Karsten Hedegaard

Economic and environmental optimization of waste treatment.

This article presents the new systems engineering optimization model, OptiWaste, which incorporates a life cycle assessment (LCA) methodology and captures important characteristics of waste management systems. As part of the optimization, the model identifies the most attractive waste management options. The model renders it possible to apply different optimization objectives such as minimizing costs or greenhouse gas emissions or to prioritize several objectives given different weights. A simple illustrative case is analysed, covering alternative treatments of one tonne of residual household waste: incineration of the full amount or sorting out organic waste for biogas production for either combined heat and power generation or as fuel in vehicles. The case study illustrates that the optimal solution depends on the objective and assumptions regarding the background system--illustrated with different assumptions regarding displaced electricity production. The article shows that it is feasible to combine LCA methodology with optimization. Furthermore, it highlights the need for including the integrated waste and energy system into the model.

Liquid Biofuels: We Lose More Than We Win

Throughout the world, nations are seeking ways to decrease CO2 emissions and to reduce their dependency on fossil fuels, especially oil, for environmental as well as geopolitical reasons. Being a renewable, CO2-reducing and easily storable energy carrier, biomass is a priority resource for fossil fuel substitution, and biomass is increasingly used for both the transport and the heat and power sectors, with increasing interest in using it for chemicals production as well. For the transport sector, the conversion of biomass to the liquid biofuels of bio-diesel and bioethanol is at present a technological pathway promoted by governments in many countries. With the increasing interest in our biomass resource, however, the issue of competition for the biomass and the need for prioritising it has become evident. For several decades ahead, we still depend heavily on fossil fuels, and we can only replace them to the extent and with the speed that alternatives become available. As the magnitude of biomass that is or can be made available for energy purposes is small compared to the magnitude of the new potential customers for it, any long-term and large-scale prioritisation of biomass for one purpose will imply a loss of alternative uses of the same biomass. If the lost alternatives are, then, significantly more efficient as well as economically more attractive in fossil fuels substitution and CO2 reduction, we lose more than we win. It is our claim that this is the case for most liquid biofuels, including first-generation bio-diesels (plant bio-diesels) as well as first- and second-generation bioethanols produced in Europe and the USA. When we prioritise biomass for these biofuels, we deprive ourselves the better alternative of using the same limited biomass for heat and power and running our cars on the fuels saved there.