HTL is similar to the geological processes that produced the fossil fuels used today, except that the technological process occurs in a time frame measured in minutes instead of geo­logical time. HTL is a chemical reforming process of biomass in a heated and usually pres­surized, oxygen deprived enclosure, where long-chain organic compounds (solid biomass) break into short — chain hydrocarbons. All fossil fuels found in nature—petroleum, natural gas, and coal, based on biogenic hypothesis—are formed through HTLs from biomass buried beneath the ground and subjected to high temperature and pressure. Simple heating without water (pyrolysis or anhydrous pyrolysis) has long been considered to take place naturally during the catagenesis of kerogens to form fossil fuels. In recent decades, it has been found that water under pressure causes more efficient breakdown of kerogens at lower temperatures than without it (Siskin and Katritzky 1991; Pennisi 1993). The carbon isotope ratio of natural gas also suggests that hydrogen from water has been added during creation of the gas, in addition to the formation of oil. The state of fossil fuels (solids, liquid, or gaseous) depends
on the composition of feedstocks and environmental conditions, including temperature, pres­sure, retention time, and presence of particular catalysts.

The exact pathways of HTL to produce crude oil from biomass remain unclear, and addi­tional research is needed. The following examples may give some hints of possible pathways of HTL of bio-waste feedstock. In a study by Appell et al. (1975) , one of the mechanisms for the conversion of carbohydrates into oil that was consistent with the results they obtained was as follows:

Sodium carbonate reacts with carbon monoxide and water to yield sodium formate:

Na2CO3 + 2CO + H2O ^ 2HCO2Na + CO2

Vicinal hydroxy groups in the carbohydrates undergo dehydration to form an enol followed by isomerization to a ketone:

H H H H H H

II I I II

—C_C_ * _C_C_ * _C_

O O O HO

H H H

The newly formed carbonyl group is reduced to the corresponding alcohol with formate ion and water:

The hydroxyl ion then reacts with additional carbon monoxide to regenerate the formate ion.

OH — + CO ^ HCO2-

A variety of side reactions may occur and the final product is a complex mixture of com­pounds. One of the beneficial side reactions occurs in alkaline conditions. Carbonyl groups tend to migrate along the carbon backbone. When two carbonyl groups become vicinal, a benzylic type of rearrangement occurs, yielding a hydroxy acid. The hydroxy acid readily decarboxyl — ates causing a net effect of reducing the remainder of the carbohydrate derived molecule.

This type of reaction is beneficial to HTL because it leads to the formation of paraffin-type structures, which has less oxygen than the original compounds. In addition, the reaction happens by disproportionation and does not require any additional reducing agent.

Aldol condensation may also be part of the reaction process. Aldol condensation occurs between a carbonyl group on one molecule and two hydrogens on another molecule with the elimination of water. The condensation product is a high-molecular weight compound typi­cally with high viscosity. Condensation reactions become a major pathway in the absence of reducing agents such as carbon monoxide and hydrogen. Reducing agents keep the carbonyl content of the reactant system sufficiently low so that liquid instead of solid products are formed.

In a study by Appell et al. (1980), the authors believed that the free hydrogen radical (H), not the hydrogen molecule (H2), participates in the chemical conversion reactions. Thus, they concluded that the addition of carbon monoxide (CO) to the process was more efficient than the addition of hydrogen gas. Based on the water-gas shift reaction, carbon monoxide reacts with water to form carbon dioxide and two hydrogen radicals.

C=O + H-O — H O=C=O + 2H •

In the presence of the hydrogen radicals, the oxygen is removed from the compounds containing carbonyl and hydroxyl groups, then form paraffin and water. A possible pathway is described in the following four reactions (He 2000).

O

I II II

— C—C— + 2H — — ► —C = C— + H2O

Hydroxyl group

The complexity of the chemical reactions involved in HTL can be attributed to the complex composition of feedstocks. According to Chornet and Overend (1985) and Vasilakos and Austgen (1985), some of the reactions that may be involved in the liquefaction of carbonaceous materials are cracking and reduction of polymers such as lignin and lipids, hydrolysis of cellulose and hemicellulose to glucose and other simple sugars, hydroge — nolysis in the presence of hydrogen, reduction of amino acids, reformation reactions via dehydration, decarboxylation, C-O and C-C bond ruptures, and hydrogenation of functional groups.