Design of Ecological CO 2 Enrichment System for Greenhouse Production using TBAB + CO 2 Semi-Clathrate Hydrate

This paper proposes an innovative CO2 enrichment system for crop production under a controlled greenhouse environment by means of tetra-n-butylammonium bromide (TBAB) + CO2 semi-clathrate hydrate (SC). In this system, CO2 is captured directly from exhaust gas from a combustion heater at night, which can be used for stimulating photosynthesis of crops in greenhouses during daytime. Although the gas capacity of TBAB + CO2 SC is less than that of CO2 gas hydrate, it is shown that TBAB + CO2 SC can store CO2 for CO2 enrichment in crop production even under moderate pressure conditions (<1.0 MPa) at 283 K.


Introduction
Sustainability of agriculture is a challenging subject because of future rapid global population increase [1].Controlled environmental horticulture in a greenhouse is one of the most modern and sophisticated forms of horticulture systems for obtaining a high yield beyond the cultivation season in the field, by maintaining an optimum temperature in every stage of the crop [2].However, temperature in the greenhouse without a heating system can fall below the optimum range for crops, especially during winter or nighttime, so an appropriate heating system is required to maintain the optimum temperature by means such as fossil-fuel combustion heating systems or heat pumps [3,4].Thus, greenhouse horticulture leads to a rise in energy demand in the agricultural industry, which faces many difficulties, such as increased fuel cost [5,6] and the emission of greenhouse gases [7].Therefore, the development of innovative technologies for production under a controlled greenhouse environment to establish a cost-effective, energy-effective, and environmentally-friendly greenhouse horticultural production system is of critical importance [8].
In greenhouse horticulture, carbon dioxide (CO 2 ) enrichment is effective to promote photosynthesis and thereby increase crop yield and grower income [9,10].A variety of CO 2 enrichment systems have been studied using CO 2 gas from a storage tank or exhaust gas of combustion heaters [11,12].A biomass-based boiler system has also been developed recently, for which energetic efficiency is improved by reusing biomass from agricultural/industrial waste [13].However, because of greenhouse air ventilation to prevent its overheating, CO 2 is released from the greenhouse.CO 2 concentration in the greenhouse decreases and cancels out the CO 2 enrichment.Therefore, a cooling system to limit ventilation time [14,15] or a combustion-type CO 2 enrichment system connected to heat pump has been studied [16].Here again, the economic benefit depends on the balance between increase in crop value and the cost of introducing the CO 2 enrichment system.For the above reasons, an innovative such system coupled with a greenhouse air-conditioning system based on a high-efficiency and low-cost cooling technique is desirable.
Owing to high CO 2 gas capacity of CO 2 gas hydrates, an application of CO 2 gas hydrates to agriculture is proposed; i.e., usage of CO 2 hydrate to increase CO 2 concentration in culturing algae [17].We also evaluated CO 2 gas hydrates as cold energy and the source of CO 2 for the greenhouse [18].It is shown that CO 2 gas supply capacity of the CO 2 gas hydrate is sufficient to stably maintain high concentrations of CO 2 for the stimulation of photosynthesis of tomato crops in greenhouses.CO 2 gas hydrate can be used as a source of stored cold thermal energy to reduce the amount of electricity consumed, which contributes to partial removal of heat from greenhouses in combination with other innovative methods such as root zone cooling [19,20] or nocturnal cooling in summer.The drawback to this approach is that CO 2 gas hydrate is formed under high-pressure and low-temperature conditions (e.g., >1.2 MPa at 273 K).It would be feasible to apply CO 2 gas hydrate as unconventional media for CO 2 transportation from industrial to agricultural areas [21], but it would not be feasible to form CO 2 gas hydrate at sites of greenhouse horticulture.
In this paper, we propose a novel concept of a CO 2 enrichment system for greenhouse production using semi-clathrate hydrate (hereafter SC).SC is formed under milder conditions (lower pressure and higher temperature) than CO 2 gas hydrate, and greater economic benefit can be expected.In this system, at night, tetra-n-butylammonium bromide (TBAB) + CO 2 SC is formed using CO 2 gas supplied from the exhaust gas of a combustion (or biomass-based) heater.During daytime, CO 2 gas and cold thermal energy from the dissociating SC is supplied in the greenhouse to stimulate crop photosynthesis.

Theoretical Background
TBAB is nontoxic and widely used as an ionic guest substance.Melting points of TBAB SC of ~285 K under atmospheric pressure are higher than those of gas hydrates and ice.Because TBAB SC forms a slurry that can be transported through a pipeline, SCs have been expected to be a new phase-change material [22,23].In addition, SCs have been anticipated as a medium for gas storage and separation [24,25].

CO 2 Gas Storage Using SC
Different SC structures form depending on the concentration of aqueous TBAB solution.The crystal structure of TBAB SC with hydration number 38 (TBAB•38H 2 O) was revealed [26] according to X-ray crystal structure analysis.The crystal structure of TBAB SC in Figure 1 shows that tetrakaidecahedral cages (5 12 6 2 ) and pentakaidecahedral (5 12 6 3 ) cages are occupied by tetra-n-butylammonium (TBA) cations, and anions (bromide; Br) form cage structures bonding to water molecules.There are also empty pentagonal dodecahedral (5 12 ) cages in the structure of TBAB SC.TBAB•36H 2 O hydrate is isostructural to the orthorhombic hydrate of TBAB•38H 2 O, but tetragonal hydrates of TBAB•32H 2 O and TBAB•26H 2 O have not been refined yet [27,28].Crystal structures of gas hydrates are comparable with that of SC.Three main structural families of gas hydrate are known, and they form depending mainly on the size of the guest molecules.These three are cubic structure I (sI)  Pure CO2 gas forms sI hydrate, in which CO2 is encaged in both 5 12 and 5 12 6 2 cages.The CO2 hydrate contains about 150 volumes of CO2 gas per volume of hydrate crystal at standard temperature and pressure (150 V/V(STP)) [33].Other structural gas hydrates of sII and sH also include CO2 in the presence of other guest molecules, such as tetrahydrofuran (THF) [34] or 3,3-dimethyl-2butanone (pinacolone) [35].Likewise, it is known that small molecules are selectively incorporated in TBAB SC, but large molecules such as ethane and propane are not [36].It is shown that small molecules such as methane (CH4), nitrogen (N2), and CO2 are encaged into 5 12 cages during the formation of TBAB SC by 13 C NMR [37], and their cage occupancies were determined by X-ray structure analysis [38][39][40].

Gas Separation Using SC
CO2 gas separation from combustion flue gas is feasible through the crystallization of SC and gas hydrates.When hydrate crystals form from a mixture of CO2 and other gases such as nitrogen (N2) or hydrogen (H2), the crystalline phase of SC or gas hydrate is enriched with CO2, while the concentration of the other gas is increased in the gas phase.Pure CO 2 gas forms sI hydrate, in which CO 2 is encaged in both 5 12 and 5 12 6 2 cages.The CO 2 hydrate contains about 150 volumes of CO 2 gas per volume of hydrate crystal at standard temperature and pressure (150 V/V(STP)) [33].Other structural gas hydrates of sII and sH also include CO 2 in the presence of other guest molecules, such as tetrahydrofuran (THF) [34] or 3,3-dimethyl-2-butanone (pinacolone) [35].Likewise, it is known that small molecules are selectively incorporated in TBAB SC, but large molecules such as ethane and propane are not [36].It is shown that small molecules such as methane (CH 4 ), nitrogen (N 2 ), and CO 2 are encaged into 5 12 cages during the formation of TBAB SC by 13 C NMR [37], and their cage occupancies were determined by X-ray structure analysis [38][39][40].

Gas Separation Using SC
CO 2 gas separation from combustion flue gas is feasible through the crystallization of SC and gas hydrates.When hydrate crystals form from a mixture of CO 2 and other gases such as nitrogen (N 2 ) or hydrogen (H 2 ), the crystalline phase of SC or gas hydrate is enriched with CO 2 , while the concentration of the other gas is increased in the gas phase.
Generally, there are two types of hydrate formation promoters: chemical additives for gas hydrate that have no effect on structures of the water cages (e.g., tetrahydrofuran, cyclopentane, and anionic/non-ionic surfactants), and SC types such as TBA salts that form SC [24,25,41].These additives reduce hydrate formation pressures.Equilibrium pressures of mixed-gas hydrate such as CO 2 + N 2 are between the respective dissociation pressures of CO 2 and N 2 hydrates.Equilibrium pressures of mixed-gas hydrates are also moderated by these hydrate formation promoters (Figure 2).
Energies 2017, 10, 927 4 of 11 Generally, there are two types of hydrate formation promoters: chemical additives for gas hydrate that have no effect on structures of the water cages (e.g., tetrahydrofuran, cyclopentane, and anionic/non-ionic surfactants), and SC types such as TBA salts that form SC [24,25,41].These additives reduce hydrate formation pressures.Equilibrium pressures of mixed-gas hydrate such as CO2 + N2 are between the respective dissociation pressures of CO2 and N2 hydrates.Equilibrium pressures of mixed-gas hydrates are also moderated by these hydrate formation promoters (Figure 2).

TBAB (+CO2 + N2) SC with Mixed Gas of CO2 + N2
In this study, we synthesized TBAB + CO2 + N2 SC samples using the same method as our previous paper [49].To synthesize the SC samples, a high-pressure cell of 223 cm 3 volume was charged with 30 g of an aqueous TBAB solution with 0.320 mass fraction for TBAB (>0.99 mass fraction, Sigma-Aldrich).The hydrate formation pressure and temperature was 1.0 MPa and 282.2 K, respectively.After the supply of aqueous solution, the reactor was sealed and vacuumed by a vacuum pump.Subsequently, it was charged and discharged with CO2 + N2 mixed gas with the aid of the vacuum pump.When residual air was removed, we supplied CO2 + N2 mixed gas (CO2/N2 = 0.151/0.849,Japan Fine Products) to the cell and started stirring for dissolution of the gas into liquid phase.For comparison, TBAB SC without CO2 + N2 gas was formed from an aqueous TBAB solution with w = 0.320 for TBAB at 283.1 K under atmospheric pressure, and grown for 17 h.Both samples were taken out from the cell after quenching at a temperature <220 K. Detailed procedures on SC formation and gas analysis are treated in our previous study [50].
During the formation of the TBAB + CO2 + N2 hydrate, an initial rapid pressure drop ended in two hours and no further pressure drop was observed over 40 h.The gas amount contained in the SC sample was calculated from the pressure drop during the formation process.There was 4.3 mmol of CO2 + N2 gas absorbed in the SC formed from 30 g of the aqueous TBAB solution.In the previous study [49], it was shown that CO2 is selectively captured in the SC, and the CO2 mole fraction in the SC phase was ~0.7 under the same experimental conditions as the present study.Thus, it is assumed that about 3 mmol of CO2 was contained in the TBAB + CO2 SC sample formed in this study.
To confirm the crystal structure of the sample formed under conditions of the proposed system, we carried out powder X-ray diffraction (PXRD) measurements at 93 K to avoid leakage of CO2+N2 molecules in the SC sample under atmospheric pressure.Figure 3 shows PXRD patterns of TBAB + CO2 + N2 SC and that of TBAB SC formed without CO2 + N2 gas compression at the same temperature, Formation conditions of gas-containing semi-clathrate hydrates and gas hydrates [28,30,31,39,[42][43][44][45][46][47][48].Double circle represents formation condition in the present study.Here, w denotes mass fraction of promoters in aqueous phase.NH: neohexane; TBAB: tetra-n-butylammonium bromide; THF: tetrahydrofuran.

TBAB (+CO 2 + N 2 ) SC with Mixed Gas of CO 2 + N 2
In this study, we synthesized TBAB + CO 2 + N 2 SC samples using the same method as our previous paper [49].To synthesize the SC samples, a high-pressure cell of 223 cm 3 volume was charged with 30 g of an aqueous TBAB solution with 0.320 mass fraction for TBAB (>0.99 mass fraction, Sigma-Aldrich).The hydrate formation pressure and temperature was 1.0 MPa and 282.2 K, respectively.After the supply of aqueous solution, the reactor was sealed and vacuumed by a vacuum pump.Subsequently, it was charged and discharged with CO 2 + N 2 mixed gas with the aid of the vacuum pump.When residual air was removed, we supplied CO 2 + N 2 mixed gas (CO 2 /N 2 = 0.151/0.849,Japan Fine Products) to the cell and started stirring for dissolution of the gas into liquid phase.For comparison, TBAB SC without CO 2 + N 2 gas was formed from an aqueous TBAB solution with w = 0.320 for TBAB at 283.1 K under atmospheric pressure, and grown for 17 h.Both samples were taken out from the cell after quenching at a temperature <220 K. Detailed procedures on SC formation and gas analysis are treated in our previous study [50].
During the formation of the TBAB + CO 2 + N 2 hydrate, an initial rapid pressure drop ended in two hours and no further pressure drop was observed over 40 h.The gas amount contained in the SC sample was calculated from the pressure drop during the formation process.There was 4.3 mmol of CO 2 + N 2 gas absorbed in the SC formed from 30 g of the aqueous TBAB solution.In the previous study [49], it was shown that CO 2 is selectively captured in the SC, and the CO 2 mole fraction in the Energies 2017, 10, 927 5 of 12 SC phase was ~0.7 under the same experimental conditions as the present study.Thus, it is assumed that about 3 mmol of CO 2 was contained in the TBAB + CO 2 SC sample formed in this study.
To confirm the crystal structure of the sample formed under conditions of the proposed system, we carried out powder X-ray diffraction (PXRD) measurements at 93 K to avoid leakage of CO 2 +N 2 molecules in the SC sample under atmospheric pressure.Figure 3 shows PXRD patterns of TBAB + CO 2 + N 2 SC and that of TBAB SC formed without CO 2 + N 2 gas compression at the same temperature, for comparison.These two PXRD patterns do not show noteworthy diffraction peaks from hexagonal ice, which suggests that nearly all the TBAB solution transformed into the TBAB SC during its formation and after quenching for sampling.The obtained PXRD pattern of the TBAB + CO 2 + N 2 SC is consistent with that of the TBAB SC with orthorhombic Pmma phase [26].This result agrees with our previous study of TBAB + CO 2 (or CH 4 ) SC [51].Under substantial pressure of the gases, the TBAB solutions with w = 0.320 are likely to form the orthorhombic phase but not the tetragonal phase.However, the TBAB SC sample formed without CO 2 + N 2 gas was a mixture of the orthorhombic and other phases.It is unclear whether these structural differences are due to the occurrence of CO 2 + N 2 gas.It could be caused by a pressure-temperature effect or capture efficiency of gases, or it could be an extrinsic effect such as sample treatment after its synthesis.For further understanding of detailed crystallographic properties of TBAB SC, other studies from a physicochemical point of view are necessary.However, the fact that TBAB + CO 2 + N 2 SC were likely to form a pure orthorhombic structure is useful to estimate and assess its gas capacity within a crystalline structure from an application standpoint of the TBAB SC.
Energies 2017, 10, 927 5 of 11 ice, which suggests that nearly all the TBAB solution transformed into the TBAB SC during its formation and after quenching for sampling.The obtained PXRD pattern of the TBAB + CO2 + N2 SC is consistent with that of the TBAB SC with orthorhombic Pmma phase [26].This result agrees with our previous study of TBAB + CO2 (or CH4) SC [51].Under substantial pressure of the gases, the TBAB solutions with w = 0.320 are likely to form the orthorhombic phase but not the tetragonal phase.However, the TBAB SC sample formed without CO2 + N2 gas was a mixture of the orthorhombic and other phases.It is unclear whether these structural differences are due to the occurrence of CO2 + N2 gas.It could be caused by a pressure-temperature effect or capture efficiency of gases, or it could be an extrinsic effect such as sample treatment after its synthesis.For further understanding of detailed crystallographic properties of TBAB SC, other studies from a physicochemical point of view are necessary.However, the fact that TBAB + CO2 + N2 SC were likely to form a pure orthorhombic structure is useful to estimate and assess its gas capacity within a crystalline structure from an application standpoint of the TBAB SC.The CO2 gas amount contained in the SC sample was assessed by comparing crystallographic structure data.The total amount of CO2 gas contained in four different hydrate crystals (sI, sII, sH and SC) as a function of gas amount encaged into 5 12 cage (so-called cage occupancy) were simulated based on their crystal structures (Figure 4).The total amount of gas in the TBAB + CO2 + N2 SC sample in this study is also plotted in this figure.Then, the overall apparent cage occupancy of 5 12 cage (~0.15) was estimated.Other results based on cage occupancies determined by X-ray structure analysis [33,34,38] are plotted for comparison.Figure 4 suggests the possibility of forming greater gas occupation in the TBAB + CO2 + N2 SC sample, which could be achieved by improving the formation process of the SC sample.The CO 2 gas amount contained in the SC sample was assessed by comparing crystallographic structure data.The total amount of CO 2 gas contained in four different hydrate crystals (sI, sII, sH and SC) as a function of gas amount encaged into 5 12 cage (so-called cage occupancy) were simulated based on their crystal structures (Figure 4).The total amount of gas in the TBAB + CO 2 + N 2 SC Energies 2017, 10, 927 6 of 12 sample in this study is also plotted in this figure.Then, the overall apparent cage occupancy of 5 12 cage (~0.15) was estimated.Other results based on cage occupancies determined by X-ray structure analysis [33,34,38] are plotted for comparison.Figure 4 suggests the possibility of forming greater gas occupation in the TBAB + CO 2 + N 2 SC sample, which could be achieved by improving the formation process of the SC sample.
Energies 2017, 10, 927 6 of 11 especially in closed greenhouses.Accordingly, we conclude that TBAB + CO2 + N2 SC is suitable for use in CO2 enrichment within a closed greenhouse.Other materials such as activated carbon, zeolite, or porous materials have been investigated, toward the development of a CO2 capture technology based on adsorption [52].Specifically, activated carbons or zeolites are also cost-effective candidate materials for CO2 storage.These have higher CO2 adsorption capacity than TBAB + CO2 SC, but also high humidity because of mist spraying for crop growth and yield, and wet exhaust gas generally reduces their capacity.Thus, overall assessment of the total system is required to evaluate and compare potentials of these media.

Operation Cycle of System
Performance of the TBAB + CO2 SC formation/dissociation cycle for CO2 enrichment depends on gas capacity in TBAB SC and thermal efficiency of the temperature cycle with alternate actions of hydrate formation and dissociation.Figure 5 conceptually illustrates the SC formation/dissociation cycle on a pressure-temperature plane together with the TBAB + CO2 + N2 SC phase equilibrium curve.Each process in the SC formation/dissociation cycle is described below.
During the night, exhaust gas of the combustion heater is trapped and cooled to near ambient temperature.Here, we can use cool energy of the environment as a supplement during the night.Pollutants such as NOx and SOx in the gas are washed out by a water scrubber.It is noteworthy that because the gas capture medium used in this system is a water-based material, a water-type scrubber can be used to clean the flue gas.The gas is compressed to 1.0 MPa or less to flow into TBAB solution within a formation reactor.Although equilibrium conditions of TBAB + CO2 + N2 SC are mild (<1.0 MPa at 283 K), supercooling of TBAB solution is generally necessary for nucleation.Thus, the TBAB solutions must be cooled to 280 K or lower for SC nucleation.Once SC nucleation occurs, temperature is set to near the equilibrium temperature for slow SC growth.The CO 2 gas capacity of sI hydrate was the greatest of the four hydrates (sI, sII, sH, and SC).However, the equilibrium pressure of CO 2 (+ N 2 ) hydrate formed from mixed gas of CO 2 + N 2 (CO 2 /N 2 ~0.15/0.85)was >10 MPa as shown in Figure 2. Additionally, since the hydrate promoters such as THF are basically volatile and harmful, they are unsuitable for use in agricultural fields, and especially in closed greenhouses.Accordingly, we conclude that TBAB + CO 2 + N 2 SC is suitable for use in CO 2 enrichment within a closed greenhouse.
Other materials such as activated carbon, zeolite, or porous materials have been investigated, toward the development of a CO 2 capture technology based on adsorption [52].Specifically, activated carbons or zeolites are also cost-effective candidate materials for CO 2 storage.These have higher CO 2 adsorption capacity than TBAB + CO 2 SC, but also high humidity because of mist spraying for crop growth and yield, and wet exhaust gas generally reduces their capacity.Thus, overall assessment of the total system is required to evaluate and compare potentials of these media.

Operation Cycle of System
Performance of the TBAB + CO 2 SC formation/dissociation cycle for CO 2 enrichment depends on gas capacity in TBAB SC and thermal efficiency of the temperature cycle with alternate actions of hydrate formation and dissociation.Figure 5 conceptually illustrates the SC formation/dissociation cycle on a pressure-temperature plane together with the TBAB + CO 2 + N 2 SC phase equilibrium curve.Each process in the SC formation/dissociation cycle is described below.During the night, exhaust gas of the combustion heater is trapped and cooled to near ambient temperature.Here, we can use cool energy of the environment as a supplement during the night.Pollutants such as NO x and SO x in the gas are washed out by a water scrubber.It is noteworthy that because the gas capture medium used in this system is a water-based material, a water-type scrubber can be used to clean the flue gas.The gas is compressed to 1.0 MPa or less to flow into TBAB solution within a formation reactor.Although equilibrium conditions of TBAB + CO 2 + N 2 SC are mild (<1.0 MPa at 283 K), supercooling of TBAB solution is generally necessary for nucleation.Thus, the TBAB solutions must be cooled to 280 K or lower for SC nucleation.Once SC nucleation occurs, temperature is set to near the equilibrium temperature for slow SC growth.During the day, the temperature controller ceases operation and the temperature of the reactor warms to the ambient temperature above the equilibrium point of SC hydrate, and CO2 + N2 gas automatically emerges for CO2 enrichment.At the same time, because the SC dissociation is an endothermic reaction, cold heat may be obtained and used to cool parts of the greenhouse.After completion of TBAB SC dissociation, reactor conditions are reset to the initial state.

Configuration of CO2 Enrichment System Using TBAB + CO2 SC
Figure 6a is a conceptual process diagram of the proposed system, which is composed of three sections.The first is a conventional combustion heater to heat the greenhouse.The heater is connected to the second section of the water scrubber system and compressor for outflow of exhaust gas.The third section is a TBAB SC reactor with a temperature control unit.During the day, the temperature controller ceases operation and the temperature of the reactor warms to the ambient temperature above the equilibrium point of SC hydrate, and CO 2 + N 2 gas automatically emerges for CO 2 enrichment.At the same time, because the SC dissociation is an endothermic reaction, cold heat may be obtained and used to cool parts of the greenhouse.After completion of TBAB SC dissociation, reactor conditions are reset to the initial state.

Configuration of CO 2 Enrichment System Using TBAB + CO 2 SC
Figure 6a is a conceptual process diagram of the proposed system, which is composed of three sections.The first is a conventional combustion heater to heat the greenhouse.The heater is connected to the second section of the water scrubber system and compressor for outflow of exhaust gas.The third section is a TBAB SC reactor with a temperature control unit.
As shown in Figure 6b, CO 2 gas supply from the storage tank has been widely used for CO 2 enrichment.In this case, some CO 2 gas is released to the atmosphere during daytime.Additionally, exhaust gas from the heater for greenhouse heating is released during nighttime.However, the advantage of the present system is that it is not necessary to provide CO 2 gas from a storage tank, and exhaust gas can be used for the stimulation of photosynthesis of crops in greenhouses.From carbon balance estimates, the environmental benefit of the proposed system is to remove the CO 2 emissions of a combustion heater.This then becomes a low-cost and environmentally friendly CO 2 enrichment and heating/cooling system.Moreover, the proposed temporally varying process can achieve additional economic benefits when balanced against the conventional heat pump system for daytime cooling.
Figure 6a is a conceptual process diagram of the proposed system, which is composed of three sections.The first is a conventional combustion heater to heat the greenhouse.The heater is connected to the second section of the water scrubber system and compressor for outflow of exhaust gas.The third section is a TBAB SC reactor with a temperature control unit.We evaluated cultivation conditions of non-everbearing strawberry (Fragaria x ananassa Duch.) cultivars of Japanese "Tochiotome".For a preliminary field study of the system in a greenhouse of 150 m 2 in size, ~4.5 kg/day of CO 2 is required.According to our results, 3 mmol of CO 2 gas was absorbed in the SC formed from 30 g of aqueous TBAB solution due to SC formation.Thus, it is estimated that 0.004 kg/L of CO 2 is captured by the TBAB solution with w = 0.32 mass fraction.That is 1125 L of TBAB solution and ~1500 L of a SC reactor needed for storing 4.5 kg of CO 2 .Our bench-scale test of TBAB + CO 2 SC using a larger reactor (1000 cm 3 ) with continuous gas flow showed similar or less CO 2 capacity in the system [49].
It is necessary to control the CO 2 enrichment system as follows.During nighttime (e.g., from 9 pm. to 6 am.), the greenhouse is heated to avoid temperature drop below 281 K.For example, the heating can be via a combustion heater (NEPON Inc., Model KA-125, Tokyo, Japan) that exhausts ~22.6 m 3 /h (STP) of flue gas containing 2.84 kg/h of CO 2 caused by the combustion of kerosene.CO 2 enrichment using the TBAB + CO 2 SC system needs 4.5 kg of CO 2 from this flue gas to provide sufficient CO 2 to a 150-m 2 field during the day, as noted above.The minimum performance for the CO 2 capture rate in our system is 4.5 kg/9 h = ~0.5 kg/h on average, and that required for CO 2 capture efficiency is ~0.18 (=0.5 kg/h/2.84kg/h).Regardless of formation rate, this value was confirmed by our previous study [49], in which it was calculated at 0.16-0.29 from data with 1 MPa operation pressure.These values would be further improved by a practical process.Upon morning, temperature maintenance of the SC reactor is terminated and the temperature of the reactor is warmed to ambient temperature above the equilibrium point of SC hydrate to dissociate the SC and release the captured CO 2 .Using CO 2 concentration monitors in the greenhouse, CO 2 concentration is controlled in the range 700-1000 ppm, which is effective for the crops [10].The suggested temporally varying CO 2 enrichment is promising for enhancing energy efficiency; i.e., at night, there is heating and CO 2 storage from a flue gas, and in daytime there is CO 2 release and enrichment.
Energies 2017, 10, 927 9 of 12 The system proposed herein is still being upgraded.However, it can be expected that the CO 2 condensation process from the flue gas before gas injection to the hydrate reactor may improve system CO 2 gas capacity.Additionally, optimization of the formation temperature of TBAB + CO 2 SC, a method for flue bubbling into the aqueous TBAB solution, or other parameters may promote kinetic hydrate formation (e.g., [53][54][55]).These procedures to improve the SC system will enable downsizing of the SC formation reactor.Further study based on bench-scale experiments in an agricultural field will be necessary to improve the concept for estimation of energy balance.This would also contribute to the reuse of CO 2 gas from the heater exhaust gas for carbon capture and utilization technology.

Summary
In this paper, a new concept of CO 2 enrichment system for crop production under a controlled greenhouse environment for horticulture was proposed, based on new and environmentally friendly tetra-n-butylammonium bromide (TBAB) + CO 2 semi-clathrate hydrate (SC).The feasibility of this system was assessed based on experimentally obtained CO 2 capacity in TBAB + CO 2 + N 2 SC formed at 1.0 MPa and 282.2 K.As a result, 0.004 kg/L of CO 2 was obtained by SC formation from TBAB solution with w = 0.320 mass fraction of TBAB.There was 1125 L of TBAB solution and SC reactor of ~1500 L needed for CO 2 enrichment for a typical greenhouse of 150-m 2 size.It is anticipated that the CO 2 condensation process from the flue gas before gas injection to the hydrate reactor will improve CO 2 gas capacity of this system.Another additive or other parameters may promote kinetic hydrate formation.These processes to improve the SC system will enable downsizing of the SC formation reactor.
We specified requirements of the SC reactor with regard for the CO 2 capture rate based on the CO 2 enrichment process in the cultivation of a Japanese strawberry cultivar as a standard case.In this system, a CO 2 storage tank is not necessary to provide CO 2 gas, and exhaust gas is effectively and efficiently used for the stimulation of crop photosynthesis in greenhouses.

Figure 4 .
Figure 4. Gas amount contained in semi-clathrate hydrate and gas hydrates as function of cage occupancy of 5 12 cage.Here, cage occupancy of 4 3 5 6 6 3 cage in sH is assumed identical to that of 5 12 cage.Each circle represents experimentally obtained CO2 gas amount in hydrate crystals, and the result of this study represents the amount of CO2 + N2 (CO2/N2~0.7/0.3)gas.

Figure 4 .
Figure 4. Gas amount contained in semi-clathrate hydrate and gas hydrates as function of cage occupancy of 5 12 cage.Here, cage occupancy of 4 3 5 6 6 3 cage in sH is assumed identical to that of 5 12 cage.Each circle represents experimentally obtained CO 2 gas amount in hydrate crystals, and the result of this study represents the amount of CO 2 + N 2 (CO 2 /N 2 ~0.7/0.3)gas.

Figure 6 .
Figure 6.Configuration of CO2 enrichment system, using (a) TBAB + CO2 SC and (b) CO2 gas from a tank.Figure 6. Configuration of CO 2 enrichment system, using (a) TBAB + CO 2 SC and (b) CO 2 gas from a tank.

Figure 6 .
Figure 6.Configuration of CO2 enrichment system, using (a) TBAB + CO2 SC and (b) CO2 gas from a tank.Figure 6. Configuration of CO 2 enrichment system, using (a) TBAB + CO 2 SC and (b) CO 2 gas from a tank.

Table 1 .
Crystallographic data of four types of clathrate hydrates.

Table 1 .
Crystallographic data of four types of clathrate hydrates.

Space Group Hydration Number Lattice Constant/Å Cage Type Number of Cages Promoter ∆H/kJ mol −1
* sH hydrates necessarily form with two different guest substances; one is a large-molecule guest substance such as neohexane (NH), and the other is a small-molecule guest substance such as CH 4 .