Research On Hydrogen Production From Biomass Via A Combined Cycle Of Hydrothermal Carbonization-electrolysis Process

Human's survival and development depend on energy,while the traditional fossil energy is not sustainable.It is extremely urgent to seek new alternative energy sources.Biomass energy becomes one of the research hot spots for its readily available.Hydrogen energy with its environmental benefits and high energy density has become a key target for energy development route in many countries.However,the low energy density of biomass,and the high cost of the hydrogen production prevent the practical application of these two technologies.This paper proposed and verified a new approach to hydrogen production from biomass.It combines two processes,which are the hydrothermal carbonization of biomass and hydrogen production by electrolysis.They are integrated through the valence change between Fe³⁺and Fe²⁺.Firstly,it is the hydrothermal carbonization of biomass.Unlike the traditional way,Fe³⁺,an oxidant,is introduced into the reaction solution to accelerate the oxidation of biomass.The reaction temperature is reduced from the conventional 200°C to about 140°C by the effect of Fe³⁺.The quality and energy density of biomass are increased a lot due to the increase of carbon content.And most of the Fe³⁺is reduced to Fe²⁺through this process.Secondly,it is the electrolytic hydrogen production process.The positive electrolyte which contains a lot of Fe²⁺is exactly the solution of biomass after the first process.The negative electrolyte could be sulfuric acid solution or salt solution.By this way,the conventional process of electrolysis of water converts into electrolysis of Fe²⁺and H⁺,which could reduce the voltage of electrolysis from about 2 V to about 1 V,greatly reducing the power consumption of the electrolytic process.After electrolysis,the Fe²⁺is oxidized into Fe³⁺,returning into the state of very beginning of hydrothermal reaction.Thus,the Fe³⁺produced during the electrolysis goes back to the hydrothermal reaction again,by which the cycles are established.Both high quality biomass and hydrogen with low energy consumption can be obtained by this innovative idea.In order to investigate the optimal reaction conditions and obtain hydrogen in the most efficient way,several experiments about key factors affecting the hydrogen production rate are proceeded,such as hydrothermal temperature,residence time and the concentration of biomass,etc.,taking pine powder as biomass sample.The hydrogen production and its efficiency increase with the increase of hydrothermal reaction temperature,while the growth trend gradually slows down.Considering the input/output factors,120°C is the most reasonable temperature.The hydrogen production and its efficiency grow linearly with the extension of hydrothermal reaction time.Since the whole process is recycle,it takes two hours for each thermal reaction The electrolysis current becomes larger firstly and then gets smaller with the increase of biomass concentration,and the optimal biomass concentration is 20g/L.the electrolysis current density of 110 mA·cm^-2 is obtained at 1V cell voltage for pine powder under such condition.Based on the calculation of capacity,1kg dried biomass can produce 545L hydrogen.The possible reaction mechanism was preliminarily speculated based on the characterization of samples before and after the hydrothermal reaction.A view by a scanning electron microscope?SEM?reveals that the sample is decomposed obviously at the surface protrusion so that a high density of spherical structure is appeared.The results of elemental analysis indicate that the carbon content of biomass increases after hydrothermal reaction.the carbon oxygen absorbing peak disappears according to FTIR.These results show that Fe³⁺accelerate oxidation of the functional groups in biomass,such as hydroxyl and ether groups,and itself is reduced to Fe²⁺for electrolysis.At the same time,the side chain structure of the biomass is broken and the degree of unsaturation increases.The reaction takes place preferentially on the surface of the biomass and its internal pores.After the surface structure of biomass is destroyed and its internal cells are exposed to the solution,the reduction reaction occurs further;therefore,the reaction is a progressive process from the surface to the inside.The hydrogen production from biomass via a combined cycle of hydrothermal carbonization and electrolysis process not only improves the quality of biomass but also reduces the cost of hydrogen production by introducing Fe³⁺/Fe²⁺redox couple.It has broad application prospects.