A Feasibility Study on Hydrate-Based Technology for Transporting CO₂ from Industrial to Agricultural Areas

Abstract

Climate change caused by global warming has become a serious issue in recent years. The main purpose of this study was to evaluate the effectiveness of the above system to quantitatively supply CO₂ or CO₂ hydrate from industrial to agricultural areas. In this analysis, several transportation methods, namely, truck, hydrate tank lorry, and pipeline, were considered. According to this analysis, the total CO₂ supply costs including transportation ranged from 15 to 25 yen/kg-CO₂ when the transportation distance was 50 km or less. The cost of the hydrate-based method increased with the transport distance in contrast to the liquefied CO₂ approach. However, the technology of supplying CO₂ hydrate had merit by using a local cooling technique for cooling specific parts of agricultural products.

1. Introduction

The climate changes caused by global warming has become a serious issue in recent years [1]. Thus, the reduction of CO₂ emissions has become increasingly important. In this sense, solutions promoting industrial activity while reducing CO₂ emissions are required. One effective way of selecting environmentally favorable CO₂ sources for CO₂ utilization is of importance [2], and we proposed an agro-industrial system [3] as is shown in Figure 1. In this system, industrial and agricultural areas are connected. The former releases CO₂ and heat as a result of production processes, while the latter can make use of air-conditioning-derived heat and residual industrial heating for agricultural production. The main purpose of this study was to evaluate the effectiveness of the above system to quantitatively supply CO₂ or CO₂ hydrate from industrial to agricultural areas located within a distance of 50 km or less.

In this analysis, we focused on the utilization of hydrate transport technology in the agriculture field, especially facility horticulture. The supply of CO₂ by hydrate has the merit that it can simultaneously supply cold heat necessary for cultivation. These simultaneous supplies can keep the CO₂ concentration relatively high under suitable temperatures in greenhouses, so it is expected to increase the yield [4,5]. As an example of a city, more than 200 thousand tons of CO₂ are discharged from one factory in a year. On the other hand, the CO₂ necessary for photosynthesis is supplied by burning a boiler, and its supply amount is around 5–10 thousand tons a year in the agricultural field [1,4]. If CO₂ is transported from industrial areas, it will be possible to reduce emissions of CO₂ by the combustion of boilers in agricultural areas.

We calculated the energy consumption and cost for separating CO₂ from an industrial exhaust gas stream via chemical absorption and transporting it to the agricultural area via boosting or hydration technologies. Several transportation methods, namely, truck, hydrate tank lorry, and pipeline, were considered. Finally, we compared the energy consumption and cost required for CO₂ separation and transportation by these methods and subsequently evaluated the effectiveness of the proposed system connecting agricultural and industrial areas.

2. Outline of the CO₂ Supply System from Industrial Areas

2.1. Outline of Some CO₂ Supply Methods

We calculated the cost of the CO₂ supply system involving the separation of CO₂ from an industrial exhaust gas stream and the subsequent transportation of this CO₂ from industrial to agricultural areas. In this calculation, the cost of the CO₂ supply process was divided into transporting and collecting costs, and the collecting cost was further divided into separation and boosting costs.

Table 1. Some CO₂ supply methods.

Type	Collecting		Transportation
	Separating	Boosting
(1)	Chemical absorption	Liquefaction	Truck
(2)	Chemical absorption	Hydration	Tank lorry
(3)	Chemical absorption	Compression	Pipe line

shows some CO₂ supply methods considered herein.

With regard to the pressurization and transportation of CO₂, three cases were considered:

(1) Truck type

CO₂ is separated from exhaust gas by chemical engineering, liquefied into the cylinder, and finally transported by truck.

(2) CO₂ hydrate type

CO₂ is separated from exhaust gas by chemical engineering, hydrated, and finally transported by tank lorry.

(3) Pipe line type

CO₂ is separated from exhaust gas by chemical engineering, compressed to 3.5 MPa, and finally transported by pipeline.

Figure 2, Figure 3 and Figure 4 show the outline for each CO₂ separation and transportation process.

2.2. Basic Principle of Various Applied Separation and Recovery Devices

The CO₂ separation method and the various compression methods considered herein are presented next. Figure 5 describes the principle of a standard amine-based CO₂ absorption–desorption process [6]. The system was comprised of a simple absorber and desorber with a reboiler and a condenser, an amine/amine heat exchanger, pumps, and a cooler. The CO₂ from an exhaust gas stream is absorbed in the absorption column containing the amine solvent. Once absorbed, the CO₂-rich amine solution is pumped from the absorption column through the amine heat exchanger where it is heated before entering the stripper for regeneration.

Figure 6 shows the liquefying system of CO₂ [7], the CO₂ was subsequently liquefied with a high-pressure pump. This system allows for a flexible operation depending on the supply while eliminating manual handling of large amounts of gases in cylinders. A self-contained storage vessel consisting of a low-temperature-grade carbon steel coded vessel and insulation and control systems equipped with approved class valves, piping, and fittings was considered.

So far, CO₂ separation and capture from waste gas by means of CO₂ hydrate has been proposed [8,9,10]. Figure 7 shows the CO₂ hydrate production system [11]. This system was conducted in a stirring-vessel-type reactor at certain given conditions for hydrate formation. The pressure and temperature conditions for hydrate formation were set as operational variables for the energy consumption estimations. Herein, the base temperature and pressure were assumed to be 275 K and 2.0 MPa, respectively, and the hydrate formation rate depends on these conditions. The heat released during the formation of the hydrate was employed for pumping brine, while the cooling derived from the CO₂ release was employed to recover the brine temperature.

3. Calculating the Cost of the CO₂ Supply System

3.1. Calculating the Energy Consumption and Cost of the CO₂ Collection System

The main conditions employed for calculating the cost of the collecting system are shown in

Table 2. Conditions for calculating energy and cost.

Condition	Value	Unit
Coal-based power generation plant	1000	(kW)
CO2 collection efficiency	90	(%)
Collected rate of CO2	700	(t/h)
Amount of collected CO2 per year	5000	(kt/y)
CO2 density in the exhaust gas	20	(%)
Temperature of the exhaust gas	90	(°C)
Absorption tower inlet gas	3000	(km3/h )
Annual operating time	8000	(h)
Electricity cost	12	(yen/kWh)
Years of depreciation	10	(years)
The number of employees	8	(-)
Thermal price	0.3	(yen/MJ)

. Here, CO₂ collection efficiency and the collected rate of CO₂ were based on the value of the literature of the chemical absorption method [6].

3.1.1. Energy and Cost of Separating CO₂

Table 3. Cost and energy consumption of the main devices in the chemical absorption method.

• Main equipment
wash-column, absorber, regenerator, packing, reflux drum, heat exchange, booster fan, pomp, CO2 pomp, CO2 compressor
• Capital (fixed) cost
Equipment cost			1.2	(yen/kg-CO2)
Labor cost			0.01	(yen/kg-CO2)
• Operating (variable ) cost
Power	0.14	(kWh/kg-CO2 )	1.7	(yen/kg-CO2)
Heat	2.5	(MJ/kg-CO2 )	0.8	(yen/kg-CO2)
Etc.	52	(MJ/kg-CO2 )	0.4	(yen/kg-CO2)

Equipment cost = (construction cost / depreciation period) / annual CO₂ collection amount; Construction cost = major equipment cost × 4.74 (Lang coefficient); Labor cost = (the number of employees × 4 Myen/ year) / annual CO₂ collection amount.

shows the cost and energy consumption of the main devices required for the operating chemical absorption method obtained from the literature [6]. With regard to the separation energy, we considered the power consumptions of main devices, such as the wash column, absorber, regenerator, reflux drum, heat exchange, booster fan, pomp, CO₂ pomp, and CO₂ compressor.

In the calculation of separation costs, capital (fixed) costs and operating (variable) costs were considered. Capital cost is the expense required to make the project commercially viable, and equipment costs and labor costs were considered in this analysis. The operating cost is the expenses related to the operation of equipment or equipment, in which we considered the electric power, the heat supplied, and the cooling water in this system. The total amount of electric power was calculated by considering the total power required for the separating equipment.

3.1.2. Boosting Energy and Cost of the Collected CO₂

(1) Liquefaction and compression

The boosting energy (i.e., the energy produced upon increasing the pressure of the exhausted gas) can be derived from Equation (1) under the assumption of an ideal gas adiabatically compressed [12].

$$E_{bst}=C_{v\_co2}・T(\frac{{p^{\left(\gamma -1/\gamma \right)}}^{\text{'}}}{p}-1)$$

(1)

Here, Cv denotes the specific heat at a constant volume of CO₂, T is the absolute temperature before compression, p is the pressure before compression, p’ is the pressure after compression, and γ is the fraction of specific heat of CO₂ in the exhausted gas.

The boosting cost of the liquefying facility requiring the pressurization of CO₂ is defined by Equation (2) obtained from the literature [7].

$$Y_{liq}=160･{\left(\frac{E_{liq}}{0.228}\right)}^{0.7}･{10}^8$$

(2)

(2) Hydration

With regard to the energy consumed in the hydrate method, the hydrating energy was obtained from the relationship between the hydrate formation energy and the temperature at various concentrations of exhaust gas (Figure 8), as previously reported in the literature [10]. As shown in Figure 8, the energy required for generating the hydrate greatly depended on the concentration of the exhaust gas. When directly generating the hydrate from the exhaust gas, the energy required was significantly large (1.6 kWh/kg-CO₂) owing to the low concentration of the exhaust gas (20%).

The formation of hydrates is a stochastic process and a subcooling is needed for hydrate formation in general. For this reason, a variety of studies on hydrate promoters has been performed to reduce the equilibrium pressure. In addition, CO₂ hydrate formation using supercooled aqueous nanodroplets [13] and using others such as ice at temperatures just below 273 K [14] or ice melting water [15] may also be a potential candidate for rapid CO₂ hydrate growth. Either way, it will be possible to reduce the energy consumed in the hydrate method.

With regard to the cost in the hydrate method, we considered the power consumptions of main devices, such as the reactor, agitator, cooling system, gas compressor, heat exchange, pomp, CO₂ pomp, and hydrate tube. The costs of the main equipment were determined from a literature value based on the material and pressure condition of the equipment [3]. Labor costs were determined according to the calculation by the chemical absorption method in Table 3.

3.2. Calculating the Cost for Transporting CO₂

3.2.1. Calculation of the Load Capacity

(1) Cylinder truck case

The CO₂ loading capacity of the cylinder truck was calculated at the conditions summarized in

Table 4. Calculation condition of CO₂ load capacity by gas bomb truck.

Condition	Value	Unit
Truck load capacity	10,000	(kg)
Gas filling capacity	7.00	(Nm3)
Molar volume	0.0224	(Nm3/mol)
CO2molecular weight	44.0	(g/mol)
Cylinder capacity	13.75	(kg)
Weight of cylinder	52.00	(kg)

. The capacity was calculated from Equation (3), where the cylinder capacity for CO₂ was calculated according to Equation (4).

$$V_{\mathrm{CO}2}=V_{tr}･\frac{V_{cyl}}{V_{cyl}+W_{cyl}}$$

(3)

$$V_{cyl}=\frac{V_{gas}}{V_{m\_CO2}}･\frac{W_{m\_co2}}{1,000}.$$

(4)

According to these equations, the CO₂ loading capacity per tank truck was calculated to be 3846 kg.

(2) Transportation of CO₂ hydrate case

The load capacity of the hydrate tank lorry was calculated with Equation (5). The container capacity was defined as the volume of the container multiplied by the density of the content. By using these equations, the loading capacity for transporting CO₂ hydrates was calculated. In this calculation, the same conditions (i.e., capacity, weight, and volume of the container) as the cylinder transportation case were used. The density of CO₂ hydrate was assumed to be 1.105 g/cm³ [16]. The CO₂ loading capacity when a hydrate tank truck was used for transportation was calculated at the conditions summarized in

Table 5. Calculation condition of CO₂ hydrate load capacity.

Condition	Value	Unit
Tanker load capacity	10,000	(kg)
Container volume	47.0	(L)
Contents density	1.105	(kg/L)
Container weight	52.0	(kg)
Contents of container	43.09	(kg)

. According to these conditions, the CO₂ loading capacity of CO₂ was calculated to be 2193 kg.

$$V_{tank}=V_{tr}･\frac{V_{cyl}}{V_{cyl}+W_{cyl}}$$

(5)

$$V_{co2H}=V_{\tan \mathrm{k}}·\frac{44}{44+103.5}.$$

(6)

Furthermore, we calculated the cooling heat of CO₂ hydrate. In this calculation, three types of heat were considered: latent heat generated upon CO₂ hydrate collapse into CO₂ gas and water, water generated when the CO₂ hydrate collapsed, and sensible heat increasing the temperature of the CO₂ hydrate increase from 5 (i.e., formulation temperature of CO₂ hydrate) to 25 °C (normal temperature). Subsequently, the amount of cooling heat supplied by CO₂ hydrate was calculated with Equation (7) at the conditions summarized in

Table 6. Calculation condition of cold load capacity.

Condition	Value	Unit
CO2 hydrate load capacity	4,531	(kg-CO2 )
Heat of formation of hydrate	65.0	(kJ/mol-CO2 )
Specific heat of water	4.217	(kJ/kg·K)
Specific heat at CO2	0.8518	(kJ/kg·K)
Cold supply	504.94	(kJ/kg·K)

. According to Equation (7), the CO₂ loading capacity per tank truck was calculated to be 2,288 (MJ).

$$H_{co2H}=\frac{H_{gas\_co2H}}{0.044･\left(\frac{44}{147.5}\right)}+C_{water}･\left(\frac{103.5}{147.5}\right)+C_{gas\_co2}･\frac{44}{147.5}･20.$$

(7)

3.2.2. Calculating the Transportation Cost

The transportation cost in the case of a cylinder truck or a CO₂ hydrate tank lorry was calculated with Equation (8) assuming that the cost per unit of transport volume was proportional to the transport distance.

$${\mathrm{Y}}_{trns}=\frac{\left({\mathrm{Y}}_{lo}/{\mathrm{Y}}_{fc}+{\mathrm{Y}}_{lab}\right)·2·N_{tr}+Y_{mnt}+Y_{tr}/Y_{rp}}{D/2·\mathrm{W}}.$$

(8)

Here, it was assumed that the gas was filled in a container similar to a 47 L gas cylinder. The loading amount was defined as the maximum weight of CO₂ that a vehicle can transport with a 10 t cylinder. Additionally, in the case of CO₂ hydrate transportation, the amount of CO₂ loaded was calculated by considering the weight fraction of CO₂ in the CO₂ hydrate based on the number of CO₂ and H₂O molecules of this compound. We considered a light oil price of 108 yen/L, a fuel consumption of 3 L/km, a labor expense of 64 yen/km, a maintenance cost (e.g., insurance and tax) of 305,800 yen/number of truck/year, a truck price of 10,000,000 yen, a recovery period of 10 years, an annual mileage of 10,000 km/year, a cylinder capacity of 47 L, and a cylinder weight of 52 kg.

Furthermore, the CO₂ emissions associated with the transportation by vehicles were calculated as shown in Equation (9).

$$W_{em\_co2}=2･\frac{F_{lo}}{Y_{fc}}/W$$

(9)

3.3. Calculating the Cost in the Case of a Pipeline System

When using a pipeline system, the annual transport cost was assumed to be proportional to the transport distance and constant with respect to the transportation volume (i.e., the cost per unit of transport volume was inversely proportional to the annual volume transported). Therefore, the cost was calculated with Equation (10) using a reference value for the transportation cost corresponding to an annual CO₂ transport volume of 200,000 t [17].

$$Y_{p-trns}=18,000/\left(w_{ann}\right).$$

(10)

4. Results and Discussion

4.1. Cost of the CO₂ Collecting System (Separation and Boosting Costs)

Table 7. Separation and boosting energies required for each method.

	Collecting Method	Separation Energy	Compression Energy
			Liquefaction	Compression	Hydrate
		(kWh/kg-CO2)	(kWh/kg-CO2)	(kWh/kg-CO2)	(kWh/kg-CO2)
(1)	CA + LQ	0.13	0.12	---	---
(2)	CA + CO2H	0.13	---	---	0.17
(3)	CA + CM	0.13	---	0.07	---

CA: chemical absorption (waste heat use); LQ: liquefaction; CM: compression; CO₂H: CO₂ hydrate.

and

Table 8. Collection cost for each method.

	Collecting Method	Separation Cost	Compressed Cost	Collection Cost
		(yen/kg-CO2)	(yen/kg-CO2)	(yen/kg-CO2)
(1)	CA+LQ	3.58	0.12	5.61
(2)	CA+CO2H		0.17	6.02
(3)	CA+CM		0.07	4.76

CA: chemical absorption (waste heat use); LQ: liquefaction; CM: compression; CO₂H: CO₂ hydrate.

display the separation and boosting energies required for each method. The separation energy for the chemical absorption method was 0.39 kWh/kg-CO₂. The thermal energy cost required to separate CO₂ from the amine solution was high. With regard to the CO₂ separation system, a chemical absorption method involving an exhaust gas stream was considered. When using this system, we suggest that it is indispensable to reduce this thermal energy by using the exhaust heat. On the other hand, the CO₂ liquefying cost was considered to be 0.12 yen/kg-CO₂, according to previous reports. In the calculation of the hydrate method, the hydrate was generated from a pure CO₂ gas stream recovered by the chemical absorption method. The hydrate generation cost was relatively high value at this setting condition.

4.2. Cost of the CO₂ Transportation System

Table 9. Transportation results (CO₂ loading capacity and the transportation cost).

Transporting Method	CO2 Load Capacity	Transportation Cost	Amount of CO2 Emission
	(kg/one truck)	(yen/kg-CO2/km)	(kg/kg-CO2)
LQ (TR)	3,846	0.139	0.00045
CM (PL)	489.4	1.089	0.00357
CO2H (HTL)	2,193	0.243	0.00080

LQ: liquid CO₂; CM: compressed gas CO₂; CO₂H: CO₂ hydrate; TR: truck; PL: pipe line; HTL: hydrate tank lorry.

shows the transportation results (i.e., the relation between the CO₂ loading capacity and the transportation cost). In particular, this table shows the amount of CO₂ emissions associated with the transportation process. The liquefied CO₂ loading capacity was large, and this method was therefore promising. Additionally, this CO₂ liquefaction system showed the lowest transportation costs among the methods studied herein. On the other hand, the transportation costs for compressed air (i.e., pipeline transportation) were large, although these values depended on the amount of CO₂ transported.

4.3. Total Cost of the CO₂ Supply System

Figure 9 shows the relation between the total cost of the CO₂ supply system and the transported distance of CO₂ for each method. As can be seen, these costs, including transportation, ranged from 15 to 25 yen/kg-CO₂ for a transportation distance of 40 km or less. The supply cost per unit of CO₂ of the pipeline transportation mode increased while the annual transport volume decreased. Figure 10 shows the relation between the total cost of the CO₂ supply system and the annual CO₂ transport volume in pipeline in the case of the distance at 40 km. According to this figure, the pipeline transportation cost at an annual CO₂ transport volume of 52,000 t was comparable to those obtained by the transportation of a cylinder truck or of a CO₂ hydrate tank truck. Additionally, the pipeline transportation cost at an annual CO₂ transport volume of about 70,000 t was significantly larger as compared to other transportation methods in this condition.

In this system, storage temperature of CO₂ hydrate in the container was assumed to be −20 °C under atmospheric pressure. Taking into account results of earlier studies on dissociation rates of CO₂ hydrate and CH₄ hydrate [18,19], CO₂ hydrate can be easily stored without its dissociation during the transportation process for several hours or up to a day. The potential of a natural gas hydrate pellet for long-term transportation of natural gas by means of gas hydrate has also been reported [20]. Further assessment will be required to compare the economic benefits of using pellet technology for CO₂ transportation.

4.4. Examining the Usefulness of the Supply of Cold via CO₂ Hydrate Transportation

In ordinary greenhouse cultivation, it is impossible to fertilize CO₂ because the cultivation house has to be ventilated in the daytime to prevent temperature from increasing inside. Since cold heat and CO₂ are simultaneously supplied, the utilization of CO₂ hydrates is expected to maintain an optimum temperature while fertilizing in the summer and without ventilating the greenhouse. Therefore, the possibility of CO₂ fertilization via CO₂ hydrate supply was evaluated by analyzing the cooling capacity of the CO₂ hydrates according to Equation (11). Generally, cold heat is also generated when CO₂ is supplied in a liquefied CO₂ cylinder. However, in view of the CO₂ supply rate and difficulty in taking out, this cold heat is not normally used at present. On the other hand, CO₂ hydrate can supply CO₂ and cold heat at the same time when hydrate is decomposed.

With regard to the amount of CO₂ fertilizer required, tomato cultivation was considered and the value was obtained from the literature [4]. The cold heat supplied by the CO₂ hydrates was calculated as the sum of the latent heat produced by CO₂ hydrate decomposition, the amount of water generated by decomposition, and the sensible heat equivalent to 20 °C. The temperature of the CO₂ gas must increase from 5 to 25 °C for CO₂ hydrate formation. At this time, the cooling capacity was 12 W/m².

$$H_{cool}=\frac{w_{dem\_co2}}{100}/3,600･H_{co2H}･\mathrm{1,000}.$$

(11)

The solar radiation is strong during summer days. Since the solar heat flux was 1200 W/m² or less, and the cooling capacity of the air conditioner was 200 W/m² or less in this season, the cold heat supplied by the CO₂ hydrates seemed to be insufficient for the cooling capacity of the greenhouse.

However, when the sun's solar radiation is weak, such as the spring and autumn season's morning and evening, there is merit for a simultaneous supply of CO₂ and cold heat by CO₂ hydrate. Local cooling technologies (Figure 11) that provide a cooling effect to agricultural crops by cooling only specific parts of crop plants have been studied in recent years [20], so the technology of supplying CO₂ by hydrate has merits. In this case, photosynthesis is accelerated by fertilization of CO₂ because it can maintain an appropriate temperature near plants by local cooling, and yield increase of about 20% can be expected. Although the transportation method by CO₂ hydrate is inferior to the CO₂ transport volume, we considered that this method has sufficient effectiveness in this field in combination with local cooling technology or for use in spring and autumn.

5. Conclusions

In this study, the feasibility of several systems using CO₂ or CO₂ hydrates supplied from industrial to agricultural areas was quantitatively evaluated from the viewpoint of profitability and CO₂ emission reduction. According to this analysis, the total CO₂ supply costs including transportation ranged from 15 to 25 yen/kg-CO₂ when the transportation distance was 50 km or less. Pipeline transportation costs at an annual CO₂ transport volume of about 70,000 t were comparable to those of a cylinder truck or a CO₂ hydrate tank truck. Among them, the cost of the hydrate-based method increased with the transport distance in contrast to the liquefied CO₂ approach. However, the technology of supplying CO₂ hydrate has merit, for example, by using a local cooling technique for cooling specific parts of agricultural products. In this case, photosynthesis is accelerated by fertilization of CO₂ because it can maintain an appropriate temperature near plants via local cooling, and a yield increase of about 20% can be expected. The CO₂ supply system presented herein supplied CO₂ at a low cost and with low CO₂ emissions, and the feasibility of the proposed system was shown from the viewpoint of profitability and CO₂ emission reduction.