Hydrothermal and Supercritical Water Processes
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Results: Thermodynamic model was validated with experimental data at different operating conditions. Conclusions: Increase in reaction temperature shows the positive effect on hydrogen mole fraction in product gas mixture. However, increase in pressure leads to decrease in hydrogen mole fraction in product. Maximum hydrogen concentration in the range of 0. Lower feedstock concentration of aqueous HTL organics is observed to lead to high hydrogen production.
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Citation and Abstract. Article Metrics PDF: The total investment cost is calculated using multiplication factors to consider indirect expenses such as labor, transportation, fees, contingencies and auxiliary facilities. The production cost is the sum of the operating cost and the depreciation cost, which is the total investment cost annualized.
The main economic assumptions in the economic analysis are shown in Table 2. Environmental model : uses the mass streams simulated in the Belsim Vali model and equipment sized in the economical model to calculate the environmental impact using the impact data developed by the IPCC Intergovernmental Panel on Climate Change , which considers a time horizon of years for the GWP global warming potential.
The environmental performance indicators in this methodology are calculated while solving the optimal design problem according to Gerber Gerber, In this context, a LCA model is defined to consider the impact contributions from the involved material and the energy flows during the operation phases and environmental effects related to the construction, operation and end of life of the process equipment.
The proposed SCWG process was evaluated by performing a multi-objective optimization using an evolutionary algorithm Molyneaux et al. The optimization algorithm consists of a bi-level optimization. The optimization sub-problem considers the linearized costs and is used to assure the maximum energy recovery potential and minimal environmental impact among process streams. The optimal process configurations that result from this methodology are available in the Pareto curve, which allows displaying the trade-off of the best solutions with respect to the considered objective functions.
The set of points identifies the bounds of the feasibility region and the non-feasibility region all the region where the value of the objectives are lower in case of minimization or higher in case of maximization than the Pareto points.
The objectives defined for the multi-objective analysis were: i SNG production; ii annual cost; iii environmental impact. The multi-objective optimization problem aimed at maximizing the SNG production, minimizing the annual costs and environmental impact. To reduce the computation time, which may limit the initial population and number of evaluations that must be performed by the optimizer, the number of variables must be accurately selected. The decision variables that are considered in this study are reported in Table 3. An initial population of cases was selected, and maximum evaluations were set as stopping criteria.
To estimate the conversion potential of different SCWG plant configurations three main performance indicators are defined:.
Hydrothermal and Supercritical Water Processes
SNG production: was estimated by the model developed with the commercial flowsheet software: Belsim Vali Belsim Vali, Annual cost: was defined as the operational cost Eq. Environmental impact: the environmental impact life cycle analysis LCA method was employed. The economic performance of the proposed SCWG process was evaluated in terms of the total investment Eq. The results of the multi-objective energetic-economic-environmental problem are depicted in Figure 2 in terms of the Pareto curve.
The decision variables that correspond to the optimized plant configurations of three selected Pareto points Figure 2 are reported in Table 4. The more relevant energy flows and the related conversion efficiency are reported in Table 5 for three Pareto points.
Supercritical fluids - Supercritical fluids
The results of the optimization presented in the Pareto frontier Figure 2 show that, for the same biomass input of 20MW, the SNG production varies between 9. The curve trend shows that the increase in the efficiency reduces environmental impact due to the fact that the production of a renewable SNG is considered, replacing the same amount of fossil natural gas from the grid. The environmental impact is negative for all the optimum values presented in the Pareto curve and decreases even more with the increase of the efficiency, while requiring more investment.
As observed in Table 4 , an increase in the gasifier pressure leads to higher biomass-to-biofuel conversion due to the reduction on the amount of crude SNG that must be burned to close the thermal energy balance of the plant. The increase in the SNG production by increasing the temperature in the salt separation unit and increasing the pressure in the gasifier results in an increase in the annual cost of the SCWG process and a decrease in the environmental impact. The decision variables that most influence the present results are the maximum temperature in the salt separator and the temperature and pressure levels of the gasification reactor.
The Pinch point of the evaluated process, i. During the optimization the gasification temperature is driven to the lower bounds to allow maximizing the energy recovery between the salt separator output and gasification reactor input. This affects the net electricity balance of the process and, as a consequence, impact contribution and profitability.
nedpaleroodi.ml In addition, high-pressure levels slightly reduce the thermal energy demand of the SCWG process. The optimization of the steam cycle regarding the pressure levels and superheating temperature showed similar pressures, around 4. It is important to highlight that, for all evaluated configurations, no external fuel was necessary to supply heat for the SCWG process, only low-quality product from the SNG production and a part of high-quality SNG when necessary were used to close the overall energy balance. In the following sections, the energetic, economic and environmental performance indicators are presented to highlight the dependencies among these performance indicators and the comparison with literature data.
Thermal process integration for each optimized point of Figure 2 was performed. No external fuel was necessary at any evaluated point. It can be observed that the proposed SCWG process from leftover ginseng presented good thermal process integration by observing the reduction of the area between the hot and cold streams in the Carnot factor-heat load diagram. This occurs because the difference in the thermal requirement temperature almost completely fulfills its energy demand by the cooling demand of the final step of the SCWG process.
The energy demand of the SCWG process after thermal process integration is supplied by the cogeneration system using as fuel the waste gases, generated by the SNG cleaning system. The set of utility streams that was activated during the optimization is included in the reported heat cascade curves i. Regarding the utility integration, it is interesting to show how the steam network was fitted in the heat cascade curve by optimizing the pressure level and steam superheating.
The achieved results of this integration, which is calculated while optimizing the remaining hot and cold utilities, are reported in Figure 4. Steam superheating allows one to increase the mean thermodynamic temperature of the steam cycle, which increases the steam network efficiency. The Pinch Point activation is achieved in the obtained optimal solution. Concerning the energy efficiency of this system In the work of Mian et al. This configuration could also be analyzed considering the current proposed valorization route for Brazilian ginseng wet residue in order to diminish the crude product burning, but as already presented by Mian et al.
Therefore, in the present analysis this option was not taken into consideration. The economic evaluation is a challenging part of this study mainly because of the difficulties in estimating the costs of the SCWG plant at large scale.
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The literature nonlinear cost function, which was considered to define the capital expenses for the global plant, led to the Pareto curve depicted in Figure 2. This today represents a cost 2. Table 6 shows the breakeven price for SNG using different wet biomass sources.
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A reduction in breakeven price for SNG is possible when considering the SNG production inside a biorefinery site, as evaluated by Albarelli et al. In that extent, the integration of the SNG production with the previous phytochemical recovery processes could be a possible way to decrease production costs of SNG. Besides, this final approach would result in mutual benefits for each production plant.
In addition, environmental benefits of the substitution of fossil fuels should also be taken into account in a decision-making process for implementing this alternative SNG production technology. A subsidiary benefit of this process is the separation of the salts that can be further recycled as fertilizer for the growth of Brazilian ginseng roots. For the life cycle analysis LCA , the emission contribution for the IPCC07 Intergovernmental Panel on Climate Change 07 impact assessment method was considered and reported in terms of GWP global warming potential considering a time horizon of years as the environmental performance indicator.
The impact is analyzed for the global warming potential of the leftover ginseng conversion into SNG, including materials for construction of the process equipment. It must be highlighted that the impact of the process equipment is not so relevant in the final results, the operation of the system being responsible for the real gain in the GWP reduction.
It was considered that the biomass is produced locally; therefore, the impact of the feedstock logistics of a big plant is not an issue. The impact reduction increased with the increase in the SNG production. Impact reduction in the same range was found when wet algae was used as an input biomass Mian et al. The multi-objective optimization procedure was useful to identify the Pareto curve for an optimized supercritical water gasification SCWG process for production of synthetic natural gas SNG from wet leftover Brazilian ginseng roots after a two-step phytochemicals recovery process, accounting for the three objective functions: energy-economic-environmental.
To that extent, it was possible to identify the main decision variables affecting the set of optimal solutions and their influence on costs, emissions and energy performances. Besides that, the possibility of fossil fuel substitution makes this conversion process very attractive from the environmental point of view.