Waste feedstock containing lignocellulose, fatty acids, and protein derivatives can be hydrothermally transformed to produce a range of products such as biocrude, methane, hydrogen, biodiesel, and biogasoline. Many waste prod­ucts such as agricultural residues, food processing wastes, and municipal and agricultural sludge contain large amounts of water. The removal of this water in gasification, pyrolysis, and other thermochemical processes con­sumes a significant amount of energy. The hydrothermal process avoids the need for this water removal. Also, the considerable variations in the physical properties of water that occur with changes in temperature and pressure can facilitate efficient separations of product and by-product streams with low energy requirements. For feedstock that contains inorganics such as sulphates, nitrates, and phosphates, the hydrothermal method can facilitate recovery and recycling of these chemicals in their ionic form for eventual use as fertilizer. The product streams from hydrothermal processing are also completely sterilized against any possible pathogens including biotoxins, bacteria, and viruses.

In general, hydrothermal processing can be divided into three regions: liquefaction, catalytic gasification, and high-temperature gasification [76]. These three regions are graphically illustrated in Figure 6.8. Both catalytic and high-temperature gasifications occur under supercritical conditions producing gases with high hydrogen content. Also, depending upon the operating conditions, a significant amount of reforming reactions can take place under gasification conditions, particularly when a suitable reforming catalyst is used. The gasification process under supercritical water is cov­ered in the later section on supercritical technology. Hydrothermal liquefac­tion generally occurs at temperatures between 200 and 370°C and pressure between 4 and 20 MPa, and it can be applied to a stream of mixed waste. A significant literature on hydrothermal processing of biomass and waste has been reported over the last two decades [78-83].

Two other variations within hydrothermal liquefaction (HTL) have also been examined in the literature [83, 84]. At low temperatures (between about 180-220°C) and at saturated pressure, hydrothermal carbonization of bio­mass occurs. Although carbonized biomass is inferior to liquid or gaseous fuels, process requirements for hydrothermal carbonization are comparably low while producing a fuel that is easy to handle and store because it is stable and nontoxic. Thus, HTC may provide some advantages when considering small-scale, decentralized applications.

HTC is produced for residence time between 1 and 72 hours and with water pH below 7. Alkaline conditions produce substantially different products

[81-83]. The process goes through numerous reactions such as hydrolysis, dehydration, decarboxylation, polymerization, and aromatization. For a variety of feedstock such as cellulose, lignin, wood, peat bog, and the like, as reaction severity increases, more carbonization occurs reducing H/C and O/C ratios of the product. In a typical HTC operation, 48-50% of HTC coal (which contains lignitelike material in a dispersed powder form), about 35-37% of water and total organic carbon (which contains sugars and deriva­tives, organic acids, furanoid, and phenolic compounds), and about 15-16% of gas (which contains mainly CO2, with some CH4 and CO and traces of H2 and CnHm) are generated.

Unlike HTC, HTL (hydrothermal liquefaction) is carried out in a tem­perature range of about 200 to 400°C, and it produces products often called bio-oil or biocrude. Although HTL is a promising technology to treat waste streams from various sources, its commercial growth is inhibited due to the high transportation costs of cellulosic waste, poor conversion efficiency, and lack of understanding of complex reaction mechanisms. Kranich [85] was one of the first to use an HTL process to convert MSW. He used three differ­ent types of materials from MSW; primary sewage sludge, settled digester sludge, and digester effluent. In a laboratory autoclave, he performed experi­ments at temperatures ranging from 570-720 K, pressure of 14 MPa, and resi­dence time ranging from 20 to 90 minutes. The results showed the organic conversion rates from 45 to 99% and oil production rates from 35 to 63.3%.

Subsequent works were reported by Suzuki et al. [86], Itoh et al. [87], and Inoue et al. [88]. Most recently, Changing World Technology Co. in Carthage, Missouri, United States [89] reported the effectiveness of hydrothermal liq­uefaction for the treatment of turkey waste.

The process of hydrothermal upgrading (HTU) combines the liquefac­tion and upgrading steps. A schematic of this HTU process is described by Demirbas [84]. In this process the feed is pretreated, preheated, and pumped into the reactor. The products are separated in gas, liquid, and solid streams. In normal operating conditions the product composition is: gaseous prod­uct (>90% CO2) about 25%, process water about 20%, water-soluble organ­ics about 10%, and biocrude about 45%. The gaseous products, which have some heating value, are catalytically combusted with air to generate flue gas. The process water stream and soluble organics go through an anaero­bic digestion to produce biogas (largely containing methane) which can go through a combustion process to generate heat and electricity. The solid biocrude is separated into light and heavy biocrude. The heavy biocrude is co-combusted with coal to generate electricity. The lighter biocrude is hydro — deoxygenated to produce upgraded products such as premium diesel fuel, kerosene, and other feedstock for biorefineries. The process has an overall thermal effciciency of about 70-90%.

The hydrothermal upgrading process is generally carried out at 575 K; however, the process can be operated in the temperature range of 575-625 K, pressure range of 12-18 MPa, and residence time range of 5-20 minutes. The process carries out a series of depolymerization, decarboxylation, dehy­dration, hydrodeoxygenation, and hydrogenation reactions. The oxygen is removed as water and carbon dioxide. Typically the feed slurry contains 25% solids in water which may include wood and forest wastes, agricultural and domestic residues, municipal solid waste, or organic industrial residues. Kumar and Gupta [90] examined the effect of temperature on molecular structure and enzymatic activity of cellulose in subcritical hydrothermal technology. They indicated that the percentage of crystallinity of microcrys­talline cellulose increased with treatment with water.

A thorough and excellent review of biofuel production in hydrothermal media was given recently by Peterson et al. [76].