بررسی کارایی اقتصادی تولید هیدروژن از تراکم زیست باقی مانده در چین

As part of Pilot Project of KIP of CAS, a feasibility study of hydrogen production system using biomass residues is conducted. This study is based on a process of oxygen-rich air gasification of biomass in a downdraft gasifier plus CO-shift. The capacity of this system is 6.4 t biomass/d. Applying this system, it is expected that an annual production of 480 billion N m3 H2 will be generated for domestic supply in China. The capital cost of the plant used in this study is 1328$/(N m3/h) H2 out, and product supply cost is 0.15$/N m3 H2. The cost sensitivity analysis on this system tells that electricity and catalyst cost are the two most important factors to influence hydrogen production cost.

1.1. Background Concerns about the depletion of fossil fuel reserves and the pollution caused by continuously increasing energy demands make hydrogen an attractive alternative energy source. Biomass is a CO2 neutral resource, which makes hydrogen from biomass a prospective technology. Till now, a lot of work has been done on hydrogen production from biomass and different researchers apply different routes. Rapagnà et al. [1] and [2] explored hydrogen production from biomass catalytic gasification, while others [3], [4] and [5] investigated hydrogen production from steam gasification of biomass-derived oil. Iwasaki [6] investigated the economic efficiency of hydrogen production from biomass pyrolysis and further reforming. The main assumption he made is that the system can supply its own power and heat by utilizing waste heat from the system itself. Lau et al. [7] have a thorough analysis on the feasibility of biomass-derived hydrogen. China is an agricultural country and has huge quantity of biomass residues. Meanwhile, China is a densely populated country, 70% of the population living in rural areas and depending largely on biomass energy. For a long history, China has been using biomass as energy, especially in its rural areas. In most areas, biomass is utilized by a way of direct burning in a traditional stove and the heat efficiency only reaches 10%. In recent years, Chinese government has been attaching high importance on advanced biomass conversion technologies and has initiated a series of National Programs for Key Science and Technology projects. Hydrogen production from biomass residues also gets much financial support from Chinese government. The study reported here is one of the working contents of Pilot Project “Biomass liquefaction and hydrogen production” of Knowledge Innovation Program (KIP) of Chinese Academy of Sciences (CAS). As a Pilot Project of KIP of CAS, the objective of this study is to present a feasibility study on economic efficiency of hydrogen production from biomass residues in China. Thus, based on a process of biomass oxygen-rich air gasification in a downdraft gasifier, plus CO-shift with commercial catalysts, the economic efficiency of this system is investigated. Meanwhile, the potential of hydrogen production from biomass residues in China is also evaluated. 1.2. Chemical processes in a downdraft gasifier As shown in Fig. 1, because gases and solids move forward with the same direction in the downdraft gasifier, different reactions occur in the different zones. In the drying zone, the temperature is about 150–300 °C; therefore, water vaporization mainly occurs in this zone. In the pyrolysis zone, temperature being about 600 °C, the pyrolysis of biomass starts and produces char, tar and gas as reaction (1) shows. In the combustion zone, because of the presence of oxygen, oxidization reactions of biomass pyrolysis products proceed here to provide required heat for the whole gasification as reactions (2) and (3) present. equation(1) biomass→char+tar+gases (CO2, CO, H2O, H2, CH4, CnHm) equation(2) C+O2=CO2+408 kJ/mol equation(3) 2C+O2=2CO+246 kJ/mol In the reduction and catalyst zones, secondary reactions of biomass pyrolysis and oxidization products proceed, such as cracking, reforming and tar decomposition as reactions (4), (5), (6), (7), (8), (9), (10) and (11) list. These reactions contribute to more H2 production. equation(4) tar→gases (CO2, CO, H2O, H2, CH4, CnHm) equation(5) CH4+H2O=CO+3H2 equation(6) CO+H2O=CO2+H2+41 kJ/mol equation(7) CH4+H2O=CO+3H2−206 kJ/mol equation(8) CH4+2H2O=CO2+4H2−165 kJ/mol equation(9) C+H2O=CO+H2−131 kJ/mol equation(10) C+H2=CH4−75 kJ/mol equation(11) C+CO2=2CO−172 kJ/mol Based on above analysis, it is concluded that a downdraft gasifier is advantageous in achieving a higher H2 content compared to other gasifiers. Therefore, this system chooses a downdraft gasifier as the reactor for biomass gasification.

A feasibility analysis on a hydrogen production system using biomass residues in China is implemented in this study. The system mainly consists of biomass gasification in a downdraft gasification and a downstream CO-shift reaction in a fixed bed. The capacity of this system is 266.7 kg biomass/h (6.4 t/d) and gas flow reaches 427 N m3/h. The energy conversion efficiency of this system is as high as 51.5% (calculated by H2 yield). Applying this system, it is expected that an annual production of 480 billion N m3 H2 will be generated for domestic supply in China. The capital cost of the plant used in this study is 1328$/N m3/h H2 out, and product supply cost is 0.15$/N m3 H2. Electricity and catalyst cost are the two most important factors to influence hydrogen production cost. When the electricity price exceeds 0.23$/kWh, this system loses economic feasibility for the current technology level. Catalyst lifetime plays a rather essential role on the total cost accounting. The sensitivity analysis results prove that its lifetime should be longer than about 60 h for good economic feasibility.