Thermo-economic and environmental model of microalgae-to-SNG conversion process through hydrothermal gasification

Due to its high photosynthesis conversion efficiency, microalgae are currently considered as a serious feedstock for next-generation biofuels production. One possible conversion process is the production of synthetic natural gas (SNG) through catalytic hydrothermal gasification which uses a supercritical water-processing for the gasification and has the advantage to avoid drying of the biomass. The whole system includes four steps of production: microalgae cultivation, dewatering, gasification and product separation. In this paper the SNG production from microalgae is analyzed considering algal biomass cultivation system with open-ponds technology. To do so, a mathematical model based on mass and energy balances has been developed in order to represent microalgae growth process, and regarding culture temperature, solar-radiation incidence, CO2 and nutrients supply. The complete physical model of the biomass conversion system is developed including the dewatering system with settling ponds and centrifuges, catalytic hydrothermal gasification with salt separation unit and the SNG separation and purification system. An economic model is carried out for investment and operation cost of the process, from the biomass cultivation to the final product, including possibilities of nutrients, CO2 and water recovery. Finally, the environmental impact regarding the CO2 emission of this process and the avoided emission from the replacement of fossil natural gas is estimated in a complete life cycle analysis. A multi-objective optimization methodology is used to find the trade-off between the total cost of the system and the environmental impact that can be linked with the equipment size and the energy conversion efficiency.

Next generation cogeneration system for industry - Combined heat and fuel plant using biomass resources

Ayse Dilan Celebi, François Maréchal, Adriano Viana Ensinas

The requirement for sustainable development has prompted the researchers to explore solutions for better utilization of renewable energy resources in the future. Biomass is a promising resource and it can be converted to multiple products and services including fuels, chemicals, heat and electricity via different conversion routes. Hence, replacement of fossil-based services with biomass-based services is critical to mitigate fossil CO2 emissions, and innovative design of new and efficient energy conversion systems is necessary. Different industries need heat for their operations at different temperature levels. Today, these demands are satisfied using conventional natural gas boilers by imposing a CO2 tax to account for their emissions. In this study, we discuss the potential of replacing conventional boilers with a combined heat and fuel (CHF) plant design which utilizes lignocellulosic biomass in thermochemical conversion to generate heat for different industrial sectors together with biofuels cogeneration. Heat is generated due to the exothermic nature of the thermochemical conversion processes that operate at high temperatures. Gasification process produces syngas which is converted into fuels such as synthetic natural gas, Fischer-Tropsch crude, methanol and dimethyl ether and electricity. Different scenarios are evaluated considering the CO2 produced via this system is either released, sequestrated, or stored and used in a co-electrolysis unit in which surplus renewable electricity available during summer is converted into additional syngas. A parametric analysis has been performed considering type and size of plants, CO2 tax, and purchase and transportation costs of wood to compare the price of heat for the industrial sectors. Natural gas and wood boilers are used as the basis to calculate the breakeven CO2 tax values for the same heat prices for the proposed CHF systems. The results of this study present a state-of-the-art renewable energy system as an alternative to conventional boilers. (C) 2019 Elsevier Ltd. All rights reserved.

2019

Optimal Design of Solar Assisted Hydrothermal Gasification for Microalgae to Synthetic Natural Gas Conversion

Gianluca Ambrosetti, François Maréchal, Alberto Mian, Adriano Viana Ensinas

Catalytic hydrothermal gasification is a promising technology which allows the conversion of wet biomass into methane rich syngas. It consists of three major steps, in which thermal energy has to be supplied at different temperature levels, leading to multiple products, such as clean water, nutrients/salts and methane rich syngas. Microalgae have an important potential as a new source of biomass, principally due to the fact that they can grow much faster than others biomass feedstock available in nature. Considering the energy balance of the algae cultivation step, the gasification process and thecrude product upgrading step, part of the converted syngas has to be used to close the energy balance. In this context, solar heat can be considered as an alternative to replace the heat that has to be generated from product or crude product burning. This would lead to higher fuel production, higher carbon conversion efficiency and in general a better sustainable use of energy sources. In this paper, the goal is to show the integration potential of solar thermal energy use in the catalytic hydrothermal gasification of microalgae. In order to maximize the fuel production, thermal energy requirements of the gasification and SNG upgrading process can be generated in concentrating solar systems, coupled with thermal energy storage. This allows to continuously provide heat for the process at different temperature levels. A superstructure of design models will permit the estimation of the optimal size and integration of the solar utility for different process configurations. The optimal design configurations are evaluated by solving a multi objective optimization problem which aims at the maximization of conversion efficiency and the minimization of operating and total production costs. Copyright © 2013, AIDIC Servizi S.r.l.

2013

Increasing Conversion Efficiency in Fuel Ethanol Production from Lignocellulosic Biomass by Polygeneration - and a Paradoxon between Energy and Exergy in Process Integration

Martin Gassner, François Maréchal

In the public and scientific debate on biofuels, ethanol from lignocellulosic biomass is generally the most popular alternative that may allow for a sustainable production. Compared to thermochemical processing of biomass which assures a complete conversion of the feedstock, it yet suffers from an inherently lower fuel yield due to the resistance of lignin to biological degradation. Based on a recently developed process model for fuel ethanol production from lignocellulosic biomass, this paper discussed the cogeneration alternatives for the conversion of the residual lignin. Whereas an integrated gasification combined cycle (IGCC) increase the power cogeneration efficiency compared to the conventional combustion and power generation in a steam Rankine cycle, it is shown that alternative gasification and methanation to Synthetic Natural Gas (SNG) allows for roughly doubling the fuel yield from biomass. The paper further demonstrates the paradox situation that conventional energy recovery is limited by the available energy, and not, as usually, the available exergy from the waste heat. In order to overcome this limitation, a more general energy integration approach that allows for increasing the cogeneration efficiency in this kind of situations is proposed.