Compared to the conversion of model components, the conversion of real bio- masses poses a series of challenges. First of all, the complexity of the real biomass compared to a model component such as cellulose or lignin is increased signifi- cantly, and second, the effect of process parameters may not be directly transferable from model studies due to various synergetic effects of the various components of the biomass. A large amount of work has been put into studies of the conversion of actual biomass and model substances in HTL and HTL-like processes [48–79], which forms a foundation of knowledge for controlling the process.
To be able to understand the processes that occur when processing actual biomass in an HTL process it is necessary to have a basic understanding of the reaction types and reaction pathways that are likely to occur in such an environment. This part will initially go over the basic reaction types that are likely to occur during pro- cessing and the implications of these reactions on the products, followed by a dis- cussion of the effect of processing parameters and addition of catalysts to the process.
9.4.1 Overall Reaction Pathways
Since many types of biomass contain significant quantities of cellulose, hemi- cellulose and lignin, these are of main interest with regards to degradation. At the high pressures and temperatures, the HTL process operates the reaction media has to be considered. In this case, water constitutes a majority of the process stream, and due to the proximity to the critical point the auto-dissociation of water (reaction9.2) must be considered.
H2O!Hỵ + OH ð9:2ị Near the critical point the auto-dissociation is approx. 3 orders of magnitude larger than that at ambient conditions [80]. This means that the increased amount of ions from the water will make acid and base catalyzed reactions far more likely to occur at these conditions than at ambient.
Also the high processing temperature of the process makes thermal cracking a risk that needs to be considered, since the formation of char is an unwanted side effect. Char may be formed by either thermal cracking (reaction9.3) or Boudouard reactions (reactions9.4and9.5)
CnH2n!CnỵnH2 ð9:3ị
2CO!C + CO2 ð9:4ị
CH4!C + 2H2 ð9:5ị
These reactions relate to the steam reforming reactions for hydrogen production from hydrocarbons, as given in reactions (9.6) through (9.8), with the water-gas shift as reaction (9.7). These reactions normally operate at high temperatures [81,82], which is why they to some extent should be considered in the HTL process.
CnHmỵnH2O!nCOỵðnỵm=2ịH2 ð9:6ị
CO + H2O!CO2 + H2 ð9:7ị
CO + 3H2!CH4 + H2O ð9:8ị
If processing larger hydrocarbons then the amount of CO formed will increase as pr. reaction (9.6), this could lead to increased char formation through the Boudouard reactions (reaction9.4), which is in accordance with observations in steam reforming [82].
Also reactions to consider are hydrolysis, hydrogenation and decarboxylation (reactions9.9 through 9.11), where especially hydrolysis is of interest when processing lipids since this is part of the transesterfication of lipids into carboxylic acids.
R1OR2ỵH2O!R1OHỵR2OH ð9:9ị H2CẳCH2ỵH2!H3CCH3 ð9:10ị RCOOH!RHỵCO2 ð9:11ị These reactions may also lead to the formation of alcohols and CO2as well as the saturation of unsaturated bonds. The hydrogenation is likely to occur due to the production of hydrogen from the steam reforming reactions and any thermal cracking.
With the formation of hydrogen and CO from the various reaction sets, the Fischer-Tropsch synthesis of hydrocarbons cannot be disregarded. Through this reaction (reaction9.12) CO and H2reacts to form alkanes and water.
nCOỵð2nỵ1ịH2!CnHð2nỵ2ịỵnH2O ð9:12ị Fischer-Tropsch synthesis of biodiesel is a known and used process, so it must be noted that the HTL process is not a FT synthesis but rather that the reactions of the FT synthesis are likely to occur during HTL processing.
Another aspect that should be considered as well is the formation of furfurals from the sugars of hydrolyzed cellulose. It has been proposed [70,83] that below the critical temperature of water, it is likely that furfurals are formed which are subsequently converted into phenols and higher molecular weight components.
This is unlike supercritical conditions where radical reactions dominate [70], which promotes gas production. This follows the general idea that liquefaction reactions are occurring below the critical temperature while gasification is running
at supercritical temperatures [51]. Also lignin itself is likely to produce a large number of aromatics, since the lignin structure contains significant amount of benzene rings.
Having gone over some of the main reactions that are likely to occur during processing these may be applied to model compounds such as cellulose, lignin and lipids.
It must be assumed that any lignocellulosic material would be subject to hydrolysis of the ether bonds in the cellulose and hemicellulose. This hydrolysis would yield significant amount of sugars which would either remain or be con- verted into oil through thermal decomposition and cracking or through formation of furfurals and phenols. It is also possible that some of these sugars would go into the steam reforming reactions, i.e. be broken down into small volatile components, which subsequently could participate in a Fischer-Tropsch synthesis of alkanes and water, thus yielding the desired oil phase. This is illustrated in Fig.9.14 including the formation of char.
Besides lignocellulosic material also the conversion of lipids should be con- sidered. Figure9.15illustrates the possibilities when fatty acids are treated at high pressures and temperatures and the possible reactions.
As it may be seen, there are several possibilities for fatty acids and lipids when looking at a wider context. In the case of triglycerides hydrolysis of the ether bonds is likely to occur thus forming glycerol and free fatty acids [48]. Decar- boxylation reactions would yield ketones and CO2 while hydrogenation would saturate any unsaturated bonds in the molecule. Finally there is a chance of simple thermal decomposition which would lead to production of methane, CO2, and possibly some ketones.
In general, the reactions happening during HTL processing makes up a complex network and pathway system where the extent of reactions and their tendency to occur depends on both process parameters and the concentration of the various components in the reaction mixture. This means, that identifying every single reaction for each component is a very demanding task. Still it is important to have estimates of the conversion of model components before venturing into the con- version of actual biomass, since this knowledge will help give overall estimates of the product from the conversion process based on the composition of the biomass.
9.4.2 Effect of Process Parameters
Having established a basis of possible reactions that may occur in the HTL process the effect of process parameters on the reactions and products is a major area of interest. Since equilibrium constants will vary with temperature, and pressure will affect equilibriums in the system the process parameters cannot be neglected when investigating a high temperature, high pressure process. At process conditions, molecules will have a high kinetic energy due to the elevated temperature as well as a narrow spatial distribution due to the high pressure. This means that molecular
collisions and interactions are far more likely to occur at process conditions for the HTL process than at ambient conditions, i.e., the system is very reactive due to process conditions.
It is generally accepted that the transition from near-critical water to super- critical water as a reaction medium changes biomass conversion processes from liquefaction to gasification processes [19,49,51,54]. Still, even though the focus is on liquefaction valuable, information may be gained from gasification processes.
When increasing temperature reaction rates must be assumed to increase, i.e., the conversion will proceed at a higher pace. However, increasing temperatures also bring other advantages. It has been reported [84] that at high temperatures the formation of tar from lignocellulosic material is reduced in gasification processes.
Since tar is a high-molecular weight aromatic compound from thermal degradation Lignocellulosic material
Sugars
Oil phase Gas phase
Char
Fischer-Tropsch Synthesis Hydrolysis
Steam reforming
Water-Gas-Shift Boudouard reaction
Cracking
Furfurals
Phenols Fig. 9.14 Reaction pathways
for lignocellulosic material [48]
Fatty acids
Carboxylic acids Alcohols
CO2
Ketones
Saturated fatty acids Methane
CO2
Ketones
Hydrolysis
Hydrogenat
ion Decarboxylation Decomposition
Fig. 9.15 Reaction pathways for fatty acids [48]
of lignin without much practical use when focusing on biofuels, the formation of this is preferably avoided. As high temperatures tends to reduce tar formation it must be assumed that tar will form in a temperature range and that a high heating rate may reduce tar formation significantly, once the ‘‘tar-band’’ is determined.
Although increasing temperature generally increases reaction rates, it must be noted that not all reactions may be promoted by increasing temperature. Any exothermic reactions in the system will inhibited due to the high temperature of the process. Furthermore, the temperature may also affect the distribution of nitrogen in the product phases [85]. As the critical temperature is approached, more and more nitrogen will be found in the oil phase, while it at lower processing tem- peratures will be mainly in the aqueous phase. This shift in nitrogen-distribution must be considered carefully, and it may be difficult to avoid nitrogen as well as tar formation due to mismatched temperature effects.
With regard to pressure it must of course be noted that reactions must be assumed to behave according to le’ Chatelier’s principle, i.e., equilibriums will shift toward the fewest molecules formed. However, most effects on reactions must be attributed to the high processing temperatures rather than the pressure.
Pressure is still needed though, to get to the near-critical region and thus achieve desired properties of water as a reaction media at these conditions.
The use of catalysts in HTL processing has been investigated as well over the years [48–50,86–90]. In processing there are the options of heterogeneous and homogeneous catalysis. In general, the homogeneous catalysts utilized are a number of alkali salts, while the heterogeneous are metal oxide catalysts. These two options will be treated in turn.
Using a homogeneous alkali catalyst such as KOH or NaOH has been shown to increase the gas, especially hydrogen, formation in supercritical water [55,73,86].
It was shown that the increased formation of hydrogen and CO2was at the expense of CO production. A reason for this may be that the alkali will promote the water gas shift (reaction 4.6) through the formation of methanoate (reactions9.13 through9.16) [48,58,71,72].
K2CO3 + H2O!KHCO3 + KOH ð9:13ị
KOH + CO!K(HCOO) ð9:14ị
K(HCOO) + H2O!KHCO3 + H2 ð9:15ị 2KHCO3!CO2 + K2CO3 + H2O ð9:16ị Evidently in this case potassium carbonate catalyzes the formation of CO2and H2at the expense of CO. In a liquefaction process, the added gas production is not an objective in itself, however the produced hydrogen could be utilized in any Fischer-Tropsch reactions occurring, and thus actually promote the production of higher alkanes.
However, when using salts as homogeneous catalysts there is a risk of pre- cipitation as the critical point of water is approached. As this happens the dielectric
constant of water decreases significantly thus rendering salts and any ionic species almost insoluble in water while nonpolar substances such as alkanes become fully miscible with water. If precipitation of the homogeneous catalyst occurs, it reduces the contact area with the catalyst and thus the efficiency of this. Also with pre- cipitate in the system there is a risk of plugs forming as well as abrasion in bends and small orifices such as valves.
With regard to heterogeneous catalysts, generally metal oxides have been attempted. Results show that a small increase in gas production is observed, however this is very limited to the increase observed for the homogeneous alkali catalysts [86]. When using metal oxides some of these contain active sites that may facilitate both acid- and base catalyzed reactions. With the increased amount of ions due to the near-critical condition of the water, these sites will promote any acid or base catalyzed reaction in the process. There is, however, a downside to using heterogeneous catalysts, since any tar and coke formation may deposit on the catalysts thus fouling/poisoning them and rendering it at a lower efficiency or fully deactivated.
It has also been proposed that the reactor wall itself has an effect on the conversion reactions [91]. Due to process conditions, reactor walls need to be made of highly alloyed steels, and it cannot be ruled out that some of the com- ponents in the alloys may actually react with material in the process stream. This must be assumed to have the functionality of a heterogeneous catalyst where minor constituents in the steel may have this functionality.
There are a lot of possibilities for tuning the process using processing param- eters as well as adding catalysts to the process. This means that this type of liquefaction may be potentially designed to achieve a specified product by tuning the process conditions and composition.