Thermodynamic evaluation of a novel solar-aided biogas polygeneration system integrated with steam network etwork

Introduction

Carbon dioxide emission via the combustion of conventional energy sources has the most contribution to the global climate crises including global warming and environmental degradation [1, 2]. On the other hand, energy efficiency plays an essential role in sustainability development [3]. Thus, renewable-based polygeneration set-ups are becoming more and more desirable and attractive [4]. Polygeneration systems serve as an offering energy-efficient technology which delivers different types of energy carriers in a specific integrated complex simultaneously [5].

Among renewable resources, solar energy is the most promising one since it is available, clean, secure, and economical [6]. It can be employed in polygeneration systems as the main source of energy including thermal collectors and photovoltaic panels. Particularly, different types of solar technologies are available which can be utilized in polygeneration set-ups for the direct conversion of the sun irradiation at various operating temperatures [7]. For instance, in low- and mid-temperature ranges, the generated heat is utilized in heating and cooling applications mostly. Solar heating and cooling technologies, which convert the sun’s rays into space heating and cooling, are considered more attractive in summer since the cooling demand is frequently concurrent to the available solar intensity [8]. In high-temperature levels, the produced heat is introduced to power plants for power generation along with other useful products [9, 10].

The agricultural and animal wastes could be used as a primary feedstock for energy production [11, 12]. In recent years, these organic materials have been implemented for biogas production which includes methane and carbon dioxide. Using biogas containing carbon dioxide is not favorable in fuel cells and combustible engines. For this matter, biogas is often converted to a mixture of carbon monoxide and hydrogen through a reformer. Two kinds of reforming reactions take place during this process which are endothermic and exothermic. Furthermore, in this way, the produced gas is improved because of the achieved hydrogen via the biogas reforming biogas [13]. In the last decade, many studies discussed different approaches for biogas reforming, according to the nature of energy system [14].

Multi-generation mostly indicates that power and valuable heat are produced at the same time, along with other products, which is conceivable by employing the additional surplus heat that would be wasted in conventional systems. In this regard, Mouaky and Rachek [15] suggested a novel hybrid polygeneration system based on solar and biomass which is capable of producing power, heating, cooling, and freshwater for a residential area with forty households.

Polygeneration has a wide-range of applications including utilities, buildings sectors, as well as in different industrial clusters such as pulp, plastic, agriculture, and chemical [16, 17]. Numerous studies have investigated a variety of polygeneration systems [18-20]. For instance, Bai et al. [21] proposed a solar-biomass gasification system consisting of a biomass-steam gasification and concentrated solar thermal (CST) tower power generation system for methanol and electricity production. The reported exergy efficiency of the proposed configuration was 51.23%.

Above all, using solar energy as the heat source has been considered a promising technology for polygeneration systems. In recent years, utilizing solar energy as upstream in polygeneration systems has been increased. In the present paper, a new polygeneration system with the combination of CST tower, biogas-steam reformer, steam network (SN), pressure swing adsorption (PSA), carbon capture and storage (CCS), gas turbine cycle (GTC), and organic Rankine cycle (ORC) has been studied from a thermodynamic viewpoint. A review of the literature shows the lack of research on the integration of renewable resources and utility steam networks for heat recovery purposes. Due to the importance of this area, this paper aims to investigate the cogeneration efficiency of the devised configuration. Also, the impacts of key parameters are assessed through a parametric study.

2. Systems description

The graphical representation of the proposed system with hydrogen production and carbon capturing is demonstrated in Fig. 1. The system includes a CST tower plant, a biogas-steam reformer, an SN, a PSA, a CCS, a GTC, and an ORC. The description of this setup is shown below.

Figure 1 Graphical representation of the proposed hybrid integrated set-up.

In this paper, Solar energy provides the reforming heat requirement and also runs the steam network. This CST tower plant comprises a receiver, heliostat fields, and a thermal storage unit. The heliostats track and concentrate the sun’s beams to a focal point which is situated on the apex of the heliostat field.  Thermal energy storage is also implemented to deposit a surplus of thermal energy. The produced high-temperature molten salt flows through a reformer (state 18) to deliver the required heat for reforming reactions. In-state 3, water is mixed with condensed water from a gas-liquid separator (GLS) (state 11). The produced steam is mixed with reformed biogas. The required thermal energy for CO₂ capturing is provided by the heat that is absorbed from water at state 39. At state 10, the low-temperature syngas flows through the GLS where the condensed water splits (state 11). Later, the reformed biogas with low-content of H₂O (state 12) moves into the PSA and CCS.

The SN unit comes with three modular heat exchangers, including an economizer, an evaporator, and a superheater. The function of NG is to be used as a fuel to deliver a surplus amount that is needed by modular heat exchangers for steam production and heating. The exploitation of the CCS unit in power plants has led to a great reduction in CO₂ emission. Based on the work published by Van-Dal and Bouallou [22], the CCS unit utilized in this study uses monoethanolamine (MEA) as a solvent with a mass concentration of 30%. Their results show that the CO₂ capture rate of devised CCS module is 85% while the energy consumption for feed compression is 44 kWh_el per tons of CO₂. In addition, the energy needed for solvent regeneration is 3200 kJ/kg of CO_2, and the pressure drop occurring in this unit is around 5%.

In GTC, Inlet air is passed through an air compressor (AC) and then a recuperator is utilized for preheating this compressed air (state 28). Leaving the CCS at state 16, the unreacted syngas and hot air are guided into a combustion chamber (CC) in order to be burnt. Afterward, the hot flue gas is implemented for heating the compressed air in a recuperator (state 30). Also, the boiler feedwater (BFW) in the economizer (state 31) is heated by recovering heat from the flue gas. In this paper, a toluene-based organic ranking cycle (ORC) is used, which is integrated with an internal heat exchanger [23]. The Toluene of the ORC evaporator retrieves waste heat within the flue gas exiting the economizer. The saturated vapor of the toluene is utilized for producing power through a turbine. Then the thermal energy of the toluene, which exits the turbine is exploited by a recuperator. Afterward, this energy is transported to the toluene stream, which is introducing into the evaporator. The cooling water is used to cool the ORC turbine outlet stream and turning it into saturated liquid in the condenser.

3. Methodology

In this paper, the devised system is analyzed and evaluated from the thermodynamic point of view. The assumptions used in this evaluation are as follow [24, 25]:

• A steady-state model is applied for scrutinizing each sub-system.
• Potential and kinetic energy effects are ignored.
• Heat loss in heat exchangers and pipelines is neglected.
• For steam turbines of the SN unit, a variable isentropic efficiency is considered.
• Water is employed for desuperheating the deaerator steam.
• Acid dew point at 120℃ temperature is assumed for flue gas.
• The pressure of deaerators is presumed 1.208 bar.
• Steam flew through steam generation part is superheated.
• The value of pressure drop in the heat exchangers is considered 1% and 2% hot and cold sides, respectively.
• PSA unit recovers Hydrogen content from syngas up to 85%_. The cross-sectional area of this unit is 1.6 cm^2, and according to literature, and its pressure drop is about 2.5% [26, 27].

The general mass and energy balances equations for these components in a volume control are as follow [28, 29]:

3.1. Solar system

The heliostat field consisted of several heliostats with total aperture area ( ) is considered in this study. The following equations are used for scheming the concentrated solar energy [30]:

After calculating , total indication absorbed energy is computed using Eq. (4).

The following relation demonstrates the total energy balance for the molten salt cavity receiver. In Eq. (5),   denotes the net energy absorbed by the receiver and   is the total of heat losses:

The relation between the average temperature of the receiver surface (  and the average molten salt temperature ( ) is illustrated by Eq. (6) [30]:

Li et al. [31] applied the Dittus–Boelter equation for computing the convective heat transfer coefficient of molten salt:

Where F_r denotes the view factor and C_r is the concentration ratio.

The receiver aperture width is presumed  [30]. The absorbed receiver energy should provide the required heat energy for biogas reforming:

Then, the relationship among a number of receiver tubes and molten salt flowrate could be described as:

In this paper, the molten salt concludes of KCl/MCl₂ (62/38 w/w) is used as heat transfer fluid [32]. The required data for solar system modeling are listed in Table 1.

Table 1: Main parameters for simulation of the CST tower [30, 32]

3.2. Biogas-Steam reformer

Since direct use of biogas in gas turbines and combustion engines is low-efficient and environmentally hazardous, an alternative method should be devised as a replacement for the utilization of biogas directly. Dry reforming (DSR), methane CO₂ reforming, and methane steam (MSR) reforming are among the main reactions of the reforming process. There are also secondary reactions such as water gas shift (WGS), which are of high importance to produce high-quality syngas and decrease CO₂ greenhouse gas emissions [33]. These reactions could be described as below [34]:

The following relations are the equations for the molar flow rate in the reformer outlet:

denote the extents of reactions presented in Eqs. (19)-(24), respectively. Also, the equilibrium compositions at the expense of the reformer can be described in the following:

In order to calculate unknown parameters, the following relations can be applied:

Where,  and  are the universal gas constant,  and Gibb’s free energy in kJ kmol^-1, respectively. Table 2 tabulated the main input parameters for the biogas reforming [25].

3.3. Gas turbine cycle

The following reaction equation describes the basis of complete combustion with excess air:

A recuperator is designed for preheating the air that flows through the combustion chamber. The following relation is applied for computing the temperature of the compressed air [35]:

Also, the flue gas temperature can be determined as follows:

Where, η_is,GT  signifies the isentropic efficiency, and γ is the specific heat ratio. Consequently, Eq. (33) is used for calculating the net electricity:

In this relation, η_G denotes the generator efficiency. Table 3 presented the key input parameters of the GTC.

Table 3: Main parameters for simulating the GTC [25, 36]

3.4. Steam network modeling

3.4.1 heat recovery unit

In this paper, a series of heat exchangers including an economizer, an evaporator, and a superheater is developed which functions as an HRSG. In modeling an HRSG unit, the most crucial point that should be considered is avoiding a drop of the outlet gas temperature below the acid dew point, because this incident can result in deterioration of the equipment. The following relation is utilized for calculating the amount of heat retrieved from the flue gases by an HRSG unit [37]:

In  these relations,  means the thermal efficiency of the HRSG unit, and its presumed value is 90% [37].

Based on the amount of fuel and flue gas entered into the HRSG, the enthalpy of the inlet gas of the superheater can be utilized for determining the amount this necessitated extra fuel:

In this relation,  is the efficiency of the duct burner (DB), which is assumed to be 93% [37]. Due to its small extracted power, the pump work is ignored.

3.4.2. Steam turbine

The following formula gives the flow rate of steam going through a turbine at each step:

According to Ref. [38], the following relation shows the relationship of the maximum isentropic power and the shaft power of the turbine at the full turbine load.

Therefore, the steam turbine efficiency at maximum loading can be calculated by the following relation:

The maximum mass flow streaming in the steam turbine can be utilized for calculating steam turbine efficiency  as the following relation presents [38]:

Which a and b are calculated as follows

In the above relations,  and  are the inlet are the pressures of the entering and exiting steam, respectively. The coefficients a and b can be found in Table 4 [38].

Table 4: Necessary coefficients used in Eqs. (41-42) [38].

3.4.3. Steam network targeting

After developing the network, the system could have the maximized mass flow rate of the steam passing from each turbine. Consequently, the mass flow rate of steam from each steam header to another is presumed to vanish completely. By investigating the potential of power generation when the power network is functioning adequately, the fuel consumption of the system can be appropriately targeted. In order to reach this target, the system should be designed in a way that sets the mass flow variation in each header in accordance with the degree of superheating of each header. In the targeting of each header, the mass balance equation can be used to calculate the flow rate of steam passing from each turbine. The relation is as follows [24]:

After calculating the mass flow rate of steam passing through the turbine, the energy balance can be employed to the low-pressure header  in order to determine the steam enthalpy at the outlet of the turbine (or at the inlet of the low-pressure header) [38]:

According to Ref. [24], the enthalpy at the outlet of the steam turbine can be determined after computing the maximum steam turbine efficiency using the following formula:

After determining the enthalpy, the new header temperature should be used for re-targeting steam consumption and achieving updated values of steam mass flow rate. This process is iterated until finding out the required properties at all headers. Throughout calculating the steam temperature at the high pressure, the temperature should not exceed the tolerable limit for the steam network system (570 ). A trial-and-error approach should be used in this process until the fulfillment of the constraints. Fig. 2. illustrates the targeting algorithm for the cogeneration system in the steam network.

Figure 2: Algorithm flowchart for steam network targeting for the proposed cogeneration system in the steam network [39].

Some of the necessitated data for simulation of the steam network are listed in Table 5. Besides, Table 6 shows the mass flue rate of steam produced and consumed by the site-wide utility.

Table 5: Key inputs for simulation of the steam network [24]

Table 6: Design considerations for simulation of the steam network [24]

3.5. Key performance criterion

The target of the devised system of this study is proposing a hybrid polygeneration system integrated with a total site polygeneration system. Thus, there are other products in this system besides the heat and power that should be taken into account for computing efficiencies.

The energy efficiency of the standalone system as the topping cycle which is not integrated with SN operated by biogas and solar energy can be written as follows:

Where LHV is lower heating value and Ẇ_{Net,standalone} represent the net power output of the standalone system.  The energy efficiency of the proposed integrated cogeneration system with CCS and hydrogen production fueled by biogas, solar energy, and NG can be expressed as follows:

Finally, the power/heating ratio for the cogeneration system can be expressed as:

4. Model Validation

In this study, the performance of the devised multi-generation system is established in Engineering Equation Solver (EES). The validity of this model is evaluated by reviewing the existing computational and experimental results found in the literature. In order to verify the model devised for the reformer, the predicted syngas composition is compared with the data reported by Su et al. [25] study (Table 7). CST-tower unit is validated by a comparison between the outlet temperature strategy of the fixed receiver in the present study, and Ref. [30] (Table 8). From these results, it can be concluded that a good correspondence is established between the model of this study and recent experimental data found in the literature.

At last, the results of the simulation of the steam network are compared with those of Sun et al. [24]. The outcomes of this validation for mass flow rate and power are demonstrated in Tables 9 and 10, respectively. It can be concluded from this comparison that there is a good agreement between the results of the two studies.

5. Results and discussions

5.1. Modeling results

Results of simulation for steam network targeting are illustrated in Fig. 3. In this simulation, it is assumed that the maximum steam temperature is 570 . All determining parameters such as the mass flow rate, temperature, pressure, and output power of turbines are taken into account. It can be deduced from Fig. 4 that the heat recovery unit has a steam consumption rate of 51.35 kg/s, which is provided by products of Steam turbines 1, 2, and 3 are 11.05, 7.481, and 6.956 MW, respectively. 14.69 kg/s steam is generated subsequently.

Figure 3: Results of simulation for steam network targeting for a maximum steam temperature of 570 .

Along with Fig. 3, some of the other results of energy analysis of the devised system are listed in Table 11.  It can be concluded that using the SN increases the energy efficiency by about 9% in comparison with systems is increased by approximately 9% compared to the standalone multigeneration system. The devised system also has a heating load of 238.087 MW and net electricity of 57.634 MW, which makes it a suitable option for energy generation purposes (Table 11). The net electricity/heating ratio is also noticed in this study. The value of this variable is obtained at 0.2642, which shows that despite the surplus heating generated, the power generation still has superiority over heating.

Table 11: Final results obtained from simulation.

5.2. Parametric study

In this sub-section, the impact of main parameters on the energy efficiency and power/heat ratio is investigated through a parametric study. The considered chief parameters include biogas flowrate, operating conditions of the reformer (T_r and P_r), and pressure of headers in the SN unit.

5.2.1. Biogas flowrate

The impact of biogas flowrate on the net power/heat ratio and energy efficiency is demonstrated in Fig. 4. According to Fig. 4, as the biogas flow rate goes up, the net power/heat ratio is increased. While the biogas flow rate changes from 5 to 15 kg/s, the net electricity of the power cycles increases by 86.6%. On the contrary, the net electricity of the steam network shows no alteration and remains constant. The reason is that the Q_consumption in the steam network is not dependent on the biogas flowrate alterations. It can be concluded from this figure that with rising up of the net power/heat ratio, the energy efficiency declines exponentially and goes from 78.63% to 49.86%. The explanation could be as follows: when the biogas flow rate increases, the input heat energy increases too -similar to output work; but as the ratio of input increase is higher, energy efficiency is dropped.

Figure 4: Effect of the biogas flowrate on energy efficiency and power/heat ratio

5.2.2. Reformer temperature

For this investigation, the temperature of the reformer is changed from 923 to 1173 K, and other parameters are considered constant. The impact of the reformer temperature on energy efficiency and net power/heat ratio is shown in Fig. 5. It can be seen that both of these parameters decrease as the reformer temperature goes up. However, the reduction rates are different since the net power/heat ratio decreases at a higher pace than the energy efficiency. This observation can be explained as follows. The net electricity of the power cycles decreases as the reformer temperature goes up. In this temperature range, the output power decreases by 17.2%. Besides, the generated heat for heating purposes declines about 42.5% resulting in a net power/heat ratio decrease.

Figure 5: Effect of the reformer temperature on energy efficiency and power/heat ratio

5.2.3. Reformer Pressure

To analyze the impact of the reformer pressure on energy efficiency and net power/heat ratio, the biogas pressure is increased 6 bar from 1 to 7 bar, while other parameters have remained unvaried (Fig. 6). The generated power increases with the increase of the reformer pressure. Because of the increase in the reformer pressure, the generated power of the cycles increases by 28.4%. In the meantime, carbon capturing increases considerably, while the production of hydrogen decreases about 31.4%.

Figure 7: Effect of the reformer pressure on energy efficiency and power/heat ratio

5.2.4. Steam network medium pressure

Based on analyzing the results shown in Fig. 8, it can be concluded that with increasing P_MP, both the energy efficiency and net power/heat ratio are decreased. According to this figure, by increasing P_MP from 15 bar to 25 bar, energy efficiency decreases about 1.5% and reaches 59.7%. The net power/heat ratio also is reduced and goes from 0.269 to 0.257. The following reasons can describe these trends. When P_MP is increased, expansion ratios of upper turbines decrease, and therefore the power of steam turbines and heating load are lowered. This process results in a decrease in energy efficiency and R_Cog, because of the higher role of net electricity.

Figure 8: Impact of the steam network medium pressure on the net power/heat ratio and energy efficiency

5.2.5. Steam network high pressure

The impact of steam network very high pressure on energy efficiency and net power/heat ratio can be deduced from Fig. 9. Based on this figure, an increase of the high-pressure results in a decrease of both of these parameters. When the high pressure is increased by 2 bar and reaches 5 bar, the energy efficiency is slightly decreased by 1.5% and goes to 59.7%. net power/heat ratio goes from 0.267 to 0.260. The following reasons can be enumerated for such a trend. As HP rises up, the power of steam turbines decreases along with the heating load; accordingly, the net power/heating ratio declines.

Figure 9: Effect of the steam network high pressure on energy efficiency and power/heat ratio

5.2.6. steam network Very high pressure

Fig. 10 is a depiction of the impact of the steam network's very high pressure on energy efficiency and net power/heat ratio. It can be concluded from this figure that both of the parameters mentioned above decrease with the increase of very high pressure. As the energy efficiency goes smoothly from 61.2 to 59.7 %, the net power/heat ratio is not changing noticeably. It is worthy of pinpointing that the heating load and power of GTC and ORC unit have remained unvaried through this analysis, and hence the trend of the net electricity of the devised multi-generation system, , and energy efficiency follows that of the steam turbine's power.

Figure 10: Effect of the steam network very high pressure on energy efficiency and power/heat ratio

6. Conclusions

In this study, an innovative multi-generation energy system based on biogas and solar energy is proposed. Besides, this system also is devised for hydrogen concurrently benefited from a CCS unit and also is designed for site-wide utility for heat recovery and heat and power generation for meeting the necessitated energy of the system. In order to produce the steam needed in the SN unit, NG is utilized as a primary fuel. Solar thermal energy, absorbed by a CTS-tower plant, is utilized in the processes which are taken place in the reformer. The necessitated heat for steam is made available through these processes. Furthermore, CO₂ and H₂ of the reformer exhaust syngas are retrieved by CCS and PSA, respectively. In this study, the energetic analysis is carried out on the devised system. Biogas flowrate, reformer temperature, reformer pressure, and steam network in the medium, high, and very high-pressure levels are among the parameters analyzed in this study. The following is the summary of the final results:

• The efficiency is increased from 51.72% to 60.55% through the integration of the steam network system with the topping system.
• Consequently, an increase from 32.147 MW to 57.634 MW is seen in the net power.
• A decrease in CO₂ conversion is visible when the reformer temperature is increased.
• As the reformer pressure rose up, overall net power output and captured carbon dioxide increases, respectively.
• Adjusting the reformer pressure design is a solution for maximizing the net power/heat ratio.
• These alterations increase the devised system efficiency: decreasing biogas flow rate, reformer temperature, P_MP, or P_HP of the SN unit. Also, increasing reformer pressure and P_VHP of the SN unit increases energy efficiency.