Supercritical water gasification: a patents review

3.1 New method or procedure

There are 41 patents focused on improving the method or procedure for generating higher efficiency in terms of the amount and quality of the fuel gas produced. One of the first published patents was a Japanese patent presented in 1998 (Ota et al. 1998). The authors presented a method and apparatus for generating fuel gas by decomposition of coal in either subcritical or supercritical water. The system comprises a gasification reactor, a desulfurizer, and composite power generation equipment. The gasification reactor is a two-stage fluidized type. The main aim is to recover the heat from gases obtained during the gasification process and use it in power generation equipment. To work at subcritical conditions, pressure and temperature values were 160–215 kg/cm² and 200–374°C, respectively. In order to work at supercritical conditions, pressure and temperature values ranging between 215 and 300 kg/cm² and between 374 and 900°C were used. Slurry concentration is preferably from 10 to 55 wt.%; this slurry should be kept at a temperature that ranges from 150 to 350°C. The resulting gas is sent to a gas turbine, where it is depressurized. Afterward, it is sent to a boiler in order to take advantage of its thermal energy by heating water. Therefore, this process generates power due to the quantity of heat stored, as high as 3000–8000 kcal/Nm³. An alkali solution may be needed in order to remove the sulphur contained as inorganic salts within the raw material.

The object of a patent by Guo and Hao (2002) is to provide a continuous gasification method from supercritical water to produce hydrogen from a solid biomass and other organic substances. To do so, sodium carboxymethyl cellulose is used as an additive to the solid feedstock, which is continuously transported and mixed under high pressure. The slurry at high pressure is added to the reaction system through a piston pump. Ogawa et al. (2000) provided a new method to treat solid and liquid organic waste with a high gasification efficiency. Hydrothermal reaction conditions of 200–600°C and 2–50 MPa and a liquid line speed of 0.1–1 m/s were used. Pumped liquid organic material is heated to 200°C by means of a heat exchanger and introduced to the hydrothermal reaction tower in the presence or absence of a catalyst. When performing hydrothermal reaction in the absence of a catalyst, temperature is generally 300°C or higher and the minimum pressure is 15 MPa. If a catalyst is used (titania composite oxide preferably), the minimum necessary temperature and pressure are 200°C and 5 MPa, respectively. When employing a fluidized bed, the catalyst was remained suspended in the slurry and precipitated after the reaction was completed. Toyama et al. (2000) presented a method to produce gas fuel composed mainly of hydrogen and methane and whose composition can be controlled by varying the residence time that the liquid organic material remains in a catalytic layer. This process is suitable for a wide range of general waste when it is dispersed in water as a slurry. During the hydrothermal reaction, the temperature ranges from 250°C to 600°C and pressure ranges from 5 MPa to 50 MPa. The supported amount of active component of the catalytic in the carrier ranges from 0.1 to 3 wt.%. Zirconium and manganese oxides are considered to be the best catalysts. The spatial velocity of the liquid is about 1–60 h^-1, and the dimensions of the catalyst are about 5–15 mm. When a fluidized bed is used, particle size is preferably between 0.15 and 0.5 mm. The reaction gas obtained is purified by desulfurization or decarboxylation.

Toyama et al. (2003) presented a new and highly efficient gasification method from solid and liquid organic wastes. It prevents any deposits of undecomposed carbon components. This method consists of triggering a hydrothermal reaction in supercritical water by adding hydrogen gas to the liquid organic materials. This invention uses a solubilization tower heated over 150°C to promote the solubilization of solid organic matter into liquid organic matter and efficiently processing a hydrothermal reaction in the subsequent gasification tower. Moreover, any inorganic material, when present, is discharged by the hydrothermal reaction equipment as sludge.

In Matsumura (2004), livestock waste is treated by SCWG in a system formed by a spouted bed gasification unit for heated organic waste. Inert particles or catalyst particles can be used in the gasifier. Operating conditions were 35 MPa, 400–600°C, and residence time <30 min. Hydrogen is separated and the combustible gas, such as methane, is burnt in the heater, thereby increasing the temperature of the organic waste that enters the spouted bed gasifier. Therefore, there is no need to supply fuel from outside the system. In addition, the fluid entering and leaving the spouted bed gasifier goes through a heat exchanger, thereby improving the thermal efficiency of the whole system. Furthermore, by installing a cyclone or switched filter before the pressure-reducing valve, particles (salt produced as a gasification residue, ash, and char) can be easily removed. This helps to maintain the reliability of that valve. In the same year of publication, Sako et al. (2005) presented a method to obtain from kraft pulp black liquor, a useful gas whose main content is hydrogen. The gasification reaction temperature is preferably 600–750°C, the pressure is 7–20 MPa, and the reaction time is in the range of 15–30 min, with a high ratio of water/organic matter. By controlling operating conditions, it is possible to control the generation ratio of hydrogen and methane. The black liquor has a high content of caustic soda or organic potassium (more than 30% of total solids). Useful hydrogen is produced at a high production rate without addition of a separate catalyst. A portion of the alkali solution may be recycled to be used as a catalyst and the remaining residue after the gasification may be directly charged into the causticization step of alkali recovery in the kraft pulping. By circulating the resulting gas mixture into an alkaline solution, carbon dioxide is absorbed and removed. It is possible to obtain a mixture of hydrogen gas and a lower hydrocarbon or a chemical feedstock such as methane.

In 2005, Matsumura et al. (2005) presented an invention that offers a more efficient gasification method and system for biomass (wastewater sludge and manure). According to this patent, biomass gasification is carried out in two steps. A first hydrothermal treatment in the presence of a non-metal-based catalyst is carried out at a temperature within the range of 100–250°C and under a pressure of 0.1–4 MPa. A highly fluid slurry of the biomass containing the non-metal-based catalyst is supplied into a second pressurized hot water treatment device preferably at 600°C and 25 MPa to generate a fuel gas. When tar or char is generated, a dust collector or filters may be used to remove them. By using a non-metal-based catalyst, it is possible to prevent the corrosion of the equipment that occurs when an alkali metal catalyst is used for a long time. Thereby, the process step of neutralizing the alkali metal catalyst becomes unnecessary. Activated carbon or zeolite with a particle diameter of 200 μm or less can be used as non-metal-based catalyst.

Another method for biomass gasification has been proposed in Matsumura et al. (2007a). In this case, there is a pretreatment step where biomass undergoes a hot-water treatment (temperature range of 100–250°C and a pressure range of 0.1–4 MPa) in the presence of a nonmetal catalyst (not specified, based on activated carbon, which is recovered after the process). The feeding step includes crushing the mixture of biomass to obtain preferably an average particle size of 300 μm or less. Moreover, it includes a control step for the amount of the slurry supplied to the supercritical reactor by controlling the operation frequency of the slurry-feeding device. The invention described the use of a fluidized bed reactor, but the authors claim that a long pipe reactor can also be used.

Junjie et al. (2009b) described a method to obtain a mixed gas with high methane content by means of two catalytic reactors in a serial system. Pulverized coal (particle size preferably <420 μm) is mixed with a catalyst and water to make up a slurry. Coal-water slurry (CWS) concentration would be preferably 20–50% and the proposed catalysts are CaO, K₂O, Na₂O, NaOH, KOH, Ca (OH)₂, Mg (OH)₂, K₂CO₃, Na₂CO₃, or a mixture thereof. The products obtained in the first reactor from the gasification of coal dust with hydrogen and subcritical or supercritical water are gasified in a second reactor with supercritical water to obtain a mixed gas with a high methane content. In 2009, Junjie et al. (2009a) also patented another method to obtain high methane content gas from coal. In this method, the pulverized coal (preferably 60–150 μm) and the supercritical water undergo a catalytic pyrolysis reaction under the action of a pyrolysis catalyst [CaO, K₂O, Na₂O, NaOH, KOH, MgO, K₂CO₃ or Na₂CO₃, Mg (OH), or a mixture thereof] to obtain semi-coke. Later, this semi-coke and supercritical water undergo a gasification reaction under the action of a methanation catalyst (the active ingredient is selected from Ru, Fe, Ni, Co, Rb, Pt, and Ir elemental or compound and the support is selected from SiO₂, Al₂Ca₃O₆, Al₂O₃, ZrO₂, etc., or a mixture thereof). A high-methane-content gas can be obtained from the two-step reaction. As a variation of that invention, Junjie et al. (2010b) proposed a similar method to obtain a methane-rich gas from coal in two steps, but in this case, the water in the second reactor would be in a subcritical state.

In 2009, Junjie et al. (2009c) proposed a method for processing slurry-like residual carbon generated by the reaction of a carbon material and supercritical water. The method comprises the reaction of the slurry-like residual carbon and the excess oxygen gas in subcritical or supercritical water. In the presence of excess oxygen, carbon residue is completely oxidized to CO₂; during this process, it releases heat that transfers to the slurry residual carbon in the water. This step improves the energy efficiency of the carbon material gasification reaction. The method saves a compression filtration step and a drying step; it also saves in equipment costs, reduces energy consumption, and is pollution-free.

Yoshida et al. (2009) presented an invention where biomass gasification is carried out in four steps. The first step consists of a thermal decomposition by pyrolysis of slurried biomass (glucose solutions) in a supercritical state. Later, an oxidation decomposition step (20 s or longer in the oxidation reactor) with a solid polymer oxidizing agent generated as a by-product from the thermal decomposition step. The formation of low-molecular-weight polymer material was promoted. This product is passed through a catalytic reactor (reduced nickel catalyst, for example) to form smaller molecules such as methane, hydrogen, etc. Later, a vaporization step reduces the temperature and the pressure of the gas. As a result, even in a low temperature range of 400°C, the slurried biomass with 5 wt.% can be gasified. Figure 8 shows a schematic diagram of this patent.

Figure 8:

Schematic configuration of the biomass gasification apparatus presented in patent 21.

Another invention (Qingfeng et al. 2010) provided a sludge gasification method, where the sludge is mixed with water to obtain a slurry. In order to hydrolyze, at least part of the cellulose, this slurry is processed under a temperature of 150–350°C and a pressure of 10–30 MPa in the presence of acid.

Another invention (Wei et al. 2012a) uses cyanobacteria algae to produce hydrogen through a direct SCWG method. The raw material is pumped at high pressure into the SCWG reactor for a 5- to 10-min gasification reaction. The product is cooled by means of a heat exchanger and the cooling fluid is recovered afterward to preheat the reactor feed. The operating temperature is 450–500°C.

Lee (2011) proposed a method to improve the treatment rate of wastewater from food waste. Wastewater obtained from the dehydration process of food waste was continuously introduced into a pulveriser and then into a vibratory separator to remove solid materials of 1 mm particle size or bigger. Later, the separated liquid phase is preheated at 150–300°C and pressurized to 10–30 MPa in order to produce a hydrothermal liquefaction that converts solids into an organic material by means of a heat exchanger that uses the product that comes out of the SCWG reactor. In order to avoid the carbonization that appears as a side reaction that increases the solid contents, high temperatures are to be avoided. A subsequent step was supercritical gasification to obtain combustible synthesis gas at 500–700°C and 22.1–30 MPa. The reactor is preferably made of nickel alloy. The generated gas was separated in the last step and stored with an energy capacity of 2500–4500 kcal/Nm³, which can be used as electrical or thermal energy as no impurities were presented.

The invention of Wang et al. (2012) discloses a hybrid method of SCWG-oxidation. First, the organic wastewater enters a preheater, and then, a high-pressure pump feeds it into a SCWG device. The fluid comes out of the SCWG device into a gas-liquid separator that uses a heat exchanger. The synthesis gas at top is recycled, and the fluid at the bottom enters first a heater and then a supercritical water oxidation device.

Yu et al. (2011a,b) related to a method to gasify coal. The method puts pulverized coal and oxygen into contact with water vapor to obtain a crude gas. The crude gas contains naphtha fraction, diesel oil, and tar fractions. Later, the resulting crude gas in contact with water is cooled to obtain purified gas and liquid products. A following step to separate the product phase oil-water generates coal gasification wastewater. The coal gasification wastewater obtained is treated free of ammonia to obtain gas. The organic matter in the water and the ammonia nitrogen (also referred to as NH₃-N) are completely oxidized by means of the oxidizing agent, which is 0.2–0.8 times the theoretical oxygen necessary for the complete oxidation of organics and ammonia nitrogen in the coal gasification wastewater. Afterward, a biochemical treatment will be necessary. This method not only enables the gasification of organic waste, but also, water content is significantly reduced, and so is the gasification of organic waste into synthesis gas. This improves waste recycling efficiency.

The invention of Yoon and Lee (2011) provides a combustion gasification apparatus and method to use supercritical water to conduct a reaction by a low-grade coal. For that, the coal and the water are mixed in a specific ratio (8–10 wt.% of water for one-part weight of coal). This mixture is pumped into a tubular reactor to perform a combustion reaction.

The invention of Song et al. (2012), like the patent of Wang et al. (2012), discloses a hybrid between SCWG and oxidation for the treatment of sludge. In this patent, the sludge is preheated to feed a gasification reactor that produces a gasified material. This product is separated to obtain a gas and a liquid. The latter one is used as the feed raw material in the oxidation reaction. The oxidized material is taken to a heat exchanger so that the heat released by the oxidation reaction is supplied to the gasification reaction.

The invention of Zhu et al. (2014a,b) describes a device and a method to recover nitrogen-phosphorus from blue algae liquid, which is brought ashore and is directly processed by SCWG. This yields a large amount of ammonia-nitrogen and active phosphorus ingredients as liquid-phase products. According to the method, nitrogen and phosphorus can be recycled by a two-step process, i.e. by adding several reactive materials such as hydrochloric acid, magnesium chloride and potassium dihydrogen phosphate; and by means of a solid phase oscillator applied to sequencing batch runs. This system achieves a nitrogen-phosphorus recovery efficiency close to 90%. Magnesium ammonium phosphate (struvite) can be produced and used as a crop fertilizer, which would solve the problem of the traditional treatment of blue algae.

Xuewu and Weihong (2010) propose a biomass gasification process in supercritical water that increases hydrogen production by removing CO₂. The process consists of a reaction between the catalyst solution (hydrogen peroxide) and the biomass slurry (larch or corn cob), in the volume ratio of 1:(10–20), where a CO₂ removing agent (calcium oxide) is fed into the high-pressure reactor. Both solutions were preheated separately and then fed into a mixer that compresses the mixed fluid into the reactor. The reaction occurs at 350–600°C and 20–50 MPa and the reaction time is controlled to be 1–60 min. In the presence of the CO₂ removing agent, the production of hydrogen increases five times, from 12.1% without calcium oxide to 60.8%; 23.7% of CO and 12.2% of methane content were also obtained.

Erwang and Wei (2011c) describe a two-stage gasification process to produce biogas. The first step consists of the processing of waste biomass (corn stover, wood residue, or forestry processing wastewater) into a semiliquid stirred slurry preheated to 90–150°C. This is fed into the steam gasification device with an alkaline metal carbonate catalyst that produces crude oil. Therefore, when the first semiliquid slurry is introduced in the catalyzed steam gasification device, it can quickly reach the desired reaction temperature. Steam gasification is carried out at 200–300°C and 6–12 MPa, and a treatment time between 15 and 40 min is to be applied. Then, in a second reactor, SCWG of the crude oil is carried out at 500–700°C, 25–32 MPa, 10–20 min, and in the presence of a second catalyst (carbonates, nickel nitrate, or earth carbonate). In general, the biogas obtained contains 26–36% of H₂, 60–70% of CO, 2–5% of CO₂, and 0.3–0.9% of CH₄ with a heating value of 46 MJ/m³. Moreover, the biogas volume ratio of H₂ and CO is 0.3–0.6:1.

The invention of Cheng et al. (2012) proposes a SCWG method and apparatus that prevent the pipelines from clogging. In the present invention, an organic solvent agent dissolves the tar after the reaction. The addition of an organic solvent improves the flow since the tar is dissolved and fluidity increases. Less clogged conducts also reduce adhesion rates to the walls. This method achieves a tar production over 9.8%.

In many of the previous methods for sludge processing, water is added to prepare a slurry that increases the sludge fluidity. This reduces the processing efficiency of the dehydrated sludge. To avoid this technical problem, in the invention of Wei et al. (2012b), before the dehydrated sludge undergoes the SCWG process, the sludge is preliquefied by using a liquefaction apparatus to increase the sludge fluidity, and a part of the inorganic substances in the sludge is settled. This makes the pumping and supercharging of the sludge easier and reduces the risk of clogging the pipelines and sludge processing efficiency is substantially improved.

The invention of Quentin (2013) presents a technology that reduces corrosion in SCWG processes by means of a seed sacrificial metal. The metal particles may be distributed in the SCWG reactor through high-pressure injection. The metal particles may corrode, rather than SCWG reactor walls. These particles may be converted into metal oxides that precipitate when water is above the supercritical point and then they may be collected downstream of the reactor.

More recently, Peppou et al. (2013) described a system and method to reduce the corrosion in a SCWG reactor vessel. The system used a protective fluid (i.e. molten salt), which has a higher density than the reactor corrosive fluid and is substantially immiscible with the reactor fluid. A mobile element is configured to rotate within the reactor vessel and generate a rotational force. That impeller rotates at about 800–1000 revolutions per minute. The dense fluid that enters the reactor vessel is forced to flow in a dense fluid vertical flow that covers the reactor surface. Therefore, the protective fluid may form a layer between the reactor fluid and the inner surface of the reactor vessel. This protecting layer reduces the corrosion by forming a barrier between the reactor fluid and the reactor’s inner surface.

3.2 New systems, devices, or apparatus

This section covers those patents that focus on the improvement of systems, devices, or apparatus. Based on the system presented in patent the patent of Matsumura et al. (2005), the authors claim a new improvement in the process. In this case, a first pressure accumulator is included between the pretreatment device and the slurry feeder, and a second accumulator is laid between the slurry feeder and the reactor (Matsumura et al. 2006a,b).

In 2013, Zhong et al. (2013) presented a patent based on a supercritical water low-temperature gasification device for the treatment of black liquor, although other wet materials can be treated. A nitrogen gas inlet pipe is connected with a fluidization bed of quartz sand. A quartz capillary spiral is arranged as a reactor in the fluidized bed. The use of this capillary quartz material allows us to avoid the use of stainless steel piping, preventing metal catalysis and the corrosive effects of the materials. Moreover, the design of the capillary spiral structure can increase the area of the SCWG reactor that is uniformly heated in a fluidized bed. Furthermore, the design of the capillary spiral, compared to upright tubes, may increase the amount of black liquor treated, reducing the gasified insufficient amount of liquid, which is difficult to analyze. The invention also discloses a method to carry out supercritical water low-temperature gasification from black liquor. A certain amount of black liquor from the inlet quartz capillary spiral is injected, and the entrance is then sealed by a flame. The following process conditions were studied: temperature 380–650°C, pressure 220–450 atm, and a reaction time of 5–200 s. After the reaction has completed, the capillary spiral is removed from the fluidized bed and rapidly cooled into water at room temperature. After the cooling process, product gas was collected and analyzed.

The invention of Harinck and Smit (2011a) describes a reaction apparatus and the process for the gasification of wet biomass, based on the process presented by Erwang and Wei (2011b). The reaction tube is configured to contain a bed of solid particles suspended in a fluid and to receive wet biomass from the feeding system at a temperature below the critical temperature of water. The fluidized bed has a length of at least 0.5 m and a maximum cross-sectional area of 20 dm². The heating device heats the feed in the presence of the bed formed by suspended solid particles at a temperature above the critical temperature of water by heat exchange with a heating fluid, which results in supercritical water and fluid gasification product. Moreover, a recovery system is connected to the reaction tube to collect the gasification product. The reaction tube is adapted to allow wet biomass to pass along the reaction tube and concurrently or counter currently with the heating fluid.

Ryu et al. (2013) describe an apparatus to produce biofuel using SCWG processes. In this technique, to convert biomass into energy, the gasifier is run at a temperature of 500–900°C and uses oxygen. A gas composed of hydrogen, carbon monoxide, etc. (syngas), is produced. The present invention uses a biomass that contains micro-algae, water, and a metal catalyst. The supercritical gasification reactor is heated by a high-temperature electric furnace. Later, the product from the condenser is cooled to the 70–80°C threshold and the cooled liquid product (liquid phase) and vapor phase product (gas phase) are separated in the separator. According to a preferred arrangement of the present invention, the biofuel production apparatus is provided before the second threshold to the gasification reactor; a high-pressure tubular reactor to preheat the biomass may be further included. Other preferred arrangements are described in the patent abstract. Figure 9 shows a diagram of the system presented in this patent.

Figure 9:

Schematic drawing of the system in patent 54.

Harinck and Smit (2011b) presented a process for the gasification of wet biomass, which comprises heating wet biomass at a pressure between 22.1 and 35 MPa from a maximum temperature of T1 to a minimum temperature of T2 by heat exchange with the heated fluid gasification product. Therefore, this gasification product is cooled down at a pressure Ps between 22.1 and 35 MPa from a minimum temperature of T3 to a maximum temperature of T4, where T1, T2, T3, and T4 are temperatures in °C and can be calculated by using a mathematical formula that depends on the pressure and a factor. This invention also includes a reaction apparatus for the gasification of wet biomass. Such a reaction apparatus comprises a wet biomass feeding system at a pressure of at least 22.1 MPa, a reactor that is fitted with a reaction tube connected to the feeding system, and a heating device. The reaction tube is configured to contain a bed of solid particles suspended in a fluid and to receive wet biomass from the feeding system at a temperature below the critical temperature of water. The heating device is arranged to heat the feed in the presence of the bed of suspended solid particles to a temperature above the critical temperature of water by heat exchange with a heating fluid. This generates supercritical water and fluid gasification product. A recovery system is connected to the reaction tube to collect the gasification product from the reaction tube. The reaction tube allows the wet biomass to move along the reaction tube either concurrently or countercurrently with the heating fluid. The bed of solid particles suspended in the fluid is a bubbling fluidized bed of at least 0.5 m long and with a maximum cross-sectional area of 20 dm². In one of the arrangements, the heating device comprises a heating jacket and/or one or more adapted heating pipes to allow the heating fluid to flow through. A schematic diagram of the system is presented in Figure 10.

Figure 10:

Patent 55 system diagram.

Qian et al. (2014) advocate for an integrated oil shale refining system associated to systems that use and produce gasified hydrogen. The system includes an oil shale retorting unit connected in turn to a shale oil and gas separation unit, to shale oil hydrogenation upgrading units, and to separators, including a combustion furnace, a supercritical water coal gasification unit, a heat exchanger, a hydrogen purification unit, and several building material production units. According to the utility model, the shale scrap discarded from the oil shale dry distillation unit and the redundant residual coal gas provide heat for the SCWG coal reaction. This saves the use of coal resources with relatively high calorific value and high economic value to maintain the SCWG coal reaction. The shale ore associated with brown coal can be used to produce hydrogen with a high economic value. This optimizes allocation of resources and at the same time increases the economic benefits of the process.

When water-containing biomass (water-soluble paper, livestock waste, or sludge) reacts under conditions of high temperature and high pressure before and after the critical point of water, it is thermally decomposed or hydrolyzed to gasify. Since, additionally to the gas, ash and char are also generated during this reaction, plain tubes tend to clog during gasification. Therefore, in order to avoid clogging problems (Matsumura et al. 2006c), biomass is introduced from a charge port into the bottom of a reaction tube, where fluidization elements (e.g. alumina particles) are fed into the upper part to obtain a fluidized bed. Biomass is loaded from the first to fourth supply pipes into the first to fourth reaction pipes in order to stir the alumina particles that form fluid layers so that the biomass goes through pyrolysis or hydrolysis to generate a gas.

The supercritical water system of the invention of Matsumura et al. (2011) obtains combustible gas by treating biomass slurry and advocates for a solution to avoid clogging problems. The system is characterized by the inclusion of a sedimentation separator for inorganic solids. The separator is placed between a second heat exchanger and a first hot-water treating device. It is in this position where the viscosity coefficient of the biomass slurry decreases to a suitable value for the separation of inorganic solids.

The invention of Xu et al. (2013) describes a reactor for supercritical water oxidation or gasification treatment of high-salt-containing organic wastewater. The reactor contains a cylinder and a top cover tightly connected to the upper end of the cylinder. The closed space between the cylinder and the top cover forms the reaction chamber. Both the inner wall of the cylinder and the lower surface of the top cover are covered by a corrosion-resistant lining.

In the patent of Anderson and Sjong (2013), a system to minimize the corrosion in SCWG is disclosed. The system includes a nonconducting pipe that is configured to receive a fluid at one end and transmit the fluid toward the other end. The system further includes a number of electrodes distributed along part of the outside surface of the nonconducting pipe. These electrodes, when connected to a power source, may be configured to apply an electrophoretic force on the ions in the fluid.

The invention of Kery (2013) relates generally to a system and method to reduce corrosion in supercritical water reactor systems. The components in a SCWG system may be configured to control the temperature and the pressure of the flowing slurry. The components in the system may maintain the temperature and the pressure of the slurry so that the ionic product of water in the slurry does not reach over corrosive ionic product level.

Cooke (2013) describes a reactor system that intends to reduce corrosion thereof and may consist of a system vessel that includes an inner surface and a subcritical zone. The corrosion-resistant layer may be formed by glass and silicon carbide, which partially protect the inner surface from corrosion. It also describes a method to manufacture a corrosion protection layer in a reactor system using crustal encapsulation or filament winding.

Millar et al. (2013) describe systems and methods to reduce corrosion in a reactor system similar to those in a patent of Anderson and Sjong (2013). Electrically loaded solenoids, rings, tubes, or rods are on the outside surface of a system. A current may be applied to the current carrying element to generate an electromagnetic field within the reactor and thus force the corrosive ions moving away from the inner surface of reactor.

Graf and Kery (2013) defended a component of a reactor system to reduce corrosion too. The reactor system includes a reactor fluid that creates a layer that acts as a barrier between the product source fluid and the surface of the reactor, thus avoiding corrosion of the reactor.

On the other hand, other patents have been focused mainly on reducing electric heating costs. Ahlbeck et al. (2008) described a system formed by two heat exchangers for supercritical or near-critical water biomass gasification. The method comprises a first step to heat the biomass in a first heat exchanger, where molten salt is used as heat transfer medium. Then the SCWG yields the reaction products, which are cooled in a second heat exchanger by heating the molten salt. The molten salt circulates between the first heat exchanger and the second heat exchanger. Since molten salt has good heat transfer properties, the heating rate can be increased and the critical temperature range can be reached rapidly, which prevents or reduces substantially the accumulation of tar, char, etc.; solids; or high-viscosity fluids (produced mainly in the range of temperature 200–400°C) on the surfaces of flow channels of the heat exchanger.

In 2007, Matsumura et al. (2007b,c) presented an invention where the biomass gasification system that has been described in patent 15 is completed by a power generation device. This device generates power from the gas produced in the hydrothermal reactor. It burns part of the gas product that contains oxygen to heat the slurry feed or to generate electric power. Moreover, a new slurry-feeding device is described. In this case, the slurry from the preprocessing device is introduced into two cylinders, and by means of a three-way valve, the slurry flows from the cylinders into the reactor. Water may also be injected into the cylinders to prevent the slurry from blocking the reactor temporarily.

Daohong and Lei (2011) described a SCWG system based on oxidation and including a heat-accumulating device. Conventional supercritical water oxidation and gasification devices use electric heating to provide the heat needed to obtain supercritical temperatures. Therefore, electric heating costs are more expensive in industrial applications, where the facilities are complex and costly. This invention intends to provide a solution to this problem, thanks to a regenerative combustion heating system and gasification in a supercritical water oxidation system. The efficiency level of the entire combustion furnace (effective heat to preheat the material and react with fuel combustion ratio of the total heat release) can reach over 93%. A generative combustion heating system includes a preheating chamber and a reaction chamber with a number of regenerative burners, which are used alternatively where the combustion of the low-calorific-value hydrogen-rich gas obtained is effectively used to reduce the dependence on external sources and thus increases energy efficiency.

Moreover, in order to greatly reduce the costs of the SCWG process, some patents have incorporated a device that uses solar energy in the SCWG process (Liejin et al. 2009a,b,c, Liejin et al. 2011a,b, Li and Guo 2014).

Liejin et al. (2009b) illustrated an absorption reactor thermally driven by solar energy. An absorption reactor cavity is connected with a conical surface that acts as a solar energy secondary condenser. The absorption reactor cavity is internally fitted with a coiler flow biomass supercritical water reactor. Biomass and supercritical water continuously flow in a reaction tube to absorb and focus solar energy direct radiation and the energy that is absorbed from secondary condenser for hydrogen production by gasification reaction. The coil flow reactor is divided into a preheating section and a reaction section, thus claiming fast heating of the biomass. The absorption reactor thermally driven by solar energy has a low cost and is safe and highly efficient.

The invention of Liejin et al. (2009c) proposes the use of solar energy to reduce the cost of the SCWG process for hydrogen production. The system includes a multiplate that concentrates heat supply. The device if fitted with a cavity absorber to carry out efficient absorption of solar energy. The reactor is arranged in a coil-type distribution to be compact and enhance heat transmission. The system includes a multiplate solar energy condenser with a highly accurate automatic tracking of the sun and provides a solar energy source with high focusing ratio and high energy current density.

In 2009, Liejin et al. (2009a) also proposed an invention for SCWG of biomass using solar energy, but in this case, it uses a concentrating system formed by a spinning-pitching tread heliostat, solar secondary condenser with a conical surface, and a solar cavity absorption reactor internally fitted with a coiled flow reactor. Compared with the plate-type coupling hydrogen production system, the invention has the characteristics of simplified system, low cost, strong stability, and favored industrial amplification.

The purpose of Liejin et al. (2011a) was to present a multidisc solar heat-collecting coupled biomass gasification in a supercritical water reactor. This cavity solar absorption reactor includes an endothermic reaction chamber in the upper part and a preheated water heating cavity in the lower chamber. The inner cavity walls are fitted with thermal insulation. The upper part of the endothermic reaction chamber is made of quartz glass sealing and this chamber has a flow reactor pipe. Moreover, the bottom of the reaction chamber is equipped with a preheating water chamber that uses a motor electric heater for sun irradiance fluctuations and can provide heat even under cloudy conditions. Then, two coolers with spiral tube high-pressure heat exchangers are used to preheat water in the lower chamber and the second one is connected to the feed material. This system heats the materials to be gasified very quickly, operates at 24–30 MPa, and residence time is 30 s.

Liejin et al. (2011b) described a method and a system where solar energy is collected in a multiplate mode and connected to a biomass SCWG system to produce hydrogen. The invention uses a cavity-type solar energy absorption reactor to absorb the heat from the sun to drive the SCWG process. Moreover, a preheater is used to preheat water when the irradiance of the sun is not enough or to compensate for substantial fluctuations. The method uses biomass material (glycerol solution 0.3 m or 0.1 m, 0.2 m, or 0.3 m glucose solution) first preheated to 180–240°C and then mixed with supercritical water (>600°C) to produce a rapid heating of the material to a supercritical state; this improves the rate of free reactions and reduces tar and coke formation. Therefore, higher hydrogen content was obtained. In a test with a multidisc condenser area of 16 m², a concentration of glucose 0.1 m, and material and preheat water ratio of 1:3 when the average solar irradiance was 600 W/m², the solar furnace temperature was 700°C and the reactor effluent temperature was 650°C.

In order to greatly reduce the costs of hydrogen production by SCWG of biomass, Li and Guo (2014) advocated for a system that combines the use of solar energy to preheat the process feed. Wang et al. (2014) disclosed a multifunctional supercritical water experimental system that integrates different reactors and can be used to perform experimental research on gasification, oxidation, and partial oxidation of supercritical water from municipal sludge and organic wastewater and can be further used to perform experimental research on flow-induced corrosion of materials from municipal sludge and organic wastewater.

3.3 Use of catalyst

Although many patents include the use of a catalyst in the SCWG process (as shown in Tables 2–4), this section includes only those inventions that are specifically based on the use of the catalyst or describe in detail its composition, preparation, and/or arrangement.

In 1996, Antal (1995) presented a method to catalytically decompose organic matter, such as wet biomass or organic waste, in supercritical water as a reaction medium, producing a hydrogen-enriched product and no substantial tar or char residues. The reaction was catalyzed by a carbon-containing catalyst (activated carbons or charcoals) in a tubular reactor. Glucose gasification over 0.6 g of activated carbon catalyst at 600°C, 34.5 MPa, and 1.2 m of glucose produced 2.24 mole of H₂/mole of reactant. In the case of cellulose gasification (a dimer of glucose), 5.22 mole of H₂/mole of reactant was obtained. Moreover, other real wastes were studied (sewage sludge and representative military wastes), producing 2.7 and 2.97 mole of H₂/mole of reactant, respectively.

The invention of Zhifeng and Runtian (2008) is based on the preparation of sludge (without drying) into a slurry or a mixture of slurry and a catalyst of a certain concentration [alkali metal hydroxides KOH or NaOH, and/or alkali earth oxides CaO or MgO, or hydroxides Ca(OH)₂ or Mg(OH)₂]. Preheated water and slurry (optimal ratio by weight of water and mud is established from 3 to 6:1) is pumped into a continuously heated reactor for 1–10 min at supercritical conditions. The reaction products are separated gradually, using a gas-liquid-solid separation tank, a gas-liquid separation tank, and an oil-water separation tank to obtain clean energy such as hydrogen-rich gas and liquid phase oil product. The separated solid products and the circulating water are discharged separately from the tank bottom, while the circulating water can be reused.

Lee (2008) describes a preparation method including Y-Ni/activated carbon catalysts capable of steadily producing hydrogen by performing a supercritical water vaporization reaction of organic materials. The reactor includes a reaction pipe that is divided into a preheating part and a vaporization part, each one with its respective cylindrical heating furnace. A solid remover is installed on a discharge pipe installed at the rear end of the reactor. It includes a high-pressure manometer and a postpressure controller to monitor the pressure in the discharge pipe and a gas-liquid separator to separate the decompressed product of gas phase and liquid phase.

Michael et al. (2009) describe a catalyst system that uses at least one metal and an oxide support. The said oxide support includes at least one of Al₂O₃, Mn_xO_y, MgO, ZrO₂, and La₂O₃ or any mixtures thereof. The system includes a pressure reactor heated to the water critical temperature point and under a pressure point or higher by means of a heating system that uses concentrated solar radiation to produce a high-pressure fuel gas.

In 2010, Junjie et al. (2010a) presented a method for transcritical catalytic gasification of coal. Figure 11 shows a diagram of the process. A CWS with a concentration of 30 wt.% was pumped into a pretreatment at 300°C. Then, CWS was introduced into the first reactor (R1) at 360°C and 23–25 MPa. Later, R1 effluent without any separation entered R2, which was at supercritical conditions (600°C and 23–25 MPa). Moreover, in the connection between R1 and R2, 30% K₂CO₃ was added to the R1 effluent. Between both reactors, a heat pump system absorbs heat from the R1 and supplies it to R2. Later, the combustible gas obtained is expanded to produce expansion work and power while the pressure is reduced. Electrical power is sent to the heat pump. At the end, final methane, hydrogen, carbon monoxide, and other gases are collected. This subcritical reactor-supercritical reactor combined arrangement makes more fully dispersed catalyst so less catalyst was needed than in a conventional system. In addition, the use of the heat pump system optimizes the internal energy process, increases energy efficiency, and reduces dependence on external energy.

Figure 11:

Schematic view of the invention from patent 82. R1 and R3 are reactors in subcritical water conditions, whereas R2 and R4 are in supercritical conditions.

Ruisheng et al. (2012) disclosed a rare earth double-perovskite catalyst used in the preparation of syngas through biomass and coal in supercritical water in cogasification mode. The catalyst contains lanthanum, cobalt, and manganese. The syngas obtained from the catalytic reaction reaches 63.4%, so that the content of carbon monoxide in the gas products increases and the content of carbon dioxide is remarkably reduced.

The invention of the National Institute of Advanced Industrial Science and Technology (2010) describes a graphite-supported ruthenium catalyst with a high surface area and a method to gasify lignin, which is one of the main components of woody biomass. The gasification reaction of the catalyst was evaluated by a batch method in a 6 ml stainless steel vessel. Different rutheniums supported (graphite, powdered activated carbon, and titania) were analyzed. The best results were obtained at 400°C, 0.1 g of lignin, 0.1 g of catalyst (graphite supported ruthenium catalyst) that was 5 wt.%, 3 g if water, and 1 h of reaction time, obtaining 84 ml of gas with 60% methane and 6% hydrogen. When hydrogen was the desired product, from 0.1 g of lignin, 0.2 g of titania-supported ruthenium catalyst, 3 g of water, and a reaction time of 1 h at 400°C, the highest yield was 45 ml of gas with 35% methane and 23% hydrogen.

Guan et al. (2014) described an organic SCWG system with Ru/CeO₂ as catalyst to increase the efficiency of the reaction. Generally, many catalysts under supercritical conditions easily lose capacity, but mesoporous ceria has a significant advantage because of its specific surface area and pore structure and can withstand temperatures over 2000°C. RuCl₃ and Ce(NO₃)₃×6H₂O as a starting material were prepared by impregnation; the catalyst powder has a black appearance. Ru load was 5 wt.% and CeO₂ was prepared in a carrier medium pore structure, with a water pore structure volume of 0.35 ml/g. This catalyst was fed during supercritical gasification of phenol in a batch reactor where 1–10 wt.% of catalyst was added at 10–100 wt.% of organic matter and 0.5–40 min of reaction time, under a pressure of 20–29.6 MPa, and at a temperature of 450–550°C. The use of the catalytic enhances water vapor reaction and increases the hydrogen production at the same time that reduces the reaction time.

Wada et al. (2013c) defended a method capable of performing the SCWG of biomass with activated carbon efficiently. In this system, before starting the gasification, the activated carbon is suspended in water and then mixed with the biomass-containing water. The suspension is adjusted to a predetermined moisture content and given concentration of activated carbon (5 wt.%). This method prevents the blockage of the apparatus by tar or activated carbon.

Zhu et al. (2014a,b) developed a composite catalyst to produce hydrogen through low-temperature SCWG (at 400°C) of a low-moisture-content (74–88%) dehydrated sludge. The composite catalyst contains the following raw materials: active nickel (Raney nickel or reduction of a nickel powder) and a carbon fixation agent [one of NaOH, KOH, Ca(OH)₂, CaO, CaSiO₃, or Na₂SiO₃]. The inventors claimed that the catalytic efficiency is high, and the process cost is greatly reduced by lowering the temperature and by using the composite catalyst (with an additional 5%). A hydrogen-rich gas (88% H₂) can be obtained.

There are two patents that use a catalyst to prevent clogging (Wada et al. 2013a,b). According to the invention of Wada et al. (2013a), in a similar system to the patent of Harinck and Smit (2011b), the product obtained from the gasification of supercritical water is used to heat a double-pipe heat exchanger. As tar is not generated at temperatures up to 350–400°C, retention time is shorter to suppress the production of tar in the inner tube of the double tube where the water-containing biomass with suspended non-metal-based catalyst is heated. Generally, the pressure inside the double-pipe heat exchanger and the gasification reactor has been adjusted to more than 27 MPa in order to prevent clogging caused by tar.

According to the invention of Wada et al. (2013b), it is possible to avoid the clogging of pipes by tar or biomass in a biomass gasification system with supercritical water. In the patents of Wada et al. (2013a,c), with similar systems, the pipes can be cleaned without stopping the operation of the system, by alternating the suspension and fresh alternatively water. As described in the patent, the switching valve (such as a three-way valve) can remove tar adhered by washing with fresh water and vibration via the double-pipe heat exchanger. This cleans the piping of the exchanger and the gasification reactor.

Finally, Erwang and Wei (2011a) depicted a method to obtain fuel ethanol from waste biomass. The invention uses a carbonyl metal-rare earth composite catalyst to obtain the fuel ethanol. In addition, the biogas is produced through SCWG of biomass under the action of a nonnoble catalyst. The gasification rate of the carbon element in the waste biomass is high. This solves the problem of tar and carbon deposits in the pipes. The catalyst adopted for the preparation process is easy to prepare, low cost, and highly active.