Multi-Objective Optimization of Biomass Conversion Technologies by Using Evolutionary Algorithm and Mixed Integer Linear Programming (MILP)

The design and operation of energy systems are key issues for matching the energy supply and consumption. Moreover, in the present context of finding ways to decrease CO2 emission, poly-generation technologies, together with the integration of renewable energy resources, have a high potential for CO2 emission reduction. An optimisation model and systematic procedure to select, size and operate a poly-generation plant are developed and presented in this paper. In the optimisation model the integration of biomass resources is mainly investigated. Several options for integrating biomass in the energy system, namely back pressure steam turbine, biomass ranking cycle (BRC), biomass integrated gasification gas engine (BIGGE), biomass integrated gasification gas turbine, production of synthetic natural gas (SNG) and biomass integrated gasification combined cycle (BIGCC), are investigated in this paper. The goal is to minimize costs and CO2 emission simultaneously with a multi-objective evolutionary algorithm (QMOO) and a Mixed Integer Linear Programming (MILP). Finally the proposed model demonstrated by means of a case study. The results shown the simultaneous production of electricity and heat with biomass and natural gas are reliable upon the established assumptions. Besides, higher primary energy savings and CO2 emission reduction are obtained through the gradual increase of renewable energy sources compare to the natural gas usage.

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.

2010

Optimal use of biomass in large-scale energy systems: insights for energy policy

Victor Codina Gironès, François Maréchal, Stefano Moret, Marco Nasato, Emanuela Peduzzi

Biomass chemical conversion processes allow the production of solid, liquid and gaseous biofuels, which can substitute almost any kind of fossil fuel and the associated greenhouse gas emissions. Despite this potential, high investment costs and conversion losses reaching up to 30-40 % of the input biomass energy content are major barriers to a higher penetration rate of the chemical conversion processes. Thus, biomass is nowadays predominantly used in direct combustion processes. However, conversion losses of chemical processes may be compensated by the fact that biofuels can be used in more efficient technologies for supplying energy services compared to standard raw biomass fuelled technologies. As an example, Synthetic Natural Gas (SNG) can be used in a cogeneration-heat pump system to produce heat, reaching an overall efficiency much higher compared to a wood boiler. In this work biomass conversion options are compared taking into account the complete energy conversion pathway, from the resource to the supply of energy services. The comparison is performed by evaluating the CO2 abatement potential of integrating these different pathways into a national energy system with a Mixed-Integer Linear Programming (MILP) modelling approach. The comparison is done with 56 scenarios, which are classified in two different groups. Scenarios belonging to the first group consider the choice of the biomass chemical conversion process as the only possible change in the system. In the second group, other changes are allowed in the energy system, such as an important deployment of efficient technologies. Results show that biofuels can allow for an overall better performance in terms of avoided CO2 emissions compared to direct combustion. This potential, however, depends on a wider deployment of efficient end-use technologies.

University of Ljubljana2016

Scenario modelling and optimisation of renewable energy integration for the energy transition

Victor Codina Gironès

A large number of countries have engaged themselves in an energy transition towards more re- newable energy in their energy systems. Motivations stem mainly from the need to reduce CO2 emissions, and from a desire of their population to phase out technologies such as nuclear. Most of these countries promote biomass, wind and solar energy sources, among other possibilities. How- ever the current rate of deployment of renewable energy systems globally is not sufficient to reach the CO2 emissions reduction that would allow to maintain the global average temperature increase below the 2°C threshold. The main barriers to a wider integration of renewable energy systems are i) their limited realisable potential, ii) their still limited competitiveness, iii) their intermittence; iv) public acceptance often related to poor level of energy literacy amongst citizens. Citizens are key decision-makers. They must decide on energy policies and on the energy technologies they use, hence they have the power to foster or halt the energy transition. This thesis presents two different strategies for addressing the problem of the integration of re- newable energy sources for energy transitions. The first one (Chapter 1) consists in developing an energy modelling tool to help decision-makers understand the energy system and find their own answers. The modelling approach also includes a new methodology for the calculation of the total cost of a national energy system. A model of the Swiss energy system has been created following this approach, which serves as basis to develop the Swiss-Energyscope online calculator. This calculator and its model present an optimal trade-off between scientific rigour and user-friendliness, which allows the reproduction of the energy transition scenarios conceived by the Swiss Government, and consequently its use for energy policy making. The second strategy (chapters 2 and 3) profits from the possibilities offered by mathematical mod- elling and optimisation to analyse national energy systems, and derive insights for policy and decision-makers. First, a methodology using a mix-integer linear programming (MILP) model analyses biomass usage pathways to determine its optimal use in Switzeland in 2035. Second, in order to study the role of biomass, non-linear optimisation is applied to create future scenarios. (Chapter 3) focuses on the solutions to deal with the variability of renewable electricity. To this end, a MILP model with hourly time resolution is conceived to study the use of flexible electricity supply and demand options for the integration of renewable electricity. The optimisation methodologies are validated on case studies for the Swiss energy system. Regarding biomass, the results reveal that woody biomass chemical conversion technologies can allow for an overall better performance in terms of CO2 avoided emissions compared to direct combustion, as long as the produced biofuels are used in efficient technologies. Results also show that the combination of the gasification-methanation process of woody biomass with the production of H2 produced from excess electricity would allow to reduce the Swiss natural gas imports to zero by 2050. Concerning the integration of variable renewable electricity, the cost difference between using flexible electricity supply- and demand-options or electricity imports to deal with variable renewable electricity is below 2.5% of the total cost of the energy system.