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Gasification processes incorporate many reactions that are fairly complex to analyse making their design difficult. In this chapter it is shown that general gasification systems are limited by consideration of mass and energy balances only. Here, a ternary Carbon-Hydrogen-Oxygen diagram is developed to represent gasification processes. The diagram incorporates basic chemistry and thermodynamics to define a region in which gasification occurs. The techniques are further validated from data obtained from pilot or laboratory experiments available in literature. In this chapter we develop graphical representation for sawdust gasification and underground coal gasification (UCG), a clean coal technology. The methods described allow for further analysis without considerations to thermodynamic equilibrium, reactor kinetics, reactor design and operation. This analysis is thus an indispensable tool for flowsheet development using gasification and an excellent tool for practitioners to rapidly understand gasification processes.
School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa
*Address all correspondence to: shehzaad.kauchali@wits.ac.za
1. Introduction
Biomass gasification processes produce a versatile fuel-gas using a thermo-chemical conversion of the biomass in a reducing environment in the presence of air, oxygen or steam. The resulting gas is cleaned and is generally suitable for heating, power generation or liquid fuel production. The important drivers towards biomass utilisation include renewable and sustainable energy sources, the Kyoto protocol addressing the need to lower carbon dioxide emissions and the CO2-neutrality of biomass emissions. However, it is argued that biomass conversion systems be as efficient as existing fossil fuel technologies [1]. It is stated that gasification is one of the least efficient processes in the biomass-to-energy value chain and a study on the gasifier alone can lead to substantial improvements [2].
Large amounts of literary work, including theoretical and experimental developments, on biomass gasification have been published [3, 4, 5, 6, 7, 8, 9, 10, 11].
The use of bond-equivalent percentages to study conversion of coal to other materials on a ternary Carbon-Hydrogen-Oxygen (CHO) diagram has been advocate by [12]. [13] have used a CHO diagram to determine the feasible operating region of a moving bed gasification reactor. In an important follow on work, by [14], it was shown that any coal gasification process can be constrained to a region, by stoichiometry, and further to a line or plane by energy considerations. Thus complex coal gasification reaction schemes can be interpreted readily before the consideration of thermodynamic equilibrium, kinetics, reactor design and operation. This work forms the basis of the sawdust gasification analysis in this paper. Recently [15] use a graphical targeting approach, on the CHO diagram, to design a biomass gasification process for methanol production. This chapter seeks to provide design options for biomass gasification, on the CHO diagram, in order to evaluate theoretical limitations of the complex reacting systems. Moreover, these options are envisaged to assist in the design of new pilot-scale experiments or commercial operation of biomass and underground coal gasification systems.
There is a lack of coherent approaches to designing gasification processes. This is partly due to the fact that most approaches rely heavily on reactor types, where the information is proprietary and partly due to non-existence of fundamental explanations based on simple chemistry and thermodynamics. It is thus useful to develop a method that enables the understanding of gasification from basic principles. Lastly, and more importantly, it would be useful to empower a designer to suggest experimental validity, for given solid-feedstock, based on preliminary designs derived from the methods discussed in this chapter. This will invariably lead to honing into final designs quicker, with less experimental effort and cost.
The chapter is ordered according to the following: first the bond-equivalent CHO diagram is introduced, followed by the determination of the important gasification reactions and stoichiometric region for sawdust and underground coal gasification, followed by the determination of autothermal operation and the representation of experimental data on the CHO diagram.
The bond-equivalent percentages, as introduced by [12], implement the bonding capability of each element in the CHO system. Bond-equivalent percentages spread data points uniformly in the CHO diagram, making analysis visually appealing, and this technique is used for the remainder of the discussions in this work.
2.1 Introduction to CHO diagram
The bond-equivalent CHO diagram is shown in Figure 1, below, where the apexes represent pure C, H and O as well as pure C, H2 and O2. The other important permanent species that need to be represented are CO2, CO, H2, CH4 and H2O [5]. For example, to obtain the bond equivalent fraction for a species CxHyOz, the contribution by carbon is 4(x), hydrogen is 1(y) and oxygen is 2(z), which is normalised for each species. Thus CH4 is represent by C = 4/(4 + 4) and H = 4/(4 + 4) and places the point midway between C and H. Similarly CO2 and H2O are midway between C-O and H-O respectively. CO is a third between C-O.
Figure 1.
Representation of chemical species on the bond equivalent CHO diagram.
2.2 Representing chemical species and reactions
Chemical species, as individual or in a mixture (such as feed to a process), can thus be represented as single points on the CHO diagram. For example, a synthesis gas of composition 33.3% CO and 67.7% H2 (CO:2H2) may be represented as a single COH4 species and is plotted in Figure 1. Dry sawdust represented by CH1.35O0.617 [16] is also shown.
A further property of the diagram is that reactions may be represented as intersections of two lines: one representing the reactants and, the other, products. For example, the line joining CH4 to O2 intersecting with the line joining CO and H2 represents partial oxidation of methane to form H2 and CO, in the ratio 2:1.
The analysis performed here utilises dry sawdust with chemical formula CH1.35O0.617 with HHV of 476 KJ/mol [16] and a calculated ∆H of formation of −107.77 KJ/mol. The nitrogen, sulphur and other elements (including ash) are considered inerts within the CHO diagram and are excluded from analysis. The theoretical development here seeks to determine the region in the CHO triangle where the gasification of sawdust is feasible and attractive energy-wise. Furthermore, the theoretical result will be compared with those from pilot scale experiments in a later section.
3.1 Stoichiometric region of operation for sawdust
It is acknowledged that gasification reactions are complex comprising of numerous reactions occurring on solid surface or in gas phase. The gasification system considered here comprises of sawdust, steam and oxygen (or air with nitrogen as inert). In contrast, [14] considers a similar system with fixed carbon, steam and oxygen to represent a coal gasification system. Furthermore, a simplified set of reactions are provided that limit the product species from the list of permanent gases (CO, CO2, H2O, CH4 & H2) that occur in appreciable amounts between 650 K–1500 K [5].
For the sawdust system, the following reactions at 650 K will thus be considered:
The reactions (r1-r16) are not chosen arbitrarily. The reactions are chosen on the basis that sawdust will react with a number of gases, some from feed (oxygen, steam) while others from primary products such as hydrogen or carbon dioxide.
3.2 Graphical representation of sawdust reactions
The reactions (r1-r16) are plotted on the CHO diagram in Figure 2. The dotted line represents combustion reactions and the dashed lines are gasification reactions. It is noted that there are no reactions with CH4 and biomass or CO and biomass as these lines (CH4-sawdust & CO-sawdust) do not intersect with any other lines since they are on the extreme edges. There are other reaction schemes plausible that have not been included as they shall not form part of the important subset shown later.
Figure 2.
Graphical representation of sawdust reactions.
3.2.1 The non-negative basis reactions
From the reactions given above, some reactions are dependent on each other. Furthermore, the gasification system, and hence the analysis, requires only those reactions to form the basis reactions which are able to: 1) obtain other reactions by positive linear combinations, and 2) do not produce the original feed reactants, in particular O2, H2O and C. The reader is directed to [14] for further clarity. The eight important basis reactions that satisfy the two conditions are given in Table 1.
A method for determining which reactions are part of the basis set can be described as follows: Firstly, connect all product species, excluding the ones that appear in the feed (steam and oxygen). Note, water-methane, water-carbon dioxide and water-carbon monoxide are thus also omitted. Secondly, connect the feed (sawdust) to the feed oxidants (steam and oxygen). The intersections that are formed (within the diagram – excluding edges) are the basis reactions where the connected points form the reactants and products respectively. Also note hydrogen is not forming part of the reactants in the basis reactions as it is not specified as a feed and thus is excluded.
Any sawdust gasification overall reaction can be obtained by positive linear combinations of the eight basis reactions inTable 1. This is translated graphically by implying that an interior point in the space (formed by the basis reactions) can be obtained by connecting any boundary points.
Table 1 summarises the important reactions between sawdust, oxygen and steam. Notice, the reactions also represent the line, which in turn, determine the reactants or products. This is shown in Figure 3. For example, a line representing the reactants (sawdust and oxygen) is obtained by connecting the sawdust point with the pure oxygen point. However, the products obtained from this reactant line (sawdust-oxygen) are dependent on which product line is intersected. The product line is one which contains two of the permanent gases listed previously. For illustration purposes, consider the two product lines obtained from H2-CO and H2-CO2 – these are strictly products as none of them feature in the feed given. Finally, to obtain the reactions, say r1, the intersection of the lines joining sawdust-oxygen and CO-H2 are considered. In Figure 3, this intersection point is presented by point A. It also represents the bond equivalent point plotted for either the feed or product. The relevant stoichiometric values are then used to balance the reaction and are listed in Table 1. The process was thus repeated for all possible intersection points and a set of balanced reactions were obtained (r1-r16). Moreover, the heat of reactions were determined based on the balanced reactions. The values have been provided in brackets after every reaction. Whilst, the reactions are not meant to represent reaction sequence or mechanism they do provide for a macro representation of the possible outputs from a sawdust gasification system. This is useful when predictions of syngas composition is critical for design.
Figure 3.
Stoichiometric region for sawdust without methane formation.
The heat of reactions listed with the various reactions are important as they provide the necessary energy for gasification processes to occur. It is noted that some heat of reactions are endothermic (positive) and some exothermic (negative). Of particular interest are the heat of reactions for r6, r9 and r10. r9 depicts the exothermic nature of steam reaction producing syngas rich in CO2 and CH4 – this has not been reported elsewhere and is of commercial interest requiring low temperature (<400C) and perhaps even the use of catalysts. r6 and r9 both demonstrate the lowest amount of oxygen and steam required to gasify sawdust, to produces syngas rich in CO and CH4, at high temperatures and non-catalytically.
The sawdust-oxygen intersection with the product lines were depicted in Figure 2. These are represented, in order from the sawdust point, by r6, r1, r5, r4, r2 and r3. Of these points, it is noted that r3 and r4 do NOT form part of the basis reactions as one of the products (water) is already accounted for in the feed. This leaves only reactions that form either one of the products: CO, CO2, H2 and CH4. The same analysis applied to sawdust-water intersections with the product lines requires that those reactions that produce only the products CO, CO2, H2 and CH4 are included.
For most gasification systems, the compositions of the syngas desired is dependent on the end use for the gas. For example for liquid chemicals production syngas rich in H2 and CO, with minimal CH4, is required. The system can be designed for low methane production. When methane is not formed, then only the first four reactions (Table 1 and region ABCD in Figure 3) will provide the possible products obtainable from the gasification system using sawdust, oxygen and steam. This fundamentally implies that any sensible gasification (conversion of sawdust to gas with significant calorific value/energy content) will occur inside the stoichiometric region ABCD. Operating outside of this region will result in material not converted in the gasification process and leave the gasifier unreacted – which is not a preferred mode of operation.
3.2.2 Stoichiometric regions without methane reactions
When methane reactions are excluded from the reactor product, such as required for liquid fuel or chemicals production, the basis reactions as from Table 1, form a region (ABCD) as shown in Figure 3, above. It is noted that these reactions, which form part of the extreme boundary, span all sensible gasification products within the region. Any interior point inside region ABCD can be obtained by linear combinations of reactions r1, r2, r7 and r8 where the final products will be a combination of H2, CO and CO2 only. Moreover, the edges of the region comprise of oxygen (air) gasification processes, on the lower side (AB), and steam gasification (CD) on the top side of ABCD. Furthermore, these reactions are chosen on the initial premise that no product should contain any reactant, hence any reaction that forms steam (or oxygen) is automatically rejected. Also, operation of a gasification system to the left of AD implies that the feed contains more sawdust than steam and oxygen, which inherently implies that unreacted sawdust should be expected at the exit of the reactor. Similarly, operating to the right of BC implies that the feed contains more steam/oxygen which will leave the gasifier unreacted, implying non-optimal usage of steam/oxygen. It is in this context that it is implied that sensible gasification occurs within the region ABCD. The case where methane is formed is omitted from further interpretation and will form part of a future publication.
When gasifiers run under adiabatic conditions, without heat loss or added heat, the system balances the exothermic reactions with the endothermic reactions. In Figure 3, the two exothermic reactions r1 and r2 can be used to balance the endothermic reactions r7 & r8. Line EF forms the thermally balanced line and the product temperature equals the inlet temperature. The thermally balanced equations for reactions E and F are given in Table 2 for the case where no methane forms.
Thermally balanced basis reactions without methane formation.
When methane is not produced, any thermally balanced process can be obtained by the linear combination of the two thermally balanced basis reactions. In Figure 3, below line EF products emerge hotter, while above the line they are colder. Furthermore, point E is preferred under low H2O/O2 ratios while F would be preferred for high H2O/O2 ratios. According to [14] practical gasification processes occur below the thermally balanced line and on the hot side. The reason is a combination of compensation for heat losses as well as methanation in real gasification systems. Operating in the colder section is an indication of external heat sources used to drive the endothermic reactions. Section 6 looks at some experimental points for sawdust gasification in relation to the thermally balanced line EF.
5. Carbon boundary and contours of higher heating value
The work of [17] studied the effect of temperature and pressure on carbon formation in gasification systems. It was identified that it is common for carbon to partially gasify and, due to kinetic limitations, solid carbon does not achieve equilibrium. Furthermore, the carbon boundary, under thermodynamic limits, may be represented on the CHO diagram as isotherms at constant pressure. Two such isotherms have been depicted in Figure 4 at 1000 K [7] and 733 K [5]. Operating a gasification process within the carbon boundary indicates that there is a propensity for unreacted carbon to occur in the product stream.
Figure 4.
Carbon boundaries at 733 K and 1000 K with HHV contours.
This results in low carbon conversions with some carbon remaining in the ash. Moreover, it is desirable to operate in a carbon-free region. In Figure 4, it is evident that operating a process with feed within the stoichiometric region ABCD, at low temperatures (733 K), will invariably lead to carbon deposition. It is therefore important to determine the average maximum temperature achievable in the gasification system in order to assess the location of the carbon boundary. Figure 4 also shows a carbon boundary for a system that operates at 1000 K. The presence of the high temperature carbon boundary further reduces the stoichiometric region in which it is desirable to operate a gasification system. For exothermic gasification, with 100% carbon conversion, it is favourable to operate in the region defined by KBFL (Figure 4). Figure 4 also shows the calorific value (HHV) contours (3–7 MJ/m3) for the idealised stoichiometric region when only air (Nitrogen 79%) is used. These contours are useful when deciding on the targeted calorific value of the product syngas as well as air and steam requirements.
6. Representation of experimental points for sawdust gasification
Tables 3–5 summarise some experimental data available for analysis on the CHO-diagram. It is notable to see that the fuels used have similar C,H and O content. In this analysis the chemical representation of [16] was used to determine the heat of reaction for sawdust and used subsequently for all the other reactions in the respective calculations. It is also noted, that the experimental results have been performed in various types of gasifiers ranging from fixed bed, circulating, entrained flow reactors and even catalytic systems.
Bond equivalent composition for syngas from various experiments.
Sawdust gasification tests in a pilot-scale air blown circulating fluidized bed gasifier have been performed by [7]. 15 runs were performed with over 6 species of sawdust (with varying moisture content) at atmospheric pressure and temperature ranging from 700–815°C. With air as gasification medium, syngas contaminated with nitrogen was produced with dry gas heating values ranging from 2.43–4.82 MJ/m3 (STP) and 3.59–6.13 MJ/m3 (STP) if tar and light hydrocarbons are produced. A CHO diagram was used to analyse the experiments relative to the carbon boundaries with the conclusion that there are kinetic limitations restricting the full conversion of carbon. Figure 5 depicts the collection of extreme experimental points from [4] as indicated by the shaded region. It is of interest to observe that the points lie within the stoichiometric boundary, and on the hot side. Moreover, there are experimental points that lie on the carbon boundary. It is noted that the sawdust used in the experiment have a slightly higher hydrogen content than the one used for the analysis so some deviations are expected.
Figure 5.
Representation of experimental points for sawdust gasification.
Zainal et al. [10] develop an equilibrium model to predict the gasification process in an adiabatic downdraft gasifier. The result is plotted in Figure 5 for an adiabatic downdraft gasification of sawdust. The downdraft gasifier lies once again in the stoichiometric region and on the hot side of the thermally balance line. Also, it is found that a downdraft gasifier can be modelled using an equilibrium model provided the gasification temperature is known.
The effects of metal salt catalyst on gasification of sawdust in a fluidized bed gasifier was studied by [18]. For sawdust it was noted that using NaCl and K2CO3 as salt catalyst increased yields of CO and CH4. Excess steam was used in the gasification system and the reported data in Table 4 was determined by analysing the dry syngas data, the feed mass balance and the WGS reaction.
Qin et al. [19] performed a laboratory scale entrained flow gasifier at temperatures of 1400C using feedstock comprising wood, straw and dried lignin. The experiments were conducted using excess steam but report the syngas on a dried basis. The values for the syngas immediately after the entrained flow gasifier reported in Table 4 have thus been recalculated based on the known feed mass balance, the gasification temperature and the syngas output composition (dried). It is noted some WGS reaction had to be included to obtain the final compositions.
A Computational Fluid Dynamics (CFD) model developed by [20] predicted the output performance of an entrained flow gasifier using biomass (sawdust and cotton trash). The output of the syngas is suitable for gas-to-liquid process such as methanol or Fischer-Tropsch liquids.
A novel pilot scale bubbling fluidized gasifier was built by [21] to study the effects of gasification oxidants. 8 such points have been included and represented by a single straight line in Figure 5. It is noted that excess steam has been used and hence the points lie out of the stoichiometric region (grey shaded region). The product syngas is then further dehydrated in an additional step to obtain the dry gas reported by [21].
The CHO diagram development and the analysis performed in this work have thus been validated by experimental data. In summary, sensible biomass gasification systems will operate in a well-defined mass balance region (grey shaded region ABCD in Figure 5). This region is further divided by the presence of the energy balance and the carbon boundary (derived from maximum temperature achievable for gasification). With the additional information of the HHV contours, a desirable operating point (for high HHV syngas) can be determined at the intersection of the thermally balanced line and the carbon boundary (maximum gasification temperature). The experimental points from literature also confirm the operational regions for sawdust gasification. Hence, preliminary designs or experimental programs can greatly benefit as a targeted approach is used prior to expensive trials.
While the basis reactions in Table 1 provide the necessary process schemes required for gasification, they do not explicitly say how the specific stoichiometry is to be obtained. The restricting factor here is thermodynamic equilibrium limitations and, in the case of [7], kinetic limitations. In general, some aspects of gasification processes may be modelled as equilibrium systems. However, thermodynamics restricts the theoretically achievable CO:H2 ratios as required by the ideal stoichiometric region. For example, if reaction F is desired at say 1000 K, the equilibrium compositions are H2O: 0.19, CO:0.19, CO2:0.16, H2:0.44 and negligible CH4. In this case we are seeking a ratio (CO:H2) of infinity instead of the one limited by thermodynamics at 2.3. In order to achieve the composition from the idealised stoichiometric region steam injection (for H2 deficient gas) or CO2 (for CO deficient gas) addition is required to adjust the ratios of the species in the Water-Gas-Shift (WGS) reaction.
7.1 Circumventing thermodynamic limitations using WGS reaction
It is possible to use steam injection to obtain the thermally balanced reaction (F) (Table 2). This is in accordance with the Water-Gas-Shift reaction: CO + H2O↔CO2 + H2. Steam is added to increase H2 content from a CO rich equilibrium steam. Conversely, CO2 may be added to increase CO content from a H2 rich stream although it is not commonly practiced. In this particular case, at 1000 K, the steam per mol of sawdust is >55. This means that a large quantity of steam needs to be raised and condensed after the gasifier. Although this ratio (55) is an extreme case, it is commonly found that ratios of up to 3–7 are used in practice. It is also noted here that the steam assists in obtaining the stoichiometry of the basis reactions and is recycled after the gasifier in a recycle loop comprising of steam generation, condensation, treatment and make-up water stream.
8. Application to underground coal gasification (UCG)
UCG, a clean coal technology, is widely understood as a disruptive mining method that is efficient and environmentally benign. This method extracts deep and stranded coal by performing complex gasification reactions in-situ within the coal seam. The products of UCG are exactly the same as a surface gasifier without the ash component which is designed to be left underground. Whilst the literature on UCG technology is vast, in this chapter the analysis is limited to the syngas products and region of gasification as demonstrated by CHO-diagram. As an example, consider two UCG projects in Australia performed on Macalister Coal Seam at Bloodwood Creek and Chinchilla.
8.1 Analysis of UCG at Bloodwood Creek and Chinchilla
Macalister Coal Seam, CH0.898O0.108, has a heat of formation of −112.27 kJ/mol. With this information it can be shown (developed elsewhere), that 8 non-negative basis reactions are possible if the coal is gasified with oxygen and steam where methane production is allowed in the product stream. Furthermore, only 4 independent reactions lead to the thermally balanced operation where the heat of reactions are zero. These reactions are represent in Table 6 and the thermally balanced region (shaded in grey) in Figure 6.
Representation of gasification region for Macalister underground coal gasification.
8.2 Representation of UCG processes at Bloodwood Creek and Chinchilla
Figure 6 represents the gasification tendencies for the Macalister Coal Seam, oxygen and steam. The shaded region indicates where the thermally balanced region is for the coal, representing a net zero input/output of energy into the gasifier – a preferred scenario for any ideal gasification process. The Chinchilla syngas output are represented by the triangles and the cross represents Bloodwood Creek respectively. Two different UCG techniques have been used: Linked Vertical Wells (LVW) and Controlled Retractable Injection Point (CRIP) [22].
The syngas compositions may be found in the works of [22]. It is noted that the output from the UCG field trials lie within the theoretically predicted shaded thermally balanced region. The choice of where to operate the UCG process depends on the final use of the syngas. In these trials, a syngas feed for liquid-fuel production was desired – hence a higher hydrogen to carbon monoxide ratio was required which is achievable around the line HI. For power generation, a syngas with a higher calorific value gas would be required and would thus operate closer to line JI which is richer in methane, carbon monoxide and hydrogen.
While gasification systems are complex, the important reactions are represented by basis reactions that span the stoichiometric region of operation on a CHO diagram. The operation of autothermal sawdust gasification systems, without methane formation, is further represented by a line within the stoichiometric region. It is verified, from pilot plant data for gasification of sawdust that the operation occurs within the stoichiometric region and on the hot-side of the thermally balanced line. The analysis in this chapter thus enables the determination of outputs from sawdust gasification which can further be used to design downstream processes. It is shown that a desirable point to operate an air–steam gasification system for power generation (syngas with highest HHV) lies at the point of intersection between the thermally balanced line and the carbon boundary. This intersection represents the point where the maximum HHV is obtained for the gasification system.
The application of the CHO-diagram has been extended to underground coal gasification processes where thermally balanced regions for a given coal was developed. Field trial data where then plotted and found to be in the theoretically predicted thermally balanced region.
The method developed in this chapter provide a high-level analysis to practitioners who are doing basic design in gasification processes – it enables some predictions of syngas possible based on the carbon source and possible oxidants. The output is independent of major parameters such as gasifier type, kinetics or reaction parameters. Lastly, the method provides predictions of syngas compositions possible from a gasification system, enabling design tasks to be completed with reasonable accuracy.
Acknowledgments are due to the NRF Grant No. 81248 for funding this work.
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Written By
Shehzaad Kauchali
Submitted: 01 October 2020Reviewed: 19 December 2020Published: 28 January 2021
Open access peer-reviewed chapter
