Hydrogen Production Technology
The source of hydrogen is now dominated by fossil fuels, and the transition to green hydrogen will be in the future
According to the source of production, hydrogen energy can be divided into three categories: “gray hydrogen”, “blue hydrogen” and “green hydrogen”. “Grey hydrogen” refers to the use of fossil fuel oil, natural gas and coal to produce hydrogen. The cost of hydrogen production is low but the carbon emissions are large; “Blue hydrogen” refers to the use of fossil fuels to produce hydrogen, combined with carbon capture and carbon sequestration technologies. , the carbon emission intensity is relatively low but the capture cost is high; “green hydrogen” is the use of wind power, hydropower, solar power, nuclear power and other renewable energy sources to electrolyze hydrogen production. The hydrogen production process has no carbon emissions at all, but the cost is high.
Based on the development of demand-side industries and the improvement of the industrial chain, it is a better way to gradually transition from gray hydrogen to green hydrogen, and give priority to the use of by-product hydrogen to achieve comprehensive utilization of resources.
The following are the 6 kinds of hydrogen production technologies:
(1) Hydrogen production from coal
The essence of coal-to-hydrogen is to replace hydrogen in water with carbon in coal, and finally generate hydrogen and carbon dioxide. Here, carbon acts as a reducing agent and provides heat for the displacement reaction.
There are two main methods for producing hydrogen-containing gas from coal:
One is the coking of coal (or high temperature dry distillation), in which coal is isolated from air at 900-1000 °C to produce coke, and the by-product is coke oven gas. The components of coke oven gas contain 55%-60% (volume) of hydrogen, 23%-27% of methane, and 5%-8% of carbon monoxide. Each ton of coal can get 300-350m3 of gas, which is used as city gas and the raw material for hydrogen production.
The second is the gasification of coal, which converts coal into gaseous products by reacting with water vapor or oxygen (air) under high temperature, atmospheric pressure or pressure. The content of hydrogen in the gaseous product varies with different gasification methods.
(2) Hydrogen production from natural gas
The main component of natural gas is methane (CH4), which itself contains hydrogen. Compared with coal-based hydrogen production, the production of hydrogen from natural gas has higher yield, lower processing costs, and less greenhouse gas emissions. Therefore, natural gas has become the main raw material for hydrogen production. Among them, steam reforming of natural gas is the most common method for producing hydrogen.
(3) Partial oxidation of heavy oil to produce hydrogen
Heavy oil is the residue from the refining process that is used to make hydrogen. During the partial oxidation of heavy oil, hydrocarbons react with oxygen and water vapor to form hydrogen and carbon dioxide. The process is carried out under pressure and catalysts can be used, depending on the choice of feedstock and process.
(4) Hydrogen production by water electrolysis
Hydrogen produced by water electrolysis has high purity and is easy to operate, but requires electricity. The efficiency of water electrolysis for hydrogen production is generally 75%-85%, and the power consumption for producing 1m3 of hydrogen and 0.5m3 of oxygen is generally 4-5kWh. According to the principle of thermodynamics, the minimum electricity consumption of 1m3 hydrogen and 0.5m3 oxygen by electrolysis of water is 2.95 kWh.
(5) Biomass to produce hydrogen
Biomass from households, agriculture, forestry, etc. can be used to produce hydrogen. Feedstocks include poplar, willow and switch branches, as well as biogas from anaerobic digestion or landfill. Biomass can be gasified using proven technologies, or even co-reacted with coal or waste plastics during gasification, potentially producing negative hydrocarbons when combined with carbon capture technology. Biogas has additional purification requirements and can be retrofitted to produce hydrogen through a process similar to steam methane reforming (SMR).
(6) Purification of industrial by-product hydrogen
Crude hydrogen from coke oven gas, chlor-alkali, propane dehydrogenation to propylene and ethane cracking to olefins can be used as a hydrogen source for fuel cell vehicles after refining processes such as desulfurization, pressure swing adsorption and cryogenic separation, and the cost is much lower than that of chemical industry. Routes such as fuel hydrogen production, methanol reforming hydrogen production and water electrolysis hydrogen production.
There are many hydrogen production methods, such as chlor-alkali by-product gas, dry gas, coke oven gas, ethane cracking by-product gas, methane, coal, natural gas, electrolyzed water and other hydrogen production methods. Among them, hydrogen production from by-product gas such as chlor-alkali by-product gas, dry gas, coke oven gas, and ethane cracking by-product gas has advantages in terms of energy efficiency, pollution emission, carbon emission and cost. When developing the hydrogen energy industry chain in various regions, they should fully combine the regional energy structure, and give priority to the utilization of by-product hydrogen and surplus energy.
From the perspective of energy efficiency, the energy efficiency of chlor-alkali by-product gas for hydrogen production, dry gas for hydrogen production, and coke oven gas extraction for hydrogen production are all above 80%. The energy efficiency of hydrogen production from hydrogen, methanol, and coke oven gas conversion is 60%-80%, the energy efficiency of coal-to-hydrogen production is 50%-60%, and the energy efficiency of hydrogen production from water electrolysis is below 50%.
From the perspective of pollutant emission, the emission intensity from small to large is: electrolysis of water to hydrogen < natural gas to hydrogen ~ methanol to hydrogen ~ by-product gas to hydrogen < coal to hydrogen.
From the perspective of carbon emissions, hydrogen production from by-product gas < hydrogen production from natural gas < hydrogen production from dry gas < hydrogen production from methanol < hydrogen production from coal electrolysis < hydrogen production from water electrolysis (based on the existing power grid power structure), if clean energy (photovoltaic, Wind power, hydropower, etc.), clean energy electrolysis water carbon emissions are close to zero.
From the cost point of view, the cost of hydrogen production has the greatest relationship with the price of raw materials. To control the price of hydrogen energy, it is necessary to control the price of raw materials; according to the set price range, from the perspective of average cost, the production of hydrogen from coke oven gas < production of hydrogen from coal < production of other by-product gas Hydrogen < methanol to hydrogen < natural gas to hydrogen < water electrolysis to hydrogen. According to local conditions, select suitable raw materials for hydrogen production, and the ex-factory price of hydrogen can be comparable to the cost of coal-to-hydrogen production.
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