Solar Hydrogen Production and Its Development in China

Energy 34 (2009) 1073–1090

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Solar hydrogen production and its development in China

L.J. Guo*, L. Zhao, D.W. Jing, Y.J. Lu, H.H. Yang, B.F. Bai, X.M. Zhang, L.J. Ma, X.M. Wu

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China article info abstract

Article history: Because of the needs of sustainable development of the mankind society and natural environment Received 28 July 2007 building a renewable energy system is one of the most critical issues that today’s society must address. In Received in revised form the new energy system there is a requirement for a renewable fuel to replace current energy carrier. 19 July 2008 Hydrogen is an ideal secondary energy. Using solar energy to produce hydrogen in large scale can solve Accepted 24 March 2009 the problems of sustainability, environmental emissions, and energy security and become the focus of Available online 20 May 2009 the international society in the area of energy science and technology. It has also been set as an important research direction by many international hydrogen programs. The Ministry of Science and Keywords: Solar energy Technology of China supported and launched a project of National Basic Research Program of China (973 Biomass Program) – the Basic Research of Mass Hydrogen Production using Solar Energy in 2003 for R&D in the areas Hydrogen production of solar hydrogen production. The current status of solar hydrogen production research is reviewed and Supercritical water gasification some significant results achieved in the project are reported in this paper. The trends of development and Photocatalytic water splitting the future research directions in the field of solar hydrogen production in China are also briefly discussed. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction can be produced from water, which is abundant. Hydrogen is also renewable when it is produced from renewable energy sources. China is one of the largest energy consumers in the world with When it is converted into useful energy in the form of electricity via a coal-dominated energy structure which produces high CO2 a fuel cell, the by-product is harmless water vapor. We believe that emissions and leads to serious environment pollution. It makes with the progress of science and technology, solar hydrogen will be China one of the largest sources of greenhouse gases in the world competitive with traditional fossil energy regarding to economics within 20–30 years. Addressing these global concerns is dictating in 2030s and finally become equally important energy carrier as the need to reduce the present reliance on fossil fuels, and to electrical power [3,4]. increase the use of sustainable and environmentally friendly In the United States, Japan, Australia, Germany, and many other energy alternatives. This results in increased recognition of the countries and regions, researchers and government officials have importance of renewable energy to satisfy future energy demands. realized the possibility of a hydrogen economy. A series of impor- China has abundant renewable energy resources. For example, tant and large hydrogen energy programs, such as the ‘‘Agreement more than 2/3rd of China receives an annual total insolation that on the production and utilization of hydrogen’’ by International exceeds 5.9 GJ/m2 (1639 kWh/m2) with more than 2200 h of Energy Association (IEA) [5], the ‘‘Multiyear Plan for the Hydrogen sunshine a year. Besides, China also has a wide range of biomass R&D Program’’ by United States [4], and the ‘‘World Energy resources that can be used for energy supply. However, today’s Network’’ by Japan [6], have been launched. The Chinese govern- renewable energy resources (except for hydro-power) account for ment has also paid great attention to the hydrogen energy R&D. The only a fraction of China’s total energy consumption. The limitations Ministry of Science and Technology of China has launched a project of renewable energy utilization are low density, instability, of the Basic Research of Mass Hydrogen Production using Solar Energy discontinuity and changing with time, season and climate [1,2]. in the National Basic Research Program of China (973 Program) at the In order to better harness renewable energy, hydrogen has been end of 2003 [7]. identified as a potential alternative fuel as well as an energy carrier In this review, the current status of solar hydrogen production for the future energy supply. Hydrogen is clean and, in practice, it R&D in China is reviewed and some significant results achieved in the project of Basic Research of Mass Hydrogen Production using Solar Energy are reported. The trends of development and the future

* Corresponding author. Tel.: þ86 29 8266 3895; fax: þ86 29 8266 9033. research directions in the ﬁeld of solar hydrogen production in E-mail address: [email protected] (L.J. Guo). China are also brieﬂy discussed.

2. Pathway of solar hydrogen production temperatures the electrode reactions are more reversible, and the fuel cell reaction can more easily be reversed to an electrolysis reaction. Hydrogen can be produced from a variety of feedstocks: from For the SOEC, the main R&D needs are related to materials develop- fossil resources such as natural gas and coal and from renewable ment and thermo-mechanical stress within the functional ceramic resources such as water and biomass with input from renewable materials, which is similar to the main challenges for the SOFC. energy resources (e.g. sunlight, wind, wave or hydro-power). Presently, hydrogen is commonly produced from fossil fuels. 2.2. Thermochemical water splitting and biomass decomposition Natural gas steam reforming is one of the economical hydrogen production processes. Only about 5% of hydrogen is produced from Solar thermochemical cycles are alternative technologies for renewable sources, they are called solar hydrogen in this paper. solar energy utilization. By using trough, dish, or tower concen- Water electrolysis that can be driven by photovoltaic (PV) cells or trators, solar energy can be collected and concentrated to high wind turbines is an important solar hydrogen production tech- energy fluxes and high temperature above 2000 K. The thermal nology today. Other promising solar hydrogen production tech- energy can be used to activate chemical reactions with feedstocks, nologies include solar thermochemical, photoelectrochemical, and such as water, biomass, and fossil fuels, to produce hydrogen. Solar photocatalytic hydrogen production. Biomass products, such as thermochemical cycles suitable for renewable hydrogen production plants, microalgaes, and organic wastes, are also renewable sources include: 1) water thermolysis, 2) thermochemical water splitting, for solar hydrogen production. The latest developments of indi- and 3) thermochemical biomass decomposition. vidual important renewable hydrogen production technologies are reviewed in the following sections. 2.2.1. Solar water thermolysis Solar water thermolysis is the direct dissociation of water into hydrogen and oxygen gas by using concentrated solar thermal 2.1. Water electrolysis energy at a high temperature about 3000 C. At this temperature 10% of the water is decomposed and the remaining 90% can be Water electrolysis is currently the most dominant technology recycled. The process is conceptually simple, but operating at such used for hydrogen production from renewable sources because of high temperature requires special material selection. The efficiency high energy conversion efficiency. Water, used as a feedstock, is split is low mainly because of re-radiation loss and energy loss in gas into hydrogen and oxygen by electricity input as in Equation (1): separation by rapid quenching. The gas separation process will also generate explosive mixtures. 1 H2O þ electricity/H2 þ O2 (1) 2 2.2.2. Solar thermochemical water splitting The total energy demand for water electrolysis is increasing To reduce the temperature many thermochemical cycles for slightly with temperature, while the electrical energy demand high temperature splitting of water have been suggested. The decreasing. There are three types of water electrolysis available in chemical reactions involved in 2-step water splitting thermo- the industry: 1) alkaline electrolysis, 2) polymer electrolyte chemical cycles for hydrogen production are shown below: membrane (PEM) electrolysis, and 3) high temperature electrolysis. Alkaline electrolysers use an aqueous KOH solution (caustic) as an y MxOy/xM þ O2 (2) electrolyte that usually circulates through the electrolytic cells. They 2 are suited for stationary applications and are available at operating / pressure up to 25 bar. Alkaline electrolysis is a mature technology xM þ yH2O MxOy þ yH2 (3) allowing remote operation with significant operating experience in or industrial applications. The major R&D challenges for the future are the design and manufacturing of electrolyser equipment at lower MxOy/Mx0 Oy0 þ O2 (4) costs with higher efficiency and large turndown ratios. PEM electrolysers require no liquid electrolyte, which simplifies Mx0 Oy0 þ H2O/MxOy þ H2 (5) the design significantly. The electrolyte is an acidic membrane. PEM electrolysers can be designed for an operating pressure up to Where M is a metal and MxOy and Mx0 Oy0 are the corresponding several hundred bars, and are suited for both stationary and mobile metal oxides. Since O2 and H2 are produced in two different steps, no applications. The main drawback of this technology is the limited gas separation is needed. However, separation of metal produced in lifetime of the membranes. The major advantages of PEM over the first step is needed to avoid re-oxidation. Candidate metal oxides alkaline electrolysers are higher turndown ratio, increased safety include TiO2,ZnO,Fe3O4,MnO,MgO,Al2O3,andSiO2. due to the absence of KOH electrolyte, more compact design due to Besides 2-step cycles, 3-step cycles, such as iodine/sulfur (I/S) higher current densities and higher operating pressures. The PEM cycles, and 4-step cycles, such as UT-3 cycles, have also been electrolysers currently available are not as mature as alkaline received extensive research interests in recent years. The maximum electrolysers with relatively high cost, low capacity, poor efficiency theoretical efficiency of solar thermochemical cycles for hydrogen and short lifetime. It is expected that the performance of PEM production was estimated to be 49.5%, while currently the electrolysers can be improved significantly by materials develop- achievable efficiency is below 2%. ment and cell stack design. High temperature electrolysis is based on technology from high 2.2.3. Solar thermochemical biomass decomposition temperature fuel cells. The electrical energy needed to split water at All energy crops, agricultural residues, organic wastes, forestry 1000 C is reduced considerably compared to hydrogen production at waste, industrial wastes, and municipal wastes are categorized as 100 C. This means that high temperature electrolysers can operate at biomass products. Biomass is potentially a reliable energy source significantly higher overall process efficiencies than regular low for hydrogen production. It is renewable, abundant, and easy to use. temperature electrolysers. A typical technology is solid oxide elec- As CO2 emitted during biomass conversion process is trapped and trolysers cell (SOEC). This electrolyser is based on the solid oxide fuel consumed by green plants via photosynthesis, biomass is CO2 cell (SOFC), which normally operates at 700–1000 C. At these neutral in a complete life cycle. L.J. Guo et al. / Energy 34 (2009) 1073–1090 1075

Pyrolysis and gasification are the two thermochemical processes CO þ H2O/CO2 þ H2 (7) for biomass energy conversion [8–10]. Pyrolysis process involves heating of biomass (solid charcoal, liquid oil, or gas compounds) at Gasification, on the other hand, is the heating of biomass 650–800 K at 0.1–0.5 MPa in the absence of air. The products of particles at above 1000 K with partial oxidation for gas production. The gases produced can be steam reformed to produce hydrogen pyrolysis include: 1) gases, including H2,CH4, CO, CO2 and other gases, 2) acetone, acetic acid, and other liquid products, and 3) solid followed by water–gas shift reaction to further enhance hydrogen products, such as char, carbon, and other inert materials [11]. production. Hydrogen gas can be obtained if operation temperature is high and When biomass has high moisture content above 35%, it is likely sufficient volatile phase residue time is maintained. The hydrogen to gasify biomass in a supercritical water (SCW) condition [12].By production rate can be further improved by steam reforming, heating water to a temperature above its critical temperature (647 K) and compressing it to a pressure above its critical pressure

CH4 þ H2O/CO þ 3H2 (6) (22 MPa), biomass is rapidly decomposed into small molecules or gases in a few minutes at a high efﬁciency. Supercritical water and water–gas shift reaction, gasiﬁcation (SCWG) is therefore a promising process to gasify

To T4 T3 T2 T1 Ti

V10 Cooler Cooler

Heater 4 Heater 3 Heater 2 Heater 1 V6 V7 V8 V9

Back-pressure regulator Feeder V5

Wet test V3 V2 meter Liquid-gas separator V12 Water tank V1 High pressure pump

Fig. 1. Schematic representation of the miniature plant. 1076 L.J. Guo et al. / Energy 34 (2009) 1073–1090 biomass with high moisture contents with a high gasification ratio hydrogen production. This view has been supported by a recent (100% achievable) and a high hydrogen volumetric ratio (50% comprehensive review on hydrogen production [17]. achievable) [13]. In recent years, extensive research has been When a semiconductor is irradiated by photons having energy carried out to evaluate the suitability of various wet biomass equal to or above the band gap energy, electrons can be promoted gasification in SCW conditions. Although, the works have been from the valence band to the conduction band, leaving a positively mostly on a laboratory scale and are still in the early development charged hole in the valence band. If the conduction band edge is stage, the technology has already shown its economic competi- more negative than the hydrogen production level and the valence tiveness with other hydrogen production methods. Spritzer and band edge is more positive than the oxygen production level, the Hong [14] have estimated the cost of hydrogen production photogenerated electron/hole pairs are able to decompose water produced by SCWG to be about US$3/GJ (US$0.35/kg). into oxygen and hydrogen. Presently, the hydrogen production rate is still low due to the following reasons: 1) quick electron/hole 2.3. Photoelectrochemical and photocatalytic water splitting recombination in the bulk or on the surface of semiconductor particles, 2) quick back reaction of oxygen and hydrogen to form According to Veziroglu [15,16] the method of photo- water on the surface of catalyst, and 3) inability to utilize visible electrochemical and photocatalytic water decomposition using light. Addition of electron donors and noble metal loading can solar energy is probably the most promising method for the enhance hydrogen production significantly due to reduction in

Fig. 2. Schematic diagrams of Bench-scale apparatus. L.J. Guo et al. / Energy 34 (2009) 1073–1090 1077 charge recombination. Addition of carbonate salts can inhibit back under active R&D, will play an important role in future hydrogen reaction on the catalyst surface. Semiconductors treated by dye economy. sensitization and metal ion-implantation can extend the useful spectrum to the visible range so that the solar energy utilization is increased signiﬁcantly. Separation of hydrogen gas is required in 2.4. Biological hydrogen production photocatalytic hydrogen production as oxygen and hydrogen are produced simultaneously. This can be achieved by employing Biological hydrogen production is presently in the early devel- a photoelectrochemical system, in which hydrogen and oxygen are opment stage of laboratory-scale testing [18]. The production produced at different electrodes. It is expected that photocatalytic processes can be divided into: 1) direct biophotolysis, 2) indirect and photoelectrochemical hydrogen production, although still biophotolysis, 3) biological water–gas shift reaction, 4) photo-

Fig. 3. Schematic diagrams of SCW ﬂuidization bed system. 1078 L.J. Guo et al. / Energy 34 (2009) 1073–1090 fermentation, and 5) dark fermentation. The processes are biophotolysis is sensitive to oxygen and thus difﬁcult to sustain controlled by hydrogen-producing enzymes, namely, reversible hydrogen production. The indirect biophotolysis can overcome this hydrogenase and nitrogenase. problem by producing hydrogen and oxygen at different stages to In direct biophotolysis, microalgae, such as green algae and resolve the issue of oxygen sensitivity. Cyanobacteria, absorb light energy and generate electrons. The In biological water–gas shift reaction, some photoheterotrophic electrons are then transferred to ferredoxin (FD) using the solar bacteria, such as Rhodospirillum rubrum [19–21], can survive in the energy absorbed by photosystem I. Then, hydrogenase can accept dark by using CO as the sole carbon source to generate adenosine electrons from FD to produce hydrogen. However, direct triphosphate (ATP) coupling the oxidation of CO with the reduction

a P T V3 To gas sample

V1 V2

To N2 gas bottle

W Heated 100ml autoclave

To vent

Autoclave lift b

Fig. 4. Autoclave reactor for SCWG. L.J. Guo et al. / Energy 34 (2009) 1073–1090 1079

þ of H to H2. In photo-fermentation, photosynthetic bacteria made of special alloy tube with the inner diameter (i.d.) of 9 mm or produce hydrogen through the activity of their nitrogenase using 6 mm and reactor #2 was made of Hastelloy C-276 tubing with solar energy and organic acid or biomass. The drawbacks of photo- 13.4 mm outer diameter (o.d.) 7.7 mm i.d. The unique feature of the fermentation are 1) high energy demand for the use of nitrogenase experimental apparatus is its ability to realize the overall continuous enzyme, 2) low solar energy conversion efficiency, and 3) reaction by two pumps, two feeders and the valves. The design of the substantial land need for anaerobic photobioreactors. apparatus has obtained a Chinese patent [23].Gasificationofmodel Alternatively, in dark fermentation, anaerobic bacteria and some compounds and real biomass was realized in the miniature plant. microalgae, such as green algae on carbohydrate-rich substrates Fig. 2 displays the bench-scale apparatus in SKLMF. The reactor [22], can produce hydrogen in a dark environment. Since solar is made of special stainless steel with 9 mm i.d., and designed for irradiation is not a requirement, dark fermentation is more versatile. temperatures up to 650 C and pressures of 35 MPa. The maximum In the project of Basic Research of Mass Hydrogen Production throughput capacity of the system is 16 kg/h. The Bench-scale using Solar Energy, the emphasis has been paid on fundamental apparatus is developed based on the miniature plant. A high theories of hydrogen production from solar energy especially heating rate of biomass was attained by mixing the feedstock with visible light using thermochemical decomposition of biomass and preheated water. Also, the thermal energy can be recycled by the water, or/and using photolysis of water. The main research contents exchanger at the reactor exit and thus the energy efficiency is of this project include the principles for the construction of novel improved. Coal can be gasified with the formation of hydrogen-rich hydrogen production system and the related fundamental science gas. The apparatus design and coal gasification method have also problems, the design, preparation, characterization and action been issued a Chinese patent [24]. mechanism of catalysts, and the theory for mass production of two In a tubular reactor, ash and carbon were easy to be deposited specific hydrogen production methods and systems. Some prog- on the reactor wall so that the reactor plugging occurred resses have been achieved and will be reported in the next section. frequently. SCW fluidization bed system can prevent reactor plugging. The first SCW fluidization bed system for biomass 3. Thermochemical water splitting and biomass gasification was successfully developed in SKLMF [25]. Fig. 3 decomposition hydrogen production research in China displays schematic representation of supercritical fluidized bed system. The system includes reactor, heat exchanger, high pres- Since 1997, a series of studies on hydrogen production from sure pumps, feeders, separators, cooler and so on. The reactor is biomass in SCW have been conducted by the group of State Key Lab constructed of type 316 stainless steel with a bed diameter of of Multiphase Flow in Power Engineering (SKLMF) in Xi’an Jiaotong 30 mm and a freeboard diameter of 40 mm, and the total length of University [23–30]. The research includes development of gasifi- reactor is 915 mm. It is designed for the temperature up to 650 C cation system and investigation of gasification rules of model and the pressure up to 30 MPa. The reactor is heated to the set- compounds and real biomass, catalysts, reaction kinetic and ther- point temperature by three 2 kW electrical heaters that are coiled modynamic analysis. around the outer surface of the reactor tube. The temperature profile of the reactor wall is measured by some fixed, type K 3.1. SCWG systems thermocouples held on the outer wall of the reactor in good thermal contact. Five type K thermocouples measure the At present, four reactor SCWG systems, including miniature temperature at the reactor centerline. Model compound (glucose) plant, bench-scale apparatus, SCW fluidization bed and autoclave and real biomass (corn cob) were gasified to generate hydrogen in reactor, are running and a solar-driven SCWG system is being the system. 30 wt% Glucose and 18 wt% corn cob feedstocks were developed in SKLMF. gasified stably during long-term operations. Fig.1 displays schematic representation of the miniature plant. It is Fig. 4 displays an autoclave reactor for SCWG [26]. The autoclave designed for the temperature up to 700 C and the pressure up to is fabricated from 316L stainless steel with a volume of 140 mL and 35 MPa. The maximum throughput capacity of the system is 1 kg/h. the lines of purging and sampling are constructed of 1Cr18Ni9Ti Two kinds of reactors were used in our laboratory. The reactor #1 was stainless steel. The system can be operated at temperatures up to

Table 1 Comparison of different SCWG systems.

Type Reactor Reaction Maximum capacity Materials Catalyst Results pressure and temperature

Miniature plant Continuous Special alloy 15–32.5 MPa; 1 kg/h Glucose, CMC, lignin, KOH, K2CO3,Na2CO3 GE >100%; maximum tube; Hastelloy 500–775 C sawdust, corn cob, solution concentration is C-276 tube cornstalk, wheat 12 wt% and reactor

strew, rice shell, rice plugging occurred; H2 stalk, peanut shell, vol% ¼ 20–49% waster water

Bench-scale apparatus Continuous Special alloy 20–27.5 MPa; 16 kg/h Glucose, corn KOH, K2CO3, R-Ni GE >100%; Maximum tube; Hastelloy 650–750 C cob, Coal solution concentration is C-276 tube 15 wt% and reactor

plugging occurred; H2 vol% > 60% (for coal)

SCW ﬂuidization bed Continuous Special alloy 23–27 MPa; 16 kg/h Glucose, corn cob K2CO3, ZnCl2 30 wt% glucose and tube 500–650 C 18 wt% corn cob feedstocks were gasiﬁed

stably; H2 vol% ¼ 40% Autoclave reactor Batch Special alloy 23–28 MPa; / Cellulose, lignin, K2CO3, OH, Ca(OH)2, Ru/C and R-Ni have better tube 350–500 C xylan, peanut Ru/C, Pd/C, R-Ni, CeO2, catalyst performance shell Coal CeO2, (CeZr)xO2, ZnCl2 1080 L.J. Guo et al. / Energy 34 (2009) 1073–1090

650 C and pressures up to 35 MPa. Catalyst performance was SCWG of biomass. Figs. 10–12 show the results of biomass gasifi- evaluated in the reactor. Table 1 shows comparison of different cation with different catalysts. Alkali was in favor of water–gas shift SCWG systems in SKLMF. reaction and improved the molar of hydrogen. The Ca(OH)2 was not only catalyst but also CO2 sorbent. Metal oxide is not usually used as 3.2. Parametric study a catalyst for biomass gasification, but CeO2, CeO2, (CeZr)xO2 can catalyze the gasification reaction. Ru/C and R-Ni have a better Biomass model compounds (glucose, cellulose and lignin) and catalyst performance, but R-Ni is more costly. real biomass were gasified in SCW system and the effects of the various parameters on biomass gasification were examined. The 3.4. Thermodynamic analysis parameters investigated include pressure, temperature, residence time, reactor geometrical configuration, reactor types, heating rate, A comprehensive thermodynamic analysis, including chemical reactor wall properties, biomass types, biomass particle size, cata- equilibrium in the reactor, gas–liquid equilibrium in the high lysts and solution concentration. Figs. 5–9 display the typical results pressure separator, and energy analysis of the whole system, was of biomass gasification in SCW system under various conditions. The performed for the new fluidization bed system built in our labo- results show that temperature is a key factor and high temperature is ratory [29]. The chemical equilibrium model is based on mini- in favor of biomass gasification in SCW. The pressure has no signif- mizing Gibbs free energy. The equation of state-excess Gibbs icant effect on gasification, but a high pressure seems to favor H 2 energy (EOS-GE) method was used in phase equilibrium analysis. production. Feedstock with high concentration is more difficult to be The influence of operation parameters on equilibrium gas yields gasified in SCW and could be gasified completely only at a higher and gas compositions was predicted by chemical equilibrium temperature. For example, when feedstock containing 6.19 wt% calculation. Comparison between experimental and calculation wood sawdust and 3.0 wt% CMC was gasified at 650 C and 30 MPa, results shows that biomass gasification reaction is far from chem- plugging took place after about 40 min, whereas feedstock con- ical equilibrium. Influence of operation parameters in high pressure taining 9 wt% corn cob and 3.0 wt% CMC was stably gasified at 750 C separator was also obtained by phase equilibrium analysis. The and 25 MPa during long-term operations. results were used to design the separator. The results from energy analysis of the whole system indicate that energy losses are mainly 3.3. Catalysts due to heat transfer, so the energy efficiencies may be increased by improving heat transfer efficiency of each unit in the system. This Alkali (K CO , KOH, Ca(OH) ), metal (Ru/C, Pd/C, R-Ni,), metal 2 3 2 study provided a thermodynamic tool for further improvement oxide (CeO , CeO , (CeZr) O ) and ZnCl were used as catalyst for 2 2 x 2 2 of design and operation of biomass gasification systems. Figs. 13 and 14 show some typical results of chemical equilibrium and a 60 phase equilibrium analysis in reactor and in separator. H2 CH4 CO2 C2Hx Some researchers from Shanxi Institute of Coal Chemistry and 50 Thermal Physics Engineering Institute conducted coal and biomass

40 a 50 H2 CO CH4 CO2 30 40 20 30 Gas Yield (mol/kg) 10 20 0 773 823 873 923 973 998 1023 1048 10 Temperature (K) Gas Yield (mol/kg) 0 17.5 20 22.5 25 27.5 30 120 b Pressure (MPa)

100 GE CE b GE CE TOC 80 120 2000

60 100 TOC (ppm) 1500 80 40 GE or CE (%) 60 1000 20

GE or CE (%) 40 500 0 20 773 823 873 923 973 998 1023 1048 Temperature (K) 0 0 17.5 20.0 22.5 25.0 27.5 30.0 Fig. 5. Effect of temperature on lignin gasification at 25 MPa in the miniature plant: Pressure (MPa) (a) gas yield and (b) gasification efficiency (GE) and CE [27]. (When the temperature is 773–923 K, the feedstock with 1.5 wt% lignin content is gasified using reactor #1 and Fig. 6. Effect of pressure on sawdust gasification in SCW in the miniature plant using the flow rate is 6 g/min; When the temperature is 973–1048 K, the feedstock with reactor #1: (a) gas yield and (b) GE, CE and TOC [28]. (Temperature, 923 K; residence 3 wt% lignin content is gasified using reactor #2 and the flow rate is 6 g/min.) time, 27 s; feedstock, 2 wt% wood sawdust þ 2 wt% CMC.) L.J. Guo et al. / Energy 34 (2009) 1073–1090 1081

a gasiﬁcation in SCW. For example, Qu [31] studied on the biomass 20 pyrolysis in SCW and change of product distribution with the CO 2 reaction temperature, pressure and residence time. Ren [32] gasi- ﬁed wood sawdust in SCW with addition of CaO. Product gas and H 15 2 bio-oil were formed.

10 4. Research on photocatalytic water splitting hydrogen production in China

CH4 Gas Yield (mol/kg) 5 Research on photocatalytic hydrogen production in China has CO been initiated in nineties of the last century and we are among the groups conducting earliest work in this ﬁeld. Scientists from Dalian 0 Institute of Chemical Physics (Chinese Academy of Sciences, CAS), 10 15 20 25 30 35 40 45 50 Lanzhou Institute of Chemical Physics (CAS) and Shanghai Jiaotong Residence time (s) University have also set their research direction in this ﬁeld in the following years. These groups are mutually independent, yet b 120 closely cooperative with one another. Following the launching of 3000 the project Basic Research of Mass Hydrogen Production using Solar 100 GE Energy in 2003, almost all the main groups active in photocatalytic hydrogen production have participated in this project. With the 2500

TOC (ppm) support of this project and other relative projects as well, the 80 CE Chinese researchers have made continuous and notable contributions in the ﬁeld. 60 2000 As pointed out in Section 2.3, the key issue for realizing the commercial application of solar water splitting hydrogen produc-

GE or CE (%) 40 TOC tion technique is to find an efficient, stable and low-cost photo- 1500 catalyst. The photocatalyst should be sensitive to visible light, 20 which is dominant in the total solar spectrum available for photocatalytic hydrogen production. Accordingly, most Chinese 0 1000 10 15 20 25 30 35 40 45 50 research groups have conducted their research along this direction. Various modifications have been applied to traditional wide band Residence Time (s) gap semiconductors, either metal oxide or sulfide, to extend their Fig. 7. Effect of residence time on wood sawdust gasification in the miniature plant: light response into visible light region. Many new photocatalysts (a) gas yield and (b) GE, CE and TOC [28]. (Temperature, 923 K; pressure, 25 MPa; with various compositions and/or structures have also been feedstock, 2 wt% wood sawdust þ 2 wt% CMC.) reported. Various photocatalytic reaction systems and reactors

60 a A — 2w t% saw dust + 2w t% CMC; H2 CH4 CO2 C2Hx 923K;25MPa;4.82g/min;Reactor #1 50 B — 4w t% saw dust + 2w t% CMC 923K;25MPa;4.44g/min;Reactor #1 40 C — 2w t% corn cob + 1w t% CMC; 30 1023K;25MPa;6g/min;Reactor # 2 D — 4w t% corn cob + 2w t% CMC; 20 1023K;25MPa;6g/min;Reactor # 2 E — 7w t% corn cob + 3w t% CMC; Gas Yield (mol/kg) 10 1023K;25MPa;6g/min;Reactor # 2 F — 9w t% corn cob + 3w t% CMC; 0 1023K;25MPa;6g/min;Reactor # 2 ABCDEF

b 120 GE CE 100

40 GE or CE (%)

0 ABCDEF

Fig. 8. Gasiﬁcation of feedstock with different biomass content in SCW: (a) gas yield and (b) GE and CE [27]. 1082 L.J. Guo et al. / Energy 34 (2009) 1073–1090

a extensive search on the ways of modification of TiO2 catalysts 100% prepared by different methods [33–42]. Only a limited number of representative examples will be given here. 80% Our group have doped Ni2þ ion into the mesoporous matrix of TiO2 and found that the doped Ni is beneficial for preserving the C2Hx collapse of mesostructure of the material in thermal calcinations 60% CO2 process. More importantly, it is found that the existence of Ni can CH4 CO promote the charge separation and thus greatly improves the 40% H2 activity of the photocatalyst [43].TiO2 was further modified by us with both anion nitrogen ion and Pt or Pd ions, forming an inter- 20% calation structure which can significantly improve its photocatalytic performance [44,45], the modification effect is schematically illus- trated in Fig. 15.WS sensitized mesoporous TiO was also prepared. 0% 2 2 ABCDEFGH Compared to bulk TiO2, mesoporous TiO2 can afford more WS2 at similar dispersion levels. The growth of WS2 particles was confined 120 3000 within quantum dimensions by the mesoporous structure avoiding b GE CE TOC their detachment due to photocorrosion [46]. 100 Lu’s series work have demonstrated that the sensitization of wide band gap semiconductor by organic dyes especially eosin has TOC (ppm) 80 2000 been demonstrated to be effective [47–50]. Eosin-sensitized CuO incorporated TiO2 catalyst has found to be active in hydrogen 60 production under visible light irradiation in the presence of electron donors such as triethanolamine (TEA), acetonitrile and trie- 40 1000 GE or CE (%) thylamine [51]. Very recently, they reported a novel and highly 20 efficient Eosin Y-sensitized Pt-loaded nanotube Na2Ti2O4(OH)2 (NTS) photocatalyst active for hydrogen generation in TEA solution 0 0 under visible light irradiation [49]. Due to its very high surface area ABCDEFGH and exclusive one-dimensional nanostructure facilitating efficient electron transfer, the composite photocatalyst showed a high Fig. 9. Comparison of gas composition (a), GE, CE and TOC (b) for gasification of quantum yield of 14.97% toward photocatalytic hydrogen produc- various biomass feedstocks in SCW [28]. (Temperature, 923 K; residence time, 27 s; pressure, 25 MPa; feedstock, 2 wt% biomass þ 2 wt% CMC; A: rice straw; B: rice shell; tion. In 2007, Lu et al. [50] reported an Eosin Y-sensitized Ti-MCM- C: wheat stalk; D: peanut shell; E: corn stalk; F: corn cob; G: sorghum stalk; H: wood 41 photocatalyst. As one can see in Fig. 16, this photocatalyst sawdust. GE is defined as the total mass of the product gas/the total mass of the dry showed quite high activity and long-term stability for hydrogen feed.) production. Eosin Y-sensitized TS-1 zeolite was studied for the photo-reduction of water into hydrogen driven by visible light too have been studied. In the following sections, progress made by by the same group. In the presence of TEA as an electron donor, the Chinese research groups in this field will be reviewed. highest rate of hydrogen generation and apparent quantum efficiency are 34 mmol h1 and 9.4%, respectively, under visible light irradiation (l 420 nm). Short-term stability test indicates that the 4.1. Development of new photocatalysts catalyst is rather stable during 50 h photoreaction [52]. Liao et al. achieved photocatalytic H2 production with multi- 4.1.1. Modification of metal oxide photocatalyst walled carbon nanotube (MWNT)-TiO2:Ni prepared by modifying TiO2 is a well known wide band gap metal oxide photocatalyst TiO2 with MWNT and Ni particle [53]. Lately, they reported a mes- without visible light response. By doping TiO2 with certain anionic oporous TiO2 photocatalyst which can improve the photocatalytic or cationic atoms, the absorption edge of the photocatalyst can be shifted to visible light region. Chinese groups have conducted

60 H2 CO CH4 CO2 H2 CO CH4 CO2 30 50

40 20 30

20 10 Gas yield (mol/kg) Gas Yield (mol/kg) 10 31.1% 38.2% 44.8% 28.8% 40.3% 0 0 None Ca(OH)2 K2CO3 K2CO3 and None K2CO3 ZnCl2 R-Ni R-Ni and Ca(OH)2 ZnCl2

Fig. 10. Comparison of gas yield for cellulose gasiﬁcation with different alkali catalysts Fig. 11. Peanut shell gasiﬁcation in SCW with different catalysts [27]. (Batch reactor; in SCW using the batch reactor [27]. (Batch reactor; water, 11 g; cellulose, 1.0 g; CMC, water, 10 g; cellulose, 1.0 g; CMC, 0.3 g; 723 K; reaction time, 20 min; initial pressure,

0.2 g; 773 K; reaction time, 20 min; initial pressure, 4.0 MPa; K2CO3, 0.2 g; Ca(OH)2, 4.0 MPa; catalyst, 1 g; the mixture of R-Ni and K2CO3 is composed of 1 g R-Ni and 0.2 g 1.6 g). ZnCl2). L.J. Guo et al. / Energy 34 (2009) 1073–1090 1083

1.0 50

H 100 Hydrogen recovery ratio (%) H2 CO CH4 CO2 2 0.8 40 80

30 0.6 60 20 0.4 40 Gas yield (mol/kg) 10 Molar fraction CO2 0.2 20 0 CH None CeO2 nCeO2 n(ZrCe)xO2 Pd/C Ru/C 4 0.0 0 Fig. 12. Comparison of gas yield for gasiﬁcation of cellulose with various catalysts in 0 5 10 15 20 25 30 SCW [26]. (Batch reactor; water, 10 g; cellulose, 1.0 g; catalyst, 0.4 g; CMC, 0.2 g; 773 K; Pressure (MPa) reaction time, 20 min; initial pressure, 4.0 MPa).

Fig. 14. Effects of operation pressure in high pressure separator on separating H2 from activity of TiO2 owning to its ultrafine size, high surface area and CO2 at 298 K [29]. controllable crystalline phase [54]. Wu et al. [55] have reported a composite photocatalyst reaction. Efforts have to be made to improve the stability of the HLaNb2O7/(Pt, Fe2O3). It is well known that Fe2O3 has an ideal band metal sulfide. Research on the modification of CdS has been very structure for water splitting, but its ability of H2 production is very weak due to the combination of electrons and holes. By incorpo- active in China. We have developed a novel two-step thermal sulfidation method for the preparation of highly stable and active CdS. rating Pt and Fe2O3 into the interlayer of HLaNb2O7, the height of As shown in Figs. 17 and 18, the surface of the CdS photocatalyst the Fe2O3 pillar in HLaNb2O7/Fe2O3 and HLaNb2O7/(Pt, Fe2O3) could was modified with nanostep structures which resulted in much be less than 0.5 nm. The photocatalytic activity of HLaNb2O7/Fe2O3 intercalated material is found to be better than those of unsup- higher hydrogen production rate than CdS prepared by common procedures. Its quantum yield at 420 nm was determined to be ported Fe2O3 and is enhanced by the co-incorporation of Pt. 24.1% and the energy conversion efficiency in the visible light Very recently, Shangguan et al. [56] prepared BiYWO6 (BYW) oxide solid solution acting as a photocatalyst for overall water region of l > 430 nm was calculated to be 6.35% [57]. Nanosized CdS splitting under visible light when loading cocatalysts. The band gap particles were confined within the channels of Ti-MCM-41 by ion- of BYW was 2.71 eV, and it absorbed visible light up to 470 nm. exchange and sulfidation methods forming a CdS@Ti-MCM-41 composite photocatalyst. Owning to the quantum confinement BYW with RuO2 has the best activity compared to other cocatalysts effect and efficient charge separation, the activity of CdS photo- such as Cr2O3–Pt, Pt, and Au. Under irradiation of l > 420 nm light, the amounts of the produced hydrogen and oxygen were about 12.3 catalyst has been greatly improved [58]. Similarly, nanosized In2S3 and 5.6 mmol in 3 h, respectively. This study indicated that the particles were confined within the channels of Ti-MCM-41 by the formation of solid solution was the feasible method to adjust the same procedure and also exhibited improved activity [59]. Shang- conduction band and valence band to obtain a visible-light-driven guan et al. have intercalated CdS into the lamellar metal oxides photocatalyst. [60,61]. The incorporation could suppress the CdS particle growth. Photogenerated electrons could thus be quickly transferred 4.1.2. Modification of sulfide photocatalyst through the nanostructure, and the recombination between the Among all the visible light sensitive photocatalysts developed to photoinduced charge carriers was effectively depressed. Conse- date, CdS (2.4 eV) loading with Pt as cocatalyst was one of the most quently, the photocatalytic evolution of hydrogen was enhanced. promising photocatalysts due to its high activity and low-cost. CdS nanoparticles were decorated on Na2Ti2O4(OH)2 nanotubes However, CdS is prone to photocorrosion in the photocatalytic through partial ion-exchange method. The results showed that CdS nanoparticles of 5–6 nm were anchored on the surface of the Na Ti O (OH) nanotubes. Under irradiation of visible light, the 100 10 2 2 4 2

CO yield (10 prepared CdS/Na2Ti2O4(OH)2 showed high photoactivity for hydrogen production [62]. 80 8 H2

60 6 -3 mol/kg biomass) CO CO2 40 4

20 2 CH4 Gas Yield (mol/kg biomass)

0 0 0 5 10 15 20 25 30 Dry biomass content (wt. %)

Fig. 13. Equilibrium gas yields in the reactor as a function of dry biomass content for Fig. 15. Schematic illustration of charge separation within TiO2 modiﬁed both with N biomass gasiﬁcation at 25 MPa, 873 K [29]. and Pt or Pd. 1084 L.J. Guo et al. / Energy 34 (2009) 1073–1090

CdS was also modified with a wide band gap ZnS to form solid prevents the loaded particles (Pt, RuO2) from aggregating to grow solution Cd1xZnxS [63]. The band gap of the photocatalyst can be into larger size. In their recent studies, the interlayer structures of continuously adjusted by changing the composition of the solid layered metal oxides were modified by ion-exchange and interlayer solution. At the optimal composition, the solid solution showed reaction, in which alkylammonium and tetraethylorthosilicate high activity toward hydrogen production even in the absence of (TEOS) were used as an interlayer exchange guest and a pillar noble metal loading. Band structure of the Cd1xZnxS solid solution precursor, respectively [75]. The layer height of intercalated was further modified with Cu doping. Quantum yield for Cu doped compounds can be controlled by the sort of alkylamine of different Cd1xZnxS photocatalyst without Pt loading is as high as 9.6% [64]. chain lengths. These layered compounds exhibited enhanced The new method for the preparation of Cd1xZnxS was also activity in UV light due to the separation of evolution sites of attempted by our group. By using a two-step thermal sulfuration hydrogen and oxygen in different parts of the compound. method, the photocatalytic activity of Cd1xZnxS was significantly More recently, a series of visible light responsive single-phase enhanced. Quantum yield for Cd1xZnxS photocatalyst without Pt metal oxide photocatalysts K4Ce2M10O30 (M ¼ Ta, Nb) were loading prepared by such method is as high as 10.23%. It is proposed prepared [76,77]. It is noted that these compounds also have that the uneven distribution of negative S2 at the surface of the parallelepiped surface structure. This is beneficial to the formation Cd1xZnxS can facilitate charge separation and contribute to the of ‘‘nest’’, where nanoparticles of Pt, RuO2 and NiOx are strongly high activity [65]. Modification of sulfide with both ion doping and associated, improving the photocatalytic activity greatly. oxide incorporation has also been attempted. Ni2þ doped Li of Dalian Institute of Chemical Physics (CAS) focus their Cd1xZnxS microsphere photocatalysts were prepared by a new attention on the development of new visible light-driven photo- hydrothermal method. Experimental results showed that the catalysts. A series of rare-earth metal containing oxynitrides doped Ni2þ can not only affect the crystal structure and responsive in visible light has been developed [78,79]. Interestingly, morphology of Cd1xZnxS photocatalysts, but also greatly improve it has been found that the activity for reduction to H2 is increased its photocatalystic activity. It was found that 0.1 wt% Ni2þ doped by the incorporation of Pt or Ru as a cocatalyst, with a significant Cd0.1Zn0.9S photocatalyst showed the highest activity with the increase in production efficiency when both Pt and Ru are present. apparent quantum yield of 15.9% at 420 nm [66]. This result indicates that the cooperative interaction between metal clusters of different species may sometimes make great 4.1.3. Development of photocatalyst with new contributions in the improvement of the activity of the photo- composition and structure catalyst. More recently, a well crystalline ternary ZnIn2S4 photo- Although modification of traditional metal oxide and sulfide catalyst was prepared by a hydrothermal method by the same photocatalyst has proven to be effective, photocatalysts suitable for group. In contrast to traditional photocatalysts, ZnIn2S4 showed these modifications are still limited. Many Chinese researchers enhanced activity with increasing reaction time. The maximum have made great efforts in finding photocatalyst with new hydrogen production amounts to 213 mmol h1 g1 over 2% Pt composition, structure or morphology. loaded ZnIn2S4 [80]. This group further developed sulfur Zou for example reported a new oxide photocatalyst In2BiTaO7 substituted Zn doped In(OH)3 composite photocatalyst active in using the solid-state reaction method. The compound showed visible light. By simultaneous substitution and doping of Sulfur and strong optical absorption in the visible region. For the photo- Zn, the absorption edges of the composite photocatalyst can be catalytic reaction, H2 or O2 evolution was observed from CH3OH/ adjusted in the range of 240 nm–570 nm, with the optimal 1 1 H2O or AgNO3 solution respectively with In2BiTaO7 as the photohydrogen production to be 223.3 mmol h g [81]. In our group, catalyst under visible light irradiation [67]. More recently, they hexagonal ZnIn2S4 photocatalysts with different morphology and prepared a novel solid photocatalyst Bi2GaVO7 which can split pure crystallinity (micro-structures) were prepared in aqueous-, meth- water into H2 and O2 under UV light irradiation [68]. anol- and ethylene glycol-mediated conditions via a solvothermal/ Shangguan et al. [69–74] prepared lamellar KCa2Nb3O10 and hydrothermal method by our group. Aqueous-mediated ZnIn2S4 KTiNbO5 as photocatalyst. By loading Pt or RuO2 the activity of the was found to be more stable than the other two ZnIn2S4 photocatalyst is significantly increased. The increased activity was attributed to the formation of a ‘‘nest’’ on the particle surface that promotes a uniform distribution of the small particles. This ‘‘nest’’

Fig. 16. Photocatalytic hydrogen production for Eosin Y-sensitized Ti-MCM-41. Fig. 17. SEM image for CdS prepared by two-step thermal sulfuration method. L.J. Guo et al. / Energy 34 (2009) 1073–1090 1085 photocatalysts [82]. Various surfactants were also employed in the inhibition kinetics is observed. When the donors are adsorbed hydrothermal process. It was found that cetyltrimethylammonium strongly on TiO2 in a state of saturated adsorption in a binary bromide (CTAB) can significantly improve the photocatalytic system, the overall rate of the hydrogen evolution is consistent with activity of the photocatalyst [83,84]. Thin films of ZnIn2S4 were that of decomposition of absorbed donors, and the system can be deposited onto indium-doped tin oxide (ITO)-coated glass treated as a single component system. substrates from a mixed aqueous solution of ZnCl2, InCl3 and Lu et al. [94] have doped TiO2 with anionic boron. They (NH2)2CS by a simple and economical spray pyrolysis technique. successfully applied this visible light-driven photocatalyst to the The results of electrochemical and photoelectrochemical charac- boron-containing aqueous medium and obtained a high hydrogen teristics show that ZnIn2S4 is a potential material for splitting water production rate. It is a very interesting example of hydrogen to produce hydrogen using solar energy [85]. production by dealing with salt lakes containing very high borate More recently, our group has succeeded in employing zeolite as concentration, which are widely distributed and easily available in visible photocatalyst by incorporating Cr or Cr, Ti together into the west part of China. Very recently, Lu reported the significant MCM-41. It is found that the photocatalyst is active for hydrogen pressure effect on the photocatalytic activity of hydrogen genera- production under visible light [86,87]. A novel composite CdS/ tion over two kinds of representative semiconductor catalysts (TiSi2 mesoporous zirconium–titanium phosphate photocatalyst and Pt/TiO2). The photothermal-catalytic hydrogen generation (CdS@ZTP) working under visible light has been successfully from water steam splitting under visible light was achieved. And prepared. By continuously adjusting the conduction band of ZTP the physicochemical properties of the photocatalysts were support, the optimal hydrogen production could be obtained. As characterized by XRD and XPS techniques. Compared to the shown in Fig. 19 by coupling CdS with ZTP, the loading amount of conventional liquid–solid-phase photocatalytic method, this gas– CdS could be significantly lowered, while still maintaining a very solid-phase photothermal-catalytic method for hydrogen releasing high activity [88]. from water steam exhibited a higher apparent quantum yield (22.01%) under the high pressure [95]. 4.1.4. Investigation on various sacrificial agents in photocatalytic reaction 4.2. Development of new evaluation instruments and reactors In the process of photocatalytic hydrogen production, sacrificial agent as electron donor is often used to consume photogenerated 4.2.1. Development of new evaluation instruments and holes and avoid oxygen formation. Photocatalytic hydrogen reaction systems production over Pt loaded TiO2 in the presence of various sacrificial In order to get more insights into the mechanism behind the agents has been investigated by Shangguan [89]. Their study photocatalytic reaction, powerful and precise evaluation instru- showed that CH3OH is a favorable sacrificial agent for TiO2. Li et al. ments are always needed. To this end, a system for quick prepa- [90–93] have paid their emphasis on the studies of photocatalytic ration and precise selection of photocatalyst has been designed in hydrogen production with organic pollutants as electron donor, our group, based on the development of a novel hydrogen gas simultaneously decomposing pollutants in the process of hydrogen sensor that can precisely and quantitatively determine the production. In this group, the photocatalytic hydrogen evolution hydrogen concentration [96]. A novel multi-channel photocatalyst and decomposition of such pollutants as oxalic acid [90], formate evaluation system has also been developed that can simultaneously acid [91], EDTA [92], chloroacetic acids [93], etc., over Pt/TiO2 have evaluate six groups of photocatalysts in a precise, convenient and been systematically investigated. Their studies showed that the in-situ manner. To the best of our knowledge, no similar system has hydrogen production properties of these organic electron donors been reported or patented in other groups [97]. are strongly dependent on their absorption affinity to the surface of As for the new reactor, our lab has designed a double bed TiO2. Consequently, oxalic acid with the greatest preference of photocatalytic system [98], with photocatalytic hydrogen produc- surface absorption exhibited maximum hydrogen production. In tion occurring on one bed and the sacrificial agent re-generated on the case of binary mixture absorption systems, competitive another. This design resulted in the formation of a stable and continuous photocatalytic hydrogen production with significantly prolonged life.

250 CdS-S 40% CdS-C mCdS@ZTP= 0.2g 200 CdS-N 30% Evacuation m = 0.2g 25% CdS 150 roduction (ml) 2 2 20% VH 100 15%

50 10%

0 5% Quantum yields for H m = 0.01g 02468101214161820 CdS Reaction time (h) 0% CdS@ZTP Pure CdS Pure CdS Fig. 18. Hydrogen production for CdS prepared by two-step thermal sulfuration method (CdS-S), precipitation method (CdS-C) and atmospheric thermal treatment Fig. 19. Hydrogen production for various CdS photocatalysts with different loading (CdS-N), respectively. amounts. 1086 L.J. Guo et al. / Energy 34 (2009) 1073–1090

4.2.2. Development of direct solar photocatalytic hydrogen reach 50 m3 of Upward-flow Anaerobic Sludge Blanket (USAB) production system [102] and 1.48 m3 of Completely Stirred Tank Reactor (CSTR) [103]. As pointed out in the previous sections, solar hydrogen In order to suppress methanogenesis, it is necessary to pretreat production by photocatalytic splitting water is a very promising the mixed cultures which are generally used to decompose technology for the final realization of mass hydrogen production in industry and agriculture wastes. Fan et al. [104–107] used cow dung a completely renewable way. However, many obstacles have to be composts as microorganisms to produce hydrogen after it was overcome before this technology becomes economically viable. One heat-treated. Yu et al. compared three methods of sludge issue is the development of efficient visible-light-driven photo- pretreatment, heat-, acid- and alkaline-treatment [108].Itwas catalyst and significant progress has been achieved over the past found that highest H2 yield of 2.00 mol H2 per mol glucose could be decades. The other key issue is how to directly and efficiently utilize obtained with the heat-treated sludge, better than the other two the solar energy itself. Two major drawbacks of solar energy must methods for enriching H2-producing inoculums from mixed be considered: 1) the intermittent and variable manner in which it anaerobic cultures. The substrates containing cellulose which could arrives at the earth’s surface and 2) efficient collection of solar light not be used as substrate for bacteria directly [109] are pretreated on a useful scale. The first drawback can be resolved by converting before fermentation. Fan et al. reported that the HCl pretreated solar energy into storable hydrogen energy. For the second, the wheat straw wastes [104], beer lees wastes [105] and cornstalk solution would be the use of solar concentrator. Consequently, solar wastes [106] played a crucial role in the conversion of these reactors equipped with efficient solar light collectors are expected substrates into biohydrogen gas by cow dung composts generating to be one natural choice for direct photocatalytic solar hydrogen hydrogen. The maximum cumulative hydrogen yields were 1 1 1 production. 68.1 mLH2 g TVS, 68.6 mLH2 g TVS, 149.69 mLH2 g TVS, As for the direct utilization of solar light as the energy source for respectively, which were 136-fold, 10-fold, 46-fold as compared photocatalytic reactions, many types of solar reactors intended for with that of raw wheat straw wastes, raw beer lees, raw cornstalk solar detoxification purposes have been developed. It is surprising wastes. that no similar attempts have been made toward photocatalytic Quite a number of researches focused on evaluating the influ- hydrogen production. Addressing both the similarity and dissimi- ence of parameters and optimizing the reaction conditions. Fan larity for these two processes and by fully considering the special optimized the initial substrates and pH levels when utilizing cow requirements for photocatalytic hydrogen production, a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector has been designed for the first time in our lab [99,100]. As shown in Fig. 20, the reactor is composed of two parts, in-door and out-door. The in-door part is for control and gas collection and the out-door part is for light collection and photocatalytic reaction. Preliminary results showed that the optical properties of the designed CPC could satisfactorily meet the designing expectations. Hydrogen production tests under direct solar radiation demonstrated that scaling-up of the photocatalytic hydrogen production technique by coupling tubular reactors with CPC concentrators is technically feasible. The maximum hydrogen production rate amounted to 1.88 L/h, corresponding to a hydrogen production rate of 0.164 L/h per unit volume of reaction solution, is larger than that of a lab-scale result, 0.126 L/h. Various parameters have been investigated to optimize the operation of the designed solar reactors. These studies are expected to be useful for further improvement of the reactor in future. It is anticipated that this first demonstration of the pilot-scale solar photocatalytic hydrogen production system would shed light on this promising research direction and, in the long run, contributes to the final realization of low-cost mass production of solar hydrogen.

5. Biological hydrogen production research in China

In China, many groups carried out the researches on anaerobic fermentative hydrogen production while only a few groups focused on photosynthetic hydrogen production [92–121].

5.1. Anaerobic hydrogen production

The microorganisms for anaerobic fermentation are mostly mixed cultures since mixed anaerobic fermentation is cost-effective and convenient. Most frequently, the cultures utilized were sludge and cow dung composts. Pure culture was also studied for its high hydrogen yields. For example, Long et al. [109] of Xiamen University, isolated two strains, Enterobacter sp. and Clostridium sp., which produced hydrogen with the yields of 2.68 mol H2/mol sucrose and 2.71 mol H2/mol maltose, respectively. The fermenta- Fig. 20. CPC based solar reactors for photocatalytic hydrogen production, in-door part tion scale is beyond the laboratory scale, the volumes of the reactor (upper) and out-door part (lower). L.J. Guo et al. / Energy 34 (2009) 1073–1090 1087 dung composts as microorganisms, and the optimal values were pollution threat caused by resultant fatty acids in dark fermenta- 1 4.0 0.5 g sucrose L and 5.4 0.2, respectively. Guo et al. [110] tion. The total hydrogen yield was 6.63 mol H2/mol sucrose. No optimized the parameters of HRT, temperature, pH and C/N ratio of butyrate, acetate, propionate, or valerate was detected in the ﬁnal anaerobic fermentative hydrogen production by orthogonal array fermentation efﬂuent after photo-fermentation. Zhu [130] tests. The optimal pH, T, HRT and C/N ratio were 5.0, 33.5–36.5 C, enhanced hydrogen production by 160% by co-culture compared 8.34 h, 112/1.55, respectively. Yu [111] applied the response surface with that by the single culture of anaerobic bacterium. methodology (RSM) to determine the optimum physiological Our group also separated a stain of Rhodospirillaceae and conditions (pH, temperature and substrate concentration) combined the photosynthetic bacteria and anaerobic bacteria synchronously for the production of volatile fatty acids (VFAs) and together to treat the organic compounds. Since the present studies H2 from sucrose-rich wastewater by mixed anaerobic culture. demonstrated that hydrogen yields can be greatly enhanced by co- The mechanisms were also studied. Ren [112] proposed an culture of fermentative and photosynthetic bacteria, more in-depth ethanol-type fermentation which was proved to be more effective researches are needed in this promising direction. for hydrogen production than propionic acid-type fermentation and butyric-type fermentation. 6. Conclusions

5.2. Photobiological hydrogen production Hydrogen is recognized as one of the most promising energy carriers in the future. Many investigations on various hydrogen Microorganisms that produce hydrogen at the expense of solar production methods have been conducted over the past several energy can be classified as follows: algae, cyanobacteria and decades. Hydrogen production using water and biomass driven by photosynthetic bacteria. Algae, e.g., Platymonas subcordiformis solar energy is one of the most promising approaches for the [113–115], Chlamydomonas reinhardtii [116,117], Spirulina platensis utilization of renewable energy resource, which is significant for [101] and photosynthetic bacteria, e.g.: Rhodobacter sphaeroides energy scarcity and environment pollution. In the recent years, [118–120], Rhodobacter capsulatus [121], Rhodopseudomonas acid- Chinese government and many researchers paid great attention to ophila [122], Rhodopseudomonas capsulate [123], and Rhodop- the solar hydrogen production research. A series of significant seudomonas palustris [124], have been isolated and studied in results have been achieved since the project of Basic Research of China. Hydrogen production by cyanobacteria was however less Mass Hydrogen Production using Solar Energy started in the end of studied. 2003. Zhang’s group [113–115] demonstrated that a significant volume In the research field of biomass SCWG solar hydrogen produc- of H2 gas could be photobiologically produced by green algaes P. tion, several gasification systems have been developed. In these subcordiformis and C. reinhardtii when an uncoupler of photo- apparatus, a series of experimental investigations on model phosphorylation, carbonyl cyanide m-chlorophenylhydrazone compounds and real biomass gasification in SCW were conducted. (CCCP), was added after anaerobic dark incubation, whereas The gasification ratio reached 100% and maximum hydrogen a negligible volume of H2 gas was produced without CCCP. The role volumetric ratio reached 50% in the gas products. The rules for the of CCCP in enhancing photobiological H2 production has been gasification of model compounds and real biomass, catalysts, investigated. CCCP as an ADRY agent (agent accelerating the deac- reaction kinetic and thermodynamic analysis have also been tivation reactions of water splitting enzyme system Y) rapidly investigated. inhibited the photosystem II (PSII) activity of algal cells, resulting in In the research for photocatalytic water splitting solar hydrogen a markedly decline in the coupled oxygen evolution. The mito- production, a series of new photocatalysts have been developed chondrial oxidative respiration was only slightly inactivated by and significant progresses have been achieved. Among them the CCCP, which depleted O2 in the light. As a result, anaerobiosis CdS photocatalyst prepared by a novel two-step thermal sulfidation during the stage of photobiological H2 evolution was established, method was developed and synthesized with very high activity and preventing severe O2 inactivation of the reversible hydrogenase in stability for photocatalytic splitting water to produce hydrogen of green algaes. 24.1% quantum yield at 420 nm and 6.35% energy conversion effi- The group also cloned and analysed the structure of hydroge- ciency in the visible light region of l > 430 nm. Several photo- nase gene from C. reinhardtii SE [117]. The hydrogenase is composed catalytic reaction systems and new reactors have been developed. of 3584 bp encoding 505 amino acids in length with a predicted In the research field of biological solar hydrogen production, M.W. of 53.69 kDa. Ten exons (including 1518 bp) and nine introns significant progress has also been achieved in the research on (including 2066 bp) have been found in the hydrogenase, and there anaerobic hydrogen production and green algae hydrogen were two potential N-glycosylate sites, eight protein kinase C production. phosphorylation site, eight casein kinase H phosphorylation site For all solar hydrogen production processes there is a need for and one sulphorylation in the sequence. great improvement in conversion efficiencies, reduced capital costs, Yu optimized the compositions of VFA for hydrogen production and enhanced reliability and operating flexibility, especially: by R. capsulate [123]. The growing conditions for photosynthetic bacteria are optimized by Zhang [118–120,125] and Qu [124,126]. 1) For solar hydrogen production from the biomass SCWG, tech- nological improvement and reaction mechanism revealed 5.3. Two-step of dark-/photo-fermentation and co-culture should be made before reactors scaled-up. Future work includes: catalyst preparation, characterization and catalytic Since the VFA products of anaerobic fermentation can be used mechanism, high-efficient and low-cost solar concentration as substrates for photosynthetic bacteria, it is cost-effective, effi- theory, high-efficient solar energy storage and release, system cient and produces no wastes through co-culture of fermentative coupled matching principle and safe and stable operation and photosynthetic bacteria [127]. Fang [128] studied the hydrogen theory. In near future, the system of biomass SCWG driven by production by co-cultures of Clostridium butyricum and R. the solar energy will be constructed and a genuine renewable sphaeroides. energy conversion process could be achieved. Tao [129] used a two-step process of dark- and photo-fermen- 2) For the photocatalytic water splitting solar hydrogen produc- tation of sucrose to increase the hydrogen yield and to relieve the tion research, the key challenges to advance the technology 1088 L.J. Guo et al. / Energy 34 (2009) 1073–1090

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