Review pubs.acs.org/EF
Technical Review on Thermochemical Conversion Based on Decoupling for Solid Carbonaceous Fuels Juwei Zhang, Rongcheng Wu, Guangyi Zhang, Jian Yu, Changbin Yao, Yin Wang, Shiqiu Gao, and Guangwen Xu* State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, P. R. China ABSTRACT: The thermochemical conversion process of solid fuels is explicitly shown as the processes of pyrolysis (including coking and carbonization), gasification, and combustion. These processes actually involve a similar complex reaction network. The so-called “decoupling” refers to the optimization approach of process performance through controlling the interactions between or among the involved individual reactions. Our previous article in Energy & Fuels (2010, 24, 6223−6232) has analyzed how the approach of decoupling applies to the gasification technologies and justified the realized effects from decoupling. This successive report is devoted to understanding the applications of decoupling to the other types of thermochemical conversion technologies (mainly including pyrolysis and combustion) so as to generalize the “decoupling” methodology for innovations of thermochemical conversion technologies. After a reiteration of the principle and implementation approaches (isolating and staging) for decoupling, reanalysis of the process design principle and its consequent technical superiorities based on decoupling is performed for a few well-known or emerging novel conversion technologies developed in the world. The concrete technologies exemplified and their realized beneficial effects include the high-efficiency advanced coal coking processes with moisture control or gentle pyrolysis of feedstock in advance, coal pyrolysis in multiple countercurrent reactors for producing high-quality tar, gasification of caking coal in fluidized bed through adopting jetting preoxidation of coal, low-NOx decoupling combustion of coal by developing the in-bed NOx reduction capabilities of pyrolysis gas and char, and coal topping combustion for the coproduction of tar and heat. These highlights further justified that the decoupling would be a viable technical choice for achieving one or more of the technical advantages among polygeneration, high efficiency, high product quality, wide fuel adaptability, low pollutant emissions in thermochemical conversions of solid carbonaceous fuels.
1. APPROACH REITERATION Thermochemical conversion provides the major technical route for utilizing solid carbonaceous fuels including coal, biomass, and municipal wastes. The conversion is generally shown with three types of process technologies: pyrolysis (including coking and carbonization), gasification, and combustion. In each of these conversion processes, not a single but a series of reactions occur to incur the explicit chemical changes. Figure 1 highlights the chemical behaviors involved in the thermochemical conversion processes of solid fuels. Explicitly, the conversion converts the treated virgin fuel into char, tar, and pyrolysis gas in pyrolysis, product gas, or syngas with CO, CO2, and H2 as its major components in gasification, and heat and flue gas consisting of CO2 and H2O in combustion. These three types of different conversion processes are in fact resulted from differentiating the oxygen amount fed to the conversion system and the typical operation temperatures. The combustion process requires an excessive O2 supply to convert all fuel C and H into CO2 and H2O, while the (autothermal) gasification refers to the conversion of a solid fuel into CO and H2 under an insufficient oxygen supply but enough to maintain the required adiabatic reaction temperatures between, for example, 1173 and 1973 K. The process of pyrolysis indicates the conversion of fuel without the presence of O2 or with certain O2 but much less than that required by gasification. All these conversion processes involve essentially the same set of reactions as listed in the middle rectangle box in Figure 1. It is the difference in © 2013 American Chemical Society
Figure 1. Chemical behaviors occurring in thermochemical conversion of solid fuels. Note: These reactions essentially occur in the process of pyrolysis, combustion, and gasification, but the occurring extent varies with different conversion process. Some reactions can be ignored in a specific process. For example, in combustion the polymerization, reforming, and hydrogenation occur to a very low extent and can be ignored completely.
the degree or depth of the reactions with O2 which causes the conversion process to explicitly show as the pyrolysis, gasification, or combustion processes. Thus, in Figure 1 the terminologies of pyrolysis, gasification, and combustion refer to Received: January 21, 2013 Revised: March 18, 2013 Published: March 18, 2013 1951
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In order to manipulate individual reaction to avoid or weaken the undesired interactive effects but strengthen the desired ones, separating the related reactions and in turn rearranging them is necessary and worthwhile. This idea of reaction control has been termed “decoupling” in our previous publication.16 Two modes or approaches for implementing the decoupling, both “isolating” and “staging”, have also been developed. Through analyzing their applications to gasification, it has been found that the “isolating” and “staging” modes or approaches correspond rightly to the conversion technologies based on “dual bed” and “staging”, respectively. The technical advantages realized by performing decoupling were fully justified in ref 16 through reanalyzing the technical principles and features of several well-noted gasification technologies. These actually show that the decoupling provides a viable idea to invent new gasification technologies with the technical superiorities of, for example, polygeneration, high efficiency, high product quality, wide fuel adaptability, and/or low pollutant emission. Figure 2 compares the two decoupling approaches. The implementation of the decoupling consists of two sequential
their individual reactions in the middle rectangle box but to their macroprocesses in the last column of the figure. The reactions in the rectangle box occur in sequence and are intercorrelated or interactive. With heating, the solid fuel is first dried and pyrolyzed to produce char, tar, steam, and uncondensable pyrolysis gas mainly consisting of H2, CO, CO2, and CH4. In turn, the other reactions in the box start to occur and lead to a series of interactions between/among the various reactions through the actions or impacts of some products on the reactions. With the same reaction numbers as shown in the rectangle box of Figure 1, the following highlights the major interactions. 1) The pyrolysis reaction (2) provides reactants, including tar and gaseous product for the reactions of decomposition (3), polymerization (4), and hydrogenation (5);1,2 2) Char is gasified via reaction (6) by reacting with the steam and CO2 generated in fuel drying (1), pyrolysis (2), and gasification (6) as well as the combustion of gas and char (7); 3) The tar and hydrocarbons (CmHn) can be reformed through reaction (8) with steam and CO2 from all reactions and participate in the reactions like combustion (7);3−5 4) The gaseous products (H2, CO, tar, etc.) of pyrolysis (2) inhibit the char gasification reaction (6);6,7 5) Char and its inherent metals catalyze the reforming reaction (8), cracking or decomposition reactions (3) of the gaseous pyrolysis products including tar and CmHn;8−10 6) All combustible matters, including char and gaseous products of pyrolysis, are combusted to supply the reaction heat required for all the other endothermic reactions including drying (1), pyrolysis (2), gasification (6), reforming (8), and cracking or decomposition (3); 7) The WGS reaction (9) occurs to adjust the concentrations of H2, CO, CO2, and steam in the produced gas products;11 and 8) The ash from combustion or gasification affects the other reactions, for example, the ash can affect the fuel reactions of pyrolysis (2) and gasification (6) mainly by the catalysis effects of the ash,12,13 and the ash also react with the flue gas of combustion (7).14,15 These interactions make the involved reactions in the thermochemical conversion processes, as listed in Figure 1, closely intercorrelated to form a complicated reaction network described in our previous publication. 16 Among these interactions, some can facilitate the conversion to lead to low pollutant emission, high efficiency, high product quality, and wide fuel adaptability, whereas the others are not. Therefore, it deserves to control the interactions for optimizing the conversion process. In most commercial and in-developing processes of pyrolysis, gasification, and combustion, all such reactions are arranged to occur in a single reaction space, thus it is impossible to control any individual reaction and its interactions with the others. This possibly causes high pollutant emission, low conversion efficiency, low product quality or value, poor fuel adaptability, and so on. Taking gasification technologies as an example, most of the existing technologies produce only syngas or fuel gas and have their respectively different fuel adaptabilities. While air is used as the gasification reagent, the produced gas has to be diluted by N2 and CO2 generated in the internal combustion, causing thus lower gas heating value. Without control of the interactions via decoupling, the low-temperature gasification such as fluidized bed gasification also has limited capability to reduce tar yield.
Figure 2. Illustration of the two implementation modes or approaches based on a simplified reaction network for the decoupling in thermochemical conversion.
actions upon the involved reactions. They are the separation of one or more reactions from the intercorrelated reaction network (see ref 16) by breaking the linkages between or among neighboring reactions and in turn the rearrangement of the separated or decoupled reactions according to the needs of reaction control. The decoupling mode or approach is the “isolating”, if the decoupled reactions are arranged into isolated reactors to separate their products and fully suppress the interactions between their products. The corresponding technologies generally have several product streams and are related to dual bed conversion, realizing the effects of polygeneration, high product quality, and wide fuel adaptability. The involved decoupling mode is the “staging” if the decoupled reactions are reorganized to facilitate the beneficial interactions or suppress the undesired interactions. The corresponding conversion technologies have only one product stream and are usually related to “staging”, which is explicitly shown as twostage processes, fuel staging, or reactant staging. These conversion technologies are most effective in lowering pollutant 1952
dx.doi.org/10.1021/ef400118b | Energy Fuels 2013, 27, 1951−1966
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Table 1. Typical Thermochemical Conversion Technologies and Their Features by Involving Decoupling process pyrolysis
decoupled reaction(s) drying
CMC
commercialized
SCOPE21
demonstrated
COED
demonstrated
see Table 4
demonstrated or commercialized
DBG
demonstrated
PG TSG
in pilot test demonstrated
pyrolysis pyrolysis combustion/ reforming pyrolysis pyrolysis
Improve tar quality and fuel adaptability by taking advantage of the interactions among pyrolysis reactions of different temperatures. Heating fuel (e.g., coal) by flue gas of gas or char combustion or by solid heat carrier particles heated by the isolated combustion to avoid presence of O2 in the pyrolyzer to have high tar yield and quality. Avoiding dilution of produced gas by N2 and combustion-generated CO2 to produce middlecaloric fuel gas using air as a gasification reagent. Coproducing pyrolysis oil and fuel gas or syngas to use coal hierarchically. Reforming/cracking of tar and pyrolysis gas by catalysis of char to produce fuel gas or syngas with little tar. Avoiding caking of softened coal or char to improve fuel adaptability of the technology. Partial hydropyrolysis of coal to raise pyrolysis product quality and increase energy efficiency. Increasing efficiency and enhancing in-bed tar elimination to produce middle-caloric fuel gas with less tar and more H2. Coproducing pyrolysis oil, pyrolysis gas, steam, and electricity to use coal hierarchically. Burning pyrolysis gas in combusting char bed to lower NO and CO emissions.
POG PHG T-DBG
demonstrated demonstrated pilot finished pilot finished commercialized
pyrolysis
Reburning pyrolysis gas in transporting bed of char in burning to lower NO to CO emissions.
TC DC (gratebase) DC (CFBbase)
combustion of pyrolysis gas or char combustion pyrolysis pyrolysis
combustion
development stage
Using exhaust gas to control coal moisture for realizing the use of weak-coking coal, high productivity and high energy efficiency. Rapidly pyrolyzing weak-caking coal to increasing its caking property for using weak-caking coal and for having high productivity and low process energy consumption.
drying and gentle pyrolysis gentle pyrolysis
gasification
typical technologya
decoupling effects
in demonstration
a
CMC: coal moisture control,17 SCOPE21: super coke oven for productivity and environmental enhancement toward the 21st century,18 COED: char oil energy development,19 DBG: dual bed gasification,20 PG: pyrolysis gasification,21 TSG: two-stage gasification,22 POG: preoxidation gasification,23 PHG: partial hydropyrolysis gasification,24 T-DBG: two-stage dual-bed gasification,25 TC: topping combustion,26 DC: decoupling combustion.27
analyzed in ref 16 in terms of the decoupling to show the necessity of decoupling for such technologies. Among them, the former two, which are the dual bed gasification (DBG) and pyrolysis gasification (PG), adopted the “isolating” mode, while the following three, the two-stage gasification (TSG), preoxidation gasification (POG), and partial hydropyrolysis gasification (PHG), are based on the “staging” mode. The listed last DCG technology, two-stage dual bed gasification (T-DBG), refers to an example of combining both modes of the decoupling. As illustrated in Figure 3, the DBG was created by decoupling the char combustion from all the other gasification reactions, and the PG, TSG, POG, and PHG technologies were all born from decoupling and then reorganizing the pyrolysis reaction. The T-DBG refers to a technical formation from isolating the char combustion (dual bed) and separating the in-bed tar reforming/cracking from the gasification reactions (two-stage gasifier). Table 1 also briefs the realized decoupling effects for all the indicated DCG technologies. These effects, as detailed in ref 16, have been well validated through pilot (bench) tests or demonstration applications. This essentially demonstrates that the DCG technologies innovated with the decoupling idea of either the isolating or staging or both of them can effectively lower pollutant emission, raise efficiency, increase product quality, or/ and allow wide fuel adaptability. Because the related details about DCG can be found in ref 16, no more analysis is presented herein regarding the decoupling in the gasification.
emission, raising efficiency and product quality, and allowing wide fuel adaptability. In practice, the decoupling may also be implemented by involving both “isolating” and “staging” to have some creatively new designs for thermochemical conversion of fuels. In succession to our previous publication,16 this article is devoted to generalizing the conception and applications of the decoupling to all types of thermochemical conversion technologies. This is ensured by further justifying the applications of the decoupling modes (or approaches) of “isolating” and “staging” to the pyrolysis and combustion technologies. Table 1 summarizes some typical and well-known thermochemical conversion technologies based on decoupling and their realized major decoupling effects (i.e., benefits from decoupling). These technologies involve three types of conversion process: pyrolysis, gasification, and combustion, and here the pyrolysis also includes coking. It can be seen that all of these technologies have been commercialized or demonstrated or in the process of commercialization. Herein, all the technologies listed in Table 1 will be analyzed in terms of the decoupling to understand their process principles and demonstrate their technology superiorities ensured by implementing the decoupling. This in turn verifies the validity of the isolating and staging decoupling modes and their resulting conversion technologies.
2. APPLICATION TO GASIFICATION The gasification technologies based on the decoupling have been named as the decoupling gasification (DCG) technologies in our previous publication.16 Up to now, the decoupling has been widely applied to the development of gasification technologies, and Figure 3 highlights the process principles of six typical DCG technologies. All the technologies have been
3. APPLICATION TO PYROLYSIS The foregoing decoupling modes or approaches are also effective in the development of new pyrolysis technologies for coal and biomass. Fuel pyrolysis has to experience the stages of drying, slight pyrolysis (i.e., low-temperature devolatilization), 1953
dx.doi.org/10.1021/ef400118b | Energy Fuels 2013, 27, 1951−1966
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Figure 3. Process schematic diagrams of the representative decoupling gasification (DCG) technologies highlighted in ref 16.
and deep pyrolysis (high-temperature devolatilization), as shown in Figure 4. Here, the slight and deep pyrolysis can be further divided into more stages or steps of successive pyrolysis, if this is needed in process design. The implicated cause for this fine division of the reaction process is that the interactions between the devolatilization reactions at two different temper-
atures obviously vary with the selected temperatures as well as with the temperature difference. The products of pyrolysis process are char/coke, pyrolysis gas, and tar. A part of these products, for example, pyrolysis gas, may be recycled into the pyrolysis reactor to develop some beneficial effects. Also, combusting a part of the generated char or pyrolysis gas is 1954
dx.doi.org/10.1021/ef400118b | Energy Fuels 2013, 27, 1951−1966
Energy & Fuels
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Figure 4. Fundamental reaction steps involved in fuel pyrolysis in terms of implementing decoupling (here, “heat” comes from combusting part of char or pyrolysis gas, the heat carrier can be char, pyrolysis gas, or/and flue gas).
justifies the realized typical decoupling effects via adopting the CMC technology. It shows that the CMC technology can increase the bulk density of the charged coal in the coke oven from 0.675 to 0.725 g/cm3, raise the blending ratio of non- or weak-caking coal from 10 wt.% to 20 wt.%, and decrease the energy consumption in coking by 8%.29,30 Three generations of CMC technologies have been developed since the 1980s,17,31−33 which are the rotary kiln CMC with oil as the heat-transfer medium, the rotary kiln CMC with steam as the heat-transfer medium, and the fluidized bed CMC based on direct interaction of coal and hot flue gas from coke oven. The first two types of technologies have been widely used in Japan, and China imported as well several sets of apparatus. The third-generation CMC technology has been applied only in Japan with the capacity of 120 t/h from 1996. A few Chinese steel companies, such as Jigang Group Co., Ltd., are developing the indigenous third-generation CMC technology,33 but these technologies are still under development. Liu et al.34 found that the fine fraction (e.g., 3.0 mm) is usually below 10.0 wt.%. Therefore, in the coking process the coal crushing should primarily be used for such a coarse fraction, whereas the moisture control should be mainly applied to the fine fraction. Based on this principle, Xu et al.32,34 proposed a coal classifying moisture control (CCMC) process which simultaneously implements the classification and coal moisture control (CMC), as shown in Figure 6. The process adopts an integrated bed consisting of a dense fluidized or moving-bed bottom section and a dilute, pneumatic conveyer top section to entrain coal particles below a given size (e.g.,
