CN108823595B

CN108823595B - Method for electrolyzing lignin at high temperature in solar STEP process

Info

Publication number: CN108823595B
Application number: CN201810760316.9A
Authority: CN
Inventors: 闫超; 王嘉琪; 朱凌岳; 王宝辉; 江泓; 绳敏
Original assignee: Northeast Petroleum University
Current assignee: Northeast Petroleum University
Priority date: 2018-07-12
Filing date: 2018-07-12
Publication date: 2020-05-01
Anticipated expiration: 2038-07-12
Also published as: CN108823595A

Abstract

The invention relates to a solar STEP process high-temperature lignin electrolysis system, wherein the electrolyte of the system is molten NaOH-KOH, the temperature of an electrolytic cell is 280-340 ℃, constant-current electrolysis or constant-voltage electrolysis can be adopted, and when the constant-current electrolysis is adopted, the current density is controlled to be 25-200 mA/cm²To (c) to (d); when constant voltage electrolysis is adopted, the voltage is controlled between 1V; after electrolysis, three-state products of solid, liquid and gas are obtained, wherein the solid product is mainly biochar, the gaseous product is mainly hydrogen, methane and a small amount of hydrocarbon, and the liquid product is mainly micromolecule organic matter. At a temperature of 340 ℃ and a current density of 200mA/cm²Under the condition, the conversion rate of the lignin reaches 83.87 percent, a large amount of hydrogen and a small amount of methane are generated, the coking rate is low, and a new way is provided for the resource utilization of the lignin and the development of new energy.

Description

Method for electrolyzing lignin at high temperature in solar STEP process

Technical Field

The invention belongs to the field of new energy development and biomass resource utilization; in particular to a method for electrolyzing lignin at high temperature in the solar STEP process.

Background

The reduction of fossil energy and the increasing environmental pollution have forced people to seek clean and renewable alternative energy sources in recent years. Among them, as an important renewable resource, the huge application potential of biomass has been highlighted due to its huge yield. It is estimated that 1.3 x 10 can be produced in the united states for one year across the country⁹Ton dry biomass. The theoretical resource amount of straws reaches 45000 million tons of dry biomass in China every year. The biomass energy is planned as thirteen five, and in many developing countries including vast rural areas in China, the biomass is still directly combusted as fuel, which causes great pollution to the environment. If biomass could be effectively recycled in an economically viable manner, the global energy shortage problem would be effectively addressed.

The lignin is one of three components of biomass, is a natural polymer which is second to natural cellulose in the second most abundant resource on earth, and has great development potential. Because the stable three-dimensional network structure becomes the recycling difficulty. Less than 2 percent of industrial lignin with about 1.5-1.8 million tons is utilized every year around the world, and most of industrial lignin is burnt out or randomly discharged as cheap fuel, so that not only is the resource waste caused, but also serious environmental pollution is caused. Therefore, how to efficiently convert lignin into high value-added chemicals has received much attention. Unfortunately, the prior art has either been directed to research on cellulose-based biomass (which does not produce lignin chemicals) such as algae, straw, etc., or has consumed significant energy in the processing of lignin.

Solar energy has the advantages of abundant resources, wide coverage, convenient conversion and utilization, no pollution, renewability and the like, and is considered to be one of the best choices for replacing the traditional fossil energy in the 21 st century. The development of solar photochemistry, proposed as early as 1912 by the italian photochemist giacomocician, will lead mankind to the real green chemical industry. The primary problem with solar energy utilization is efficient conversion of solar energy. The STEP theory was first proposed by Licht et al, university of Washington, USA. The solar energy photo-thermal and photoelectric coupling utilization device realizes a high-efficiency chemical process and generates energy-containing molecules, thereby improving the solar energy utilization efficiency. In the following researches, including STEP iron making, STEP hydrogen production, STEP wastewater treatment, STEP ammonia synthesis, STEP organic synthesis of STEP nano-fiber, STEP carbon capture and the like, the process is more and more widely applied.

Therefore, it remains a great challenge to develop an environmentally friendly and economically viable solution for large-scale disposal of waste lignin.

Disclosure of Invention

The invention provides a lignin resource utilization method which is energy-saving, simple in system, low in cost and high in efficiency.

The purpose of the invention is realized by the following technical scheme: a method for electrolyzing lignin at high temperature in a solar STEP process utilizes the photo-thermal effect of solar energy to provide a high-temperature environment for a system, the photo-electrical effect provides electric energy, molten NaOH-KOH is used as electrolyte, and constant-current electrolysis or constant-voltage electrolysis is carried out on the lignin in an electrolytic cell with a cathode and an anode.

Further, a solar STEP process high-temperature lignin electrolysis system is adopted for electrolysis, the system comprises a solar photo-thermal conversion unit, a solar photo-electrical conversion unit and a STEP lignin conversion unit, the solar photo-thermal conversion unit heats the lignin conversion unit, the solar photo-electrical conversion unit supplies electric energy to the lignin conversion unit, the STEP lignin conversion unit comprises a cathode, an anode, an electrolytic cell and an electrolyte, the electrolyte is molten NaOH-KOH, the temperature of the electrolytic cell is 280-340 ℃, constant current electrolysis or constant voltage electrolysis can be adopted, and when constant current electrolysis is adopted, the current density is controlled to be 25-200 mA/cm²In the electrode area of 2cm²For example, the current is between 0.05 and 0.4A; when constant voltage electrolysis is adopted, the voltage is controlled between 1V; after electrolysis, three-state products of solid, liquid and gas are obtained, wherein the solid product is mainly biochar, the gaseous product is mainly hydrogen, methane and a small amount of hydrocarbon, and the liquid product is mainly micromolecule organic matter. The method for electrolyzing the lignin at high temperature based on the system for electrolyzing the lignin at high temperature comprises the following steps:

(1) constructing an electrolysis unit consisting of a cathode, an anode, an electrolysis cell and an electrolyte;

(2) parabolic solar concentrators are equipped with a two-dimensional tracking mirror control system to maintain the sun's focus. At the temperature tuned by the tracking control system, the reactor is positioned at the focusing point of the solar concentrator, and the solid electrolyte in the electrolytic cell is heated to form molten electrolyte;

(3) controlling the temperature of the lignin conversion unit to be constant at 280-340 ℃;

(4) use of silicon-based photovoltaic modules with lithium storage batteries for converting solar energyInto electrical energy. Controlling the current density to be constant at 25-200 mA/cm²Reacting for a certain time to generate solid, liquid and gas tri-state products.

The reaction mechanism of the solar STEP high-temperature electrolytic lignin is as follows:

thermochemical reaction of lignin:

Lignin→[Lignin]^*(Activated State)+C_xH_y+Thermo-products

the lignin electrochemical reaction is as follows:

at the anode:

[Lignin]^*+OH^--e^-→Oxidized products+C_xH_y+H₂O

at the cathode:

H₂O+e^-→H₂+OH^-

and (3) net reaction:

Lignin→Oxidized products+C_xH_y+H₂

further, when the electrolyte is in a solid state, the solar photo-thermal conversion unit provides heat energy required for the electrolyte to reach a completely molten state.

Furthermore, the solar photoelectric conversion unit utilizes a solar condenser to match the temperature of the thermal-electrochemical reactor, and the heating temperature is regulated and controlled by adjusting the angle of the condenser.

Further, the electrode material of the lignin conversion unit is a nickel electrode.

Further, the lignin conversion unit adopts a high-purity corundum crucible, high-purity nickel or other high-temperature corrosion-resistant reactors.

The invention has the following beneficial technical effects:

1. in the electrolytic reaction process, the electrolyte is heated by the solar photo-thermal conversion unit and is changed into a molten state to form an electrolytic environment; meanwhile, the solar photoelectric conversion unit is used for providing electric energy, required electrolysis voltage or current is regulated and controlled, and lignin is electrolyzed to obtain available fuel and organic matters with high added values, so that conversion and storage from solar energy to chemical energy are realized, and resource utilization of the lignin is realized.

2. The lignin dissolved in the molten alkali sodium hydroxide-potassium hydroxide electrolyte is converted into high value-added fuels such as solid biochar, liquid micromolecule organic matters, gaseous hydrogen and methane and the like through electrolysis, so that high value-added conversion and resource utilization of the lignin are realized.

3. The system treats the lignin by controlling the current density and the electrolysis temperature or the voltage and the electrolysis temperature, the electrolysis temperature is 280-340 ℃, the constant current electrolysis or the constant voltage electrolysis is adopted, and when the constant current electrolysis is adopted, the current density is controlled to be 25-200 mA/cm²Meanwhile, when constant voltage electrolysis is adopted, the voltage is controlled to be about 1V, and the total conversion rate of the lignin reaches 83.87% after three hours of electrolysis.

4. Compared with the prior art, the system has the following outstanding characteristics: firstly, the reaction process utilizes solar energy without utilizing other resources, and is clean and environment-friendly; secondly, the reaction time is greatly reduced, the temperature is 340 ℃, and the current density is 200mA/cm²The total lignin selectivity reaches 83.87% after three hours of reaction; thirdly, the reaction temperature is greatly reduced, and compared with thermal cracking, the system can effectively convert lignin in an environment of 340 ℃; fourthly, the reaction environment is relatively simple, the conditions are relatively mild, and molten alkali sodium hydroxide-potassium hydroxide is taken as electrolyte, so that the relatively mild environment can be achieved, and the electrolyte is also taken as electrolyte to support electrolysis; fifthly, the electrode material can be a cheap nickel sheet electrode, so that the production cost is effectively reduced; sixth, the system can greatly reduce the color of the aqueous lignin solution.

Drawings

FIG. 1 shows the absorption change rate of a lignin aqueous solution treated by a solar STEP process measured by an ultraviolet spectrophotometer, wherein the lignin content in the water sample is 1 g/l;

figure 2a is the solid product conversion after treatment of the solar STEP process at 0.4A, different temperatures, figure 2b is the solid product conversion after treatment of the solar STEP process at 340 ℃, different currents;

FIG. 3 is the hydrogen, methane production of a solar STEP process and a separate pyrolysis process;

figure 4A is the hydrogen, methane production after treatment by the solar STEP process at 340 c, different currents, figure 4 b-the hydrogen, methane production after treatment by the solar STEP process at 0.4A, different temperatures;

FIG. 5 is a graph of methane production and current change in a solar STEP process at constant voltage of 1V;

figure 6 is a gas chromatographic analysis of the liquid phase product of a solar STEP process and the liquid phase product of a pyrolysis process only;

FIG. 7 is the particle size distribution of lignin as such, the product after STEP process reaction and the product after pyrolysis process reaction only in 5g/L aqueous solution;

FIG. 8 is a photograph of aqueous solutions of lignin as received (right), after the STEP process (middle), and after the pyrolysis process only (left), with a lignin sample concentration of 1g/L in the water.

Figure 9 is a schematic view of the system of the present invention,

in fig. 9: 1, a solar photo-thermal conversion unit; 2 solar photoelectric conversion unit; 3a lignin conversion unit;

FIG. 10 is a schematic view of a lignin conversion unit of the present invention,

in fig. 10: 1 an anode; 2a cathode; 3 solar photoelectric conversion unit; 4, an air guide outlet pipe; 5a carrier gas inlet pipe; 6, an electrolytic cell; 7 an electrolyte; 8 metal device.

Detailed Description

Example 1: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; keeping the temperature constant at 260 deg.C, 280 deg.C, 300 deg.C, 320 deg.C and 340 deg.C, keeping the current constant at 0.05A, 0.1A, 0.2A, 0.3A and 0.4A, reacting for three hours, and keeping the reaction solutionThe residue was dissolved in water, and the lignin content was measured by an ultraviolet spectrophotometer, and the lignin conversion was shown in FIG. 1.

The parabolic solar concentrator in this embodiment is equipped with a two-dimensional tracking mirror control system to maintain the solar focus. At the temperature tuned by the tracking control system, the reactor is positioned at the focusing point of the solar concentrator, and the solid electrolyte in the corundum crucible is heated to form molten electrolyte; silicon-based photovoltaic modules equipped with lithium storage batteries are used to convert solar energy into electrical energy.

As can be seen from fig. 1, the characteristic peak of lignin after being treated by the solar STEP process gradually decreases with the increase of current and temperature, as measured by the uv spectrophotometer. Indicating that the STEP process is very effective for lignin degradation. Under the conditions of 0.4A and 340 ℃, the conversion rate of the lignin is as high as 83.87 percent.

Example 2: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; keeping the temperature constant at 260 ℃, 280 ℃, 300 ℃, 320 ℃ and 340 ℃ respectively, keeping the current constant at 0.05A, 0.1A, 0.2A, 0.3A and 0.4A respectively, after three hours of reaction, dissolving the reaction residue in water, carrying out solid-liquid separation, washing and drying the filter residue, and obtaining the biochar, wherein the yield is shown in figure 2.

As can be seen from FIG. 2, the biochar yield increased significantly with increasing temperature, from 3.94% to 7.72% (260 ℃, 0.4A) (340 ℃, 0.4A). Far lower than the coking rate obtained by common pyrolysis. The explanation shows that in the solar STEP process, molten NaOH-KOH not only serves as electrolyte, but also can ensure that lignin is heated more uniformly and dispersed more thoroughly, and conversion reaction is facilitated. In different current reactions, the coking rate does not change obviously with the increase of the current, but the fluctuation is larger.

Example 3: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; the temperature was kept constant at 340 c and the current was kept constant at 0A and 0.4A, respectively, for three hours, and gaseous products were collected during the reaction and analyzed for hydrogen and methane content by gas chromatography, as shown in fig. 3.

As can be seen from fig. 3, the gas yield of the STEP lignin conversion process is much greater than the gas yield of the pyrolysis reaction alone. The volume of methane during the STEP lignin reaction decreased over time, but the hydrogen increased. While the only major gaseous product of the pyrolysis reaction is methane, the amount of methane increases with the reaction time. It is presumed that methane is probably generated by thermal desorption of methoxy groups in lignin molecules, and in the STEP reaction, lignin is affected by the coupling action of a thermal electric field, and the oxidative desorption of methoxy groups is more obvious in the early stage of the reaction. During the subsequent reaction, the STEP lignin reacts at the anode, most likely with lignin molecules directly broken in the middle, to be decomposed into organic carbon oxides. With further oxidation, these organics would be oxidized to much smaller molecules, rather than just pyrolytically, breaking the chains from the edges of lignin molecules, forming organics such as methane. It appeared that the amount of methane produced by the pyrolysis reaction alone (19.814mL) was higher than the amount of STEP lignin conversion reaction (7.53 mL).

Example 4: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

17g KOH, 13g NaOH and 0.5g wood were added separatelyUniformly mixing the substances, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; the temperature was made constant at 260 deg.C, 280 deg.C, 300 deg.C, 320 deg.C and 340 deg.C, the current was made constant at 0.05A, 0.1A, 0.2A, 0.3A and 0.4A, respectively, the reaction was carried out for three hours, gaseous products were collected during the reaction, and the contents of hydrogen and methane were analyzed by gas chromatography, as shown in FIG. 4.

As can be seen from fig. 4, the generation of hydrogen is promoted by the increase in temperature and current. Methane production increases with increasing temperature, which is in contrast to the effect of current, which is higher, the lower the methane production. The results show that gas conversion is selective, varying with temperature and current, i.e. high selectivity of the gaseous products can be achieved by adjusting the temperature (solar thermochemical process) and the electrolytic current (solar electrochemical process).

Example 5: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; keeping the temperature constant at 340 deg.C and voltage constant at 1V, reacting for three hours, collecting gaseous product during reaction, analyzing methane content by gas chromatography, monitoring real-time current with multimeter, and obtaining the result shown in FIG. 5

As can be seen from fig. 5, initially a larger current density is present due to the fresh electrode surface, which then drops off rapidly, indicating that oxidation reactions do occur in the lignin at high temperatures. Subsequently, it is presumed from the amount of methane produced during the electrolysis that the average oxidation state of lignin in the experiment may change with time, indicating that the conversion mechanism of lignin changes with time.

Example 6: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; allowing reaction at 340 deg.C and current of 0A and 0.4A respectively for three hours, dissolving the reaction residue in water, adjusting pH to about 3 with hydrochloric acid, filtering, extracting the filtrate with diethyl ether, and analyzing the liquid product by gas chromatography, the result is shown in FIG. 6

As can be seen from FIG. 6, the organic substances extracted after the reaction are complicated and various. The pyrolysis-only reaction products produce more higher boiling point organics than the STEP lignin conversion. Whereas STEP lignin conversion produces more low boiling point organics (c, d). This directly confirms the previous inference of liquid products. It must be mentioned that this is also only the organic matter that was driven out in the first 19 minutes. The STEP lignin conversion plays a role in promoting the conversion of lignin into small molecular water-soluble organic matters.

Example 7: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; the reaction was carried out for three hours at a constant temperature of 340 ℃ and a constant current of 0A and 0.4A, respectively, and the reaction residue was dissolved in water to prepare 100mL of a solution, and the particle size was measured by a laser particle sizer, respectively, as shown in FIG. 7.

As can be seen from fig. 7, there are three aggregation peaks in the pyrolysis product, one of which is larger in size than the original sample, which means that the particle size distribution of the product during pyrolysis is broad, and lignin is not only irregularly depolymerized, but also irregularly depolymerized. However, the polymer size is significantly reduced compared to pyrolysis alone, but there is still one aggregation peak. The results show that stepwise lignin conversion can effectively reduce the particle size of lignin aggregates in aqueous solutions.

Example 8: in the embodiment, a high-temperature environment is provided for a system by utilizing the photo-thermal effect of solar energy, electric energy is provided by the photo-electrical effect, molten NaOH-KOH is used as an electrolyte, and lignin is electrolyzed by taking a nickel sheet electrode as a cathode and an anode. The method for electrolyzing the lignin at high temperature in the solar STEP process is carried out according to the following STEPs:

respectively and uniformly mixing 17g of KOH, 13g of NaOH and 0.5g of lignin, and putting the mixture into a corundum crucible; respectively dividing the effective area to 2cm²The nickel sheet of (2) is used as an electrode; the temperature was kept constant at 340 ℃ and the current was kept constant at 0A and 0.4A, respectively, for three hours, and the reaction residue was dissolved in water to prepare 500mL of a solution, the color of which is shown in FIG. 8.

As can be seen from fig. 8, the lignin solution after pyrolysis only became significantly deeper, whereas the color of the lignin solution was significantly reduced after the STEP lignin conversion process. The reason may be that chromophoric groups such as-O, C-C and the like are increased in lignin after high-temperature heat treatment, -OH, -CH, -OCH, and the like are also changed, and thus induced discoloration is caused. However, after solar electricity is coupled, most of chromophoric groups and auxochrome groups are oxidized, aromatic rings are destroyed, and quinone structures are removed, so that the solution chromaticity is remarkably reduced.

Claims

1. A method for electrolyzing lignin at high temperature in a solar STEP process is characterized in that the electrolysis method provides a high-temperature environment for a system by utilizing the photo-thermal effect of solar energy, provides electric energy by utilizing the photo-electrical effect, takes molten NaOH-KOH as electrolyte, and carries out constant current electrolysis on the lignin in an electrolytic cell with a cathode and an anode; wherein the temperature of the electrolytic cell is 280-340 ℃; the current density during constant current electrolysis is controlled to be 25-200 mA/cm²In the meantime.

2. The method of claim 1, wherein the ratio of KOH to NaOH by mass is 17: 13.

3. the method of claim 1, wherein the electrolytic cell is a high temperature corrosion resistant reactor.

4. The method of claim 3, wherein the solar STEP process high temperature electrolysis of lignin is performed using a high purity corundum crucible and high purity nickel as an electrolytic cell.

5. The method of claim 1, wherein the cathode and the anode are nickel electrodes.

6. The method of claim 1, wherein a silicon-based photovoltaic module with lithium storage batteries is used to convert solar energy into electrical energy.

7. The method of claim 1, wherein the STEP process is performed by using a solar concentrator to convert solar energy into heat energy.