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Using solar thermal power generation technology to produce ammonia with zero emission! American scientists are studying and designing new solar reactors

作者:山西常晟集团时间:2020-01-15

Cspplaza photothermal power generation network news: ammonia (NH3) is an important part of chemical fertilizer and the second largest industrial chemical in the world, with an annual output of about 180 million tons, of which nearly 90% is used to meet the global agricultural production demand.

At the same time, in view of its many advantages over green hydrogen, ammonia is increasingly regarded as a potential green energy, which can be used in some industries that are difficult to decarbonize, such as power production and shipping industry. However, Haber Bosch (H-B), the main traditional method of ammonia production, is incompatible with the theme of human response to climate change.

In the H-B process, hydrogen (H2) and nitrogen (N2) generate ammonia through exothermic catalytic reaction in a reactor that can withstand high temperature (350-500 ° C) and high pressure (150-300 bar). In the industrial H-B process, the basic raw material of this ammonia production process is obtained by consuming hydrocarbons: H2 is usually obtained from methane (CH4) through steam reforming, and N2 comes from the air after CH4 combustion to remove oxygen (O2).

In this process, hydrocarbon fuel is burned to produce the heat and mechanical energy required for the reaction process, but a large amount of carbon dioxide (CO2) will be emitted at the same time. According to statistics, every 3 tons of ammonia produced in the world will produce one ton of carbon dioxide.

Can we use renewable energy to produce ammonia in a greener way? Like solar energy?

In order to design such a new process, scientists from many countries have carried out a large number of sustainable energy research, which has also been funded by governments. However, so far, most researchers have studied how to convert the H-B process into green (no fossil fuel) or blue (fossil fuel, with carbon capture and storage functions), but these ideas have not changed the high-pressure operation conditions required for ammonia catalytic reaction.

However, a multi agency project funded by the U.S. Department of energy, composed of Sandia laboratory, Georgia Institute of technology and Arizona State University, puts forward an innovative idea that distinguishes most research, that is, using solar concentrating and heat collection technology as energy for ammonia production.

A new solar thermochemical ammonia production process

It is reported that the multi agency team led by Dr. Andrea Ambrosini of Sandia laboratory is currently studying this carbon neutral ammonia production method that does not involve H-B process at all. The team is evaluating the feasibility of a unique solar thermochemical ammonia production process, which does not emit carbon dioxide at all.

 

 

Dr. Alberto de la calle, an assistant research scientist from Arizona State University who participated in the writing of solar driven air nitrogen separation process based on two-step thermochemical cycle, introduced: "We put forward a sustainable ammonia production idea, which does not need fossil fuels, but uses concentrated solar radiation. This advanced solar thermochemical cycle technology under development can produce and store nitrogen from the air, and then produce ammonia through advanced two-stage process. More importantly, it can reduce the pressure required for ammonia synthesis."

The reaction process is mainly composed of the following four steps: the first step is to reduce the metal oxide; the second step is to separate nitrogen from the air and re oxidize the metal oxide; the third step is to synthesize ammonia through the reaction between hydrogen and metal nitrides; the fourth step is to re nitride the nitride lacking nitrogen with the generated nitrogen.

 

Figure: reaction process (the sequence of steps from top to bottom is 1,2,4,3)

According to the ideas put forward by the research team, the process can be divided into two stages, and each stage has two steps

In this stage, the research team will separate nitrogen from the air through a two-step thermochemical metal oxide cycle. The first step is the thermal reduction of metal oxide, releasing some oxygen from its structure. Since this reduction reaction is endothermic, photothermal power generation technology will be used to provide energy in the form of heat. In the second step, the reduced metal oxide is reoxidized in air to consume oxygen and produce a high-purity nitrogen flow. Once the reaction is complete, the metal oxide returns to step and goes back and forth.

In the second stage, they will produce ammonia through a two-step thermochemical metal nitride cycle. The step of the second cycle is ammonia synthesis reaction. In this step, the metal nitride is reduced by H2 (nitrogen is removed) to directly produce ammonia. In the second step, nitrogen deficient metal nitrides are re nitrided with stage purified nitrogen to regenerate nitrides. Once the reaction is completed, the regenerated nitride can be reduced again, so as to realize the cyclic reaction.

At present, research on nitrides that can work effectively in this novel cycle is in progress.

It can significantly reduce costs and avoid carbon dioxide emissions

Alberto de la calle said: "in the traditional H-B process, separating nitrogen from the air will produce a large amount of carbon dioxide. The traditional process is to remove oxygen from the air by burning CH4, and then produce more hydrogen by steam reforming, but it will also increase carbon dioxide emissions. Our idea is to use solar energy to reduce a metal oxide (thermal reduction), and then this oxide will consume oxygen in the air without producing any carbon dioxide. At the same time, this technology can also produce high-purity nitrogen without post-treatment (separation of carbon dioxide) like H-B process. "

Alberto de la calle further explained: "we can use concentrated heat energy to reduce metal oxides at 800 ℃, sweep the oxygen released by metal oxides out of the reactor with air, and separate nitrogen at 500 ℃."
Another advantage of this technology is that they can store reduced metal oxide particles (solids are easier to store than gas), so that nitrogen can be produced as needed. By storing reduced particles instead of gas, there is no need to use expensive pressurized storage and compression work to introduce gas into pressure vessels.

Similar to the polygonal heliostat field built by heliogen company in the figure below, the solar radiation energy of the scheme of producing zero emission ammonia by using photothermal power generation technology will also be provided by a solar heliostat field to focus thousands of highly concentrated solar beams on the receiver / reactor on the top of the tower. However, the solar thermochemical reactor for producing ammonia does not need to generate electricity, so it is not necessary The first step of the above idea can be carried out in the reactor.

The second step of the process is to produce ammonia through thermochemical cycle at a much lower working pressure than H-B process. H-B process requires a pressure of 150-300bar to drive the reaction. Alberto de la calle believes that the innovative ammonia production process using photothermal technology can work under a pressure of less than 30 bar.

The high pressure required by H-B process greatly increases the cost of almost all components such as reactor, heat exchanger, pipeline and compressor. In addition, the cost of energy required in the compression process is also very considerable (accounting for about 20% of the total energy consumption of H-B process). Therefore, the benefits of low-pressure working conditions are obvious. It can not only save costs, but also avoid carbon dioxide emissions.
However, the reaction of photothermal ammonia production needs a higher working temperature than that of H-B process (the working temperature of catalytic reaction of H-B process only needs 350 to 500 ° C). At present, the research team is still screening suitable materials for thermochemical cycle reaction.

Alberto de la calle pointed out: "our goal is that the working temperature of ammonia synthesis and denitrification reaction is close to 500 ℃, and the high pressure target is 30 bar. I believe that if we have a well-designed heat recovery system, we can fully recover the heat discharged from the nitrogen production process and meet all other heat needs."

It is understood that the relevant reactions proposed in this project (metal oxide reduction, nitrogen production, ammonia synthesis and re nitridation) are currently in the early stage of technical maturity. The Arizona State University team has now started system modeling and detailed thermodynamic and technical and economic analysis to find good operating conditions or system scale.

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