus, despite the accumulated knowledge, we still need a comprehensive understanding and a way to describe the kinetics quantitatively over the entire range of char conversion.

generation system, best performances of the steam gasifier are required with respect to the tar elimination. 4.2. Use of Char Instead of Biomass or Lignite. Type II gasifiers are candidates for those implemented in the super IGFC. An option for type III gasification is to employ char, which is potentially an excellent solid fuel for the steam gasification, instead of its parent fuel. It is known for the pyrolysis of biomass and lignite that the tar formation is nearly completed at 500−600 °C.129,130 The pyrolysis at higher temperatures will produce char with less tar precursors but also lower the char reactivity with steam. The tar-free nature of the fuel also leads to no necessity of considering VCI in designing the reactor. Yang et al.131 investigated the emission of aromatic compounds ranging from mono- to hepta-aromatics (i.e., benzene to coronene) from biomass- and lignite-derived chars during heating to 920 °C. The total emission of tar, which was defined as the aromatics, except benzene, toluene, and xylene (BTX), from the biomass chars was 0.03−0.08 wt % of char, which was prepared at 450 °C, but decreased to around 0.01 wt %, by increasing the char preparation temperature to 600 °C. The emission of BTX was more than any greater aromatics and oxygenates (e.g., phenol) but was also suppressed to a level as low as 0.01−0.02 wt %. Figure 14 shows the emission of aromatic hydrocarbons of different classes from chars. Even a very low emission of aromatics may still need reforming of the syngas at the gasifier downstream when the inbed reforming is not extensive, but the use of char minimizes the load to the reformer and catalyst and guarantees the quality of syngas. Char from biomass, often called biochar, has potential as a high quality solid fuel,132−137 while the liquids from the pyrolysis, i.e., bio-oils, are expected as fuels and chemical feedstock.138,139 The feasibility of applying biochar to type III gasification and advanced IGFC is associated with that of bio-oil/biochar use in the future. As mentioned previously, AAEM species in biochars and those in lignite-derived chars catalyze the steam gasification, but their contents may not be enough for fast and complete char

5. APPLICATION OF A MULTI-STAGE REACTOR TO TYPE IV GASIFICATION 5.1. Gasification of Biomass in a Two-Stage Reactor. Type IV gasification is a proposal of this paper, and its particular feature is that Texit is substantially lower than the temperature for the gasification of char and reforming of L


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Figure 15. Schematic illustration of (a) two-stage gasifier consisting of a pyrolyzer and fixed (or moving) bed gasifier and (b) type IV gasifier.

that employed by Hosokai et al.72 The K-loaded biomass (K content, 2 wt %) was pyrolyzed at 550 °C, and the pyrolysis products were converted at a bed temperature around 700 °C (equal to the peak temperature) with S/C = 0.55−1.1 and O2/ C = 0. It was found that the heavy tar concentration in the syngas was lower than 100 mg/Nm3 dry, while that of lighter tar (phenols, naphthalene, and triaromatics) was ca. 1000 mg/ Nm3. More importantly, at S/C > 0.8, the net rate of char gasification was equivalent to that of char feeding from the pyrolyzer. This result suggests a possibility of complete conversion of the biomass by applying the char bed temperature as low as 700 °C. Sueyasu at al.160 also reported that K was retained in the char bed without being carried downstream. 5.2. Proposal of the Type IV Gasifier. Results of the above-mentioned studies72,157−160 are a basis of the proposal of type IV gasification. Its concept and an example of the reactor configuration are visualized in Figures 16 and 15b, respectively.

volatiles. The original configuration of the reactor for type IV gasification is that of a two-stage biomass gasifier, as illustrated in Figure 15a.72,157−160 The solid fuel is pyrolyzed in the first reactor (pyrolyzer), and the volatiles and char formed are reformed and gasified, respectively, in the second reactor (gasifier). The gasifier consists of a fixed or moving bed of char, and the headspace above the bed plays a role of the reactor for the partial oxidation of the volatiles with air or oxygen. The oxidation heats the vapor and char, while the steam reforming of the volatiles and steam gasification of the char both take place to some extent. The fixed or moving bed of char is automatically formed. The volatiles in the hot gas enter the char bed and decomposed there in a sequence of deposition of coke and its steam gasification, while the char is gasified with steam by a non-catalytic and/or catalytic mechanism. A feature of this two-stage gasification is that the char bed provides a relatively long residence time of the volatiles, which is favored by extensive reforming of the volatiles and also higher conversion of steam. This feature also gives an important role to the char bed, that is, quenching of the hot gas (formed in the headspace) by the endothermic reactions, in other words, thermal-to-chemical energy recuperation. The degree of temperature reduction by the quenching, in other words, rapidity of the steam reforming and gasification, determines Texit, and therefore, higher reactivity of the char with steam is favorable. Steady-state operation requires at least maintenance of the height of the char bed and, if possible, steady-state conversion of char as well as the volatiles, i.e., 100% carbon conversion to syngas in the bed. According to previous studies,157−159 it seems to be difficult to attain steady-state conversion of biomass unless excess air (oxygen) is fed. Hosokai et al.72 investigated the air−steam gasification of a woody biomass in a simulated two-stage gasifier, in which the pyrolyzer and gasifier were heated externally (by electric heaters) at 550 and 850 °C, respectively, at S/C = 0.13 and O2/ C = 0.11. The concentration of heavy tar (gravimetric tar) in the syngas was as low as 100 mg/Nm3 dry, but that of di-/ triaromatics (mainly naphthalene) was as much as 1300 mg/ Nm3. The yield of fresh char from the pyrolysis was 37% on a carbon basis, about 1/4 of which was converted to gas in the gasifier. Such a low char conversion was mainly due to small O2/C and S/C ratios. Sueyasu et al.160 loaded a precursor of the K catalyst (K2CO3) on a woody biomass and studied the conversion of the K-loaded biomass in a simulated two-stage gasifier similar to

Figure 16. Concept of two-stage quenching of hot gas (formed by partial oxidation) by steam gasification/reforming and then pyrolysis.

The solid fuel (biomass or lignite) and steam are fed into the first reactor (pyrolyzer) that is also a part of the counter-current heat exchanger accepting sensible heat of the syngas. The volatiles from the pyrolysis are introduced into the second reactor (gasifier) and partly reformed with oxygen and steam in the headspace above the char bed, while the gas is heated to a M


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Figure 17. Results of thermodynamic analysis of two-stage pyrolysis gasification of biomass and lignite (type IV gasification). Tref, temperature of syngas at the exit of reformer/gasifier. S/C ratio = 0.5 for both biomass and lignite. The analysis assumes that the latent heat of water is given externally, but the heat for heating the generated steam is given by the syngas.

temperature over 1000 °C. The nascent K-loaded char may undergo fast steam gasification (i.e., pyrolysis-driven char gasification) while dropping in the headspace. The hot gas enters the char bed and is decomposed over K-loaded char via deposition of carbon and its K-catalyzed steam gasification. The catalytic steam gasification of char takes place simultaneously with the reforming of the volatiles. These endothermic reactions quench the gas to a temperature of 700 °C (equal to Tref) or even lower. The heat of the syngas is then transferred to the pyrolysis by a way of heat exchange for driving the pyrolysis with a peak temperature of 500−600 °C. Thus, type IV gasification not only involves concepts for lowtemperature gasification (i.e., intensification of VCI, intensification of catalysis of AAEM species, and pyrolysis-driven char gasification) but also introduces staged quenching of hot gas. The isolation of the pyrolysis from the subsequent conversion enables physical use of the syngas and, thereby, Texit lower than Tref. Assuming progress of endothermic reactions in the fixed or moving bed of char, there occurs a large temperature gradient along with the bed axis. This temperature gradient is effective in maximizing the overall rates of steam reforming/gasification and a clear advantage over a fluidized-bed gasifier, in which the gasification has to be operated at the same temperature as Texit. K is expected to be highly mobile on the char surface and also in the bed because of repeated transfer among char particles. Recycling of the K-loaded char may enable retention of K inside the gasifier. It would also be reasonable to discharge the K-loaded char, recover K from the char by washing with water,105,106 load K to the feedstock, and then recycle the washed char to the gasifier. Thermodynamic analyses of type IV gasification were carried out. Conditions and assumptions are as follows: (1) Elemental compositions and calorific values of biomass and lignite are the same as those shown in the caption of Figure 2. (2) The reactor system is adiabatic. (3) Inlet temperatures of solid fuel and steam are 25 and 100 °C, respectively. The steam is generated using heat from a neighbor process for water−gas-shift conversion of CO, gas-to-liquid conversion [methanol, dimethyl ether (DME), Fischer−Tropsch oil, etc.], or power generation. (4) The temperature of pyrolysis products is 550 °C (biomass) or 600 °C (lignite) at the pyrolyzer exit.160,161 (5) Pyrolysis products are H2, H2O, CO, CO2, C1−C4 hydrocarbons, CH3OH and CH3CHO, tar (crude), and char.

Yields of the individual pyrolysis products were given from unpublished experimental data obtained using a pyrolyzer.161 (6) Heat required for the pyrolysis, i.e., difference between the enthalpy of the pyrolysis products at 550 or 600 °C and that of the solid fuel and steam at the pyrolyzer inlet, is given by the syngas. (7) Enthalpy of tar at the pyrolyzer exit (the gasifier inlet) was given from its elemental composition (unpublished experimental data) and equations proposed by Yang et al.161 (8) Solid fuel is completely converted into syngas consisting of CO, CO2, H2, H2O, and CH4. (9) CO, CO2, H2, and H2O are chemically equilibrated at the gasifier exit. (10) CH4 yields from the biomass and lignites are 2 and 4% on a respective carbon basis. The assumption of steam generation using external heat is reasonable, because any processes using the syngas consists of exothermic reactions. The quantity of the sensible heat of the raw syngas at Tref is enough to drive the pyrolysis. Figure 17 shows CER and Texit as a function of Tref for the same biomass and lignite as considered in the other analyses (see Figures 2 and 8). CER, if complete gasification is performed with Tref = 700 °C, is around 0.97. Such high CER shows efficacy of decreasing Texit, which is even lower than 300 °C. For both biomass and lignite, the endothermic pyrolysis “fortunately” starts at temperatures even at 150 °C and takes place over a wide temperature range, changing the product composition. Such a characteristic enables the transfer of the heat of syngas to the pyrolysis. However, decreasing Texit to such a level is a technical challenge. A most important technical subject is enhancement of heat transfer from the syngas to the pyrolyzing solid. Yang et al.161 developed equations for estimating thermodynamic properties of crude tar such as standard enthalpy of formation and enthalpy as a function of temperature from the overall elemental composition. They reported that heat required for the pyrolysis with peak temperature of 550 °C was in a range from 6−8%-HHV.161 Driving the endothermic pyrolysis by the sensible heat of syngas is thus effective for improving CER. Oike et al.162 developed a two-stage reactor system and investigated characteristics of conversion of Kloaded woody biomass. They performed continuous pyrolysis and oxygen-steam gasification at 550 and 720 °C, respectively, with S/C = 0.46 and O2/C = 0.20, and achieved steady gas yield of around 100% and total tar concentration of 120 mg/ N


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Nm3-dry. This result proves the concept of type IV gasification (Figure 16).

Use of an alkaline aqueous medium is believed to be a most reasonable way to subject lignin to catalytic gasification in subcritical water, because the dissolution process enables recovery of cellulose that can be applied to further processing in various industries. Development of catalysts that are highly active in alkaline aqueous media is important. It is also necessary to prepare a solution of lignin at high concentrations to achieve a high CER, because practical CER depend largely upon the amount of water to be heated to the desired temperature. There is no need of steam generation in type V gasification. On the other hand, the molar heat capacity of subcritical water at 200−300 °C and 30 MPa is about 2.2 times that of steam at 0.1 MPa. To minimize the impact of such large heat capacity and result in the loss of CER, technical development is mandatory for not only the increase in the lignin concentration in alkaline media but also the efficient heat exchange/recovery and, if needed, the introduction of exothermic reactions that can supply heat internally. Technical subjects for the subcritical water gasification of lignite are similar to that for the lignin. Rather, more effort may be necessary for achieving a high degree of dissolution of the lignite and also a high lignite concentration in alkaline aqueous media.

6. TYPE V GASIFICATION WITH SUBCRITICAL WATER Gasification of biomass in highly compressed water (HCW) has been drawing the attention of researchers because of particular features, such as no need of drying of heavily wet biomass, no need of oxygen for oxidation, high solvent power of super- or subcritical water that has a dielectric constant similar to organic solvents, etc. Comprehensive reviews of catalytic and noncatalytic gasification in HCW are available.163,164 This section gives a brief account for type V gasification that converts organics in subcritical water at temperatures of 350 °C or lower, which results in the formation of CH4-rich syngas. According to previous reports,165−175 complete or nearly complete gasification in subcritical water needs heterogeneous catalysts, such as Ni, Ru, Rh, and Pt, supported by ZrO2, TiO2, or carbon, and therefore, the organics to be gasified should be dissolved in the aqueous medium prior to the catalytic gasification. Lignin, a major component of biomass, is soluble to hot alkaline aqueous media, and this nature is a principle of delignification in conventional Kraft pulping processes, in which the lignin dissolved in a so-called black liquor is burned for generation of steam and regeneration of alkali and other reagents. It is also known that a substantial portion of lignite (brown coal) is dissolved in aqueous alkaline media at temperatures to 200 °C.176−178 Figure 18 shows stoichiometric

7. CONCLUDING REMARKS This review discusses gasification of biomass and lignin at temperatures well below 1000 °C, the implementation of which is expected in future industries. In the sense of thermodynamics, CER in the gasification is determined by not either the temperature for the char gasification or reforming of volatiles but the temperature of syngas at the reactor exit, Texit. In the sense of kinetics, it is effective to recognize chemical events, such as the VCI, behavior of inherent and extraneous AAEM species, and very fast pyrolysis-driven char gasification (PDCG). Some recently proposed reactor systems already involve or can introduce one or more concepts of intensification and/or elimination of VCI, intensification of catalysis of AAEM species, and acceleration of PDCG. Those concepts enhance steam reforming of tar or steam gasification of char if thermochemical reactions (i.e., pyrolysis, steam reforming, steam gasification, partial combustion, and/or full combustion) are properly isolated, rearranged, and/or integrated in the reactor system with a particular configuration. Type II and III gasification employing a fluidized bed effectively transform externally introduced thermal energy to chemical energy of syngas, and these are therefore candidates for gasifiers in advanced power generation systems, which minimize exergy loss in the gasification by lowering Texit and also thermal-tochemical energy recuperation. Type IV gasification can realize Texit as low as 300 °C, much lower than that for the steam reforming/gasification, by transferring the sensible heat of syngas to the endothermic pyrolysis. Theoretical CER of type IV gasification is as high as 0.96−0.97 on LHV of the solid fuel. Type V gasification is an extreme gasification that converts lignin and lignite in subcritical water. It needs at least a catalyst that maintains high activity in alkaline subcritical water and a lignin/lignite solution at a high concentration for realizing CER near the theoretical CER around 1.0.

Figure 18. Yields of gaseous products and CER for the gasification of lignin and lignite.

product yields and CER of complete gasification of lignin or lignite as a function of the consumption of water. The relative yields of CH4 and H2 depend upon the catalyst activity toward gasification to form CO/H2, water−-gas-shift reaction, and methanation (CO + 3H2 = CH4 + H2O). CER of the gasification is around 1.0 for both the lignin and lignite. Such high CER is, however, a theoretical CER. Even achievement of 90% of this theoretical CER is very challenging. In comparison to catalytic gasification in neutral and acidic aqueous media, reports on that in alkaline ones are much fewer. Elliott at al.179 investigated gasification of phenolic and carboxylic compounds in alkaline aqueous media at 350 °C and 20 MPa over a Ni catalyst. The conversion of those compounds was much lower than that in acidic media because of the stability of phenolate (−O−Na+) and carboxylate (−COO−Na+) and/or other reasons.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Some of the process simulations (thermodynamic analyses of gasification) were carried out within a project that was supported by the Funding Program for Next Generation World-Leading Researchers (NEXT Program) established by the Japan Society for the Promotion of Science (JSPS).

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