Elin Svensson

Benefits of using an optimization methodology for identifying robust process integration investments under uncertainty--A pulp mill example

This paper presents a case study on the optimization of process integration investments in a pulp mill considering uncertainties in future electricity and biofuel prices and CO2 emissions charges. The work follows the methodology described in Svensson et al. [Svensson, E., Berntsson, T., Stromberg, A.-B., Patriksson, M., 2008b. An optimization methodology for identifying robust process integration investments under uncertainty. Energy Policy, in press, doi:10.1016/j.enpol.2008.10.023] where a scenario-based approach is proposed for the modelling of uncertainties. The results show that the proposed methodology provides a way to handle the time dependence and the uncertainties of the parameters. For the analyzed case, a robust solution is found which turns out to be a combination of two opposing investment strategies. The difference between short-term and strategic views for the investment decision is analyzed and it is found that uncertainties are increasingly important to account for as a more strategic view is employed. Furthermore, the results imply that the obvious effect of policy instruments aimed at decreasing CO2 emissions is, in applications like this, an increased profitability for all energy efficiency investments, and not as much a shift between different alternatives.

Tailoring Charge Recombination in Photoelectrodes Using Oxide Nanostructures

Optimizing semiconductor devices for solar energy conversion requires an explicit control of the recombination of photogenerated electron-hole pairs. Here we show how the recombination of charge carriers can be controlled in semiconductor thin films by surface patterning with oxide nanodisks. The control mechanism relies on the formation of dipole-like electric fields at the interface that, depending on the field direction, attract or repel minority carriers from underneath the disks. The charge recombination rate can be controlled through the choice of oxide material and the surface coverage of nanodisks. We provide proof-of-principle demonstration of this approach by patterning the surface of Fe2O3, one of the most studied semiconductors for light-driven water splitting, with TiO2 and Cu2O nanodisks. We expect this method to be generally applicable to a range of semiconductor-based solar energy conversion devices.

Converting a kraft pulp mill into a multi-product biorefinery: techno-economic analysis of a case mill

In this case study, we investigated the conversion of an existing Swedish kraft pulp mill to the production of dissolving pulp, with export of electricity, lignin, and a hemicellulose stream suitable for upgrading. By increasing the level of heat integration of the mill, it was possible to achieve self-sufficiency in terms of steam and to produce significant amounts of excess steam. The excess steam could facilitate the integration of a lignin separation plant or be used for power generation. The production of dissolving pulp requires a higher input of wood that is required for the same level of pulp production as is achieved with kraft pulp. For the studied mill, the batch digester was the main limitation for pulp production. Nevertheless, if the digester capacity was increased, then the level of pulp production could be maintained. In addition, the recovery boiler, causticization plant, and evaporation plant had sufficient capacities for preserving the same production level upon conversion, and could easily be upgraded to a certain degree through relatively simple measures for an increase in pulp production. However, increasing pulp production beyond that limit required extensive upgrades or investments in new equipment, which negatively affected annual earnings. Annual earnings were found to be also dependent upon the level of heat integration, type of by-product, and the costs for lignin and electricity. However, our results suggest that the optimal process configuration is more dependent upon other factors, such as the long-term vision of the company and policy instruments.

High Solids Loading in Ethanol Production – Effects on Heat Integration Opportunities and Economics

Production of lignocellulosic ethanol still suffers the drawback of high production cost. Methods for making the process more cost efficient are therefore of high importance for commercialising large-scale production. One way of improving the process economics is to ensure efficient heat integration within the process, thereby reducing process energy costs. Another opportunity to improve production costs is to design the process for high solids loading in the bioreactors. By operating the process with higher solids concentrations (so-called high-gravity), water flows through the process will be reduced, which would enable smaller equipment and lower energy demand for downstream separation and purification of the ethanol product. However, problems with pumping and mixing are also likely to occur due to higher viscosities. Furthermore, the yield will be lower due to increased concentration of inhibitors. In addition, the opportunities for heat integration within the process will be affected. In this paper, the effect on heat integration opportunities of the high gravity concept is investigated for ethanol production in a stand-alone process in which fractionation of the softwood raw material is assumed to be achieved with steam explosion. Estimations of how the process economics will be affected are also provided.

Optimal Multi-Period Investment Analysis for Flexible Pulp Mill Utility Systems

The techno-economic potential for new technologies in pulp mills is conventionally studied assuming annual averages, based on the assumption that the pulp mill is continuously operated at a constant production rate close to its design capacity. Recent work has shown, however, that a technology, such as lignin extraction, which can improve the operating flexibility of the pulp mill utility system, has a value associated with its flexibility that is not captured in such average-value models. These previous results indicated a risk of significant errors if heat load variations were not properly modelled. This paper presents a multi-period optimization model for the planning of design and operating decisions connected to pulp mill utility systems, which in this work has been extended with back-pressure and condensing turbine models. The model optimizes technology selection, equipment capacities and operating loads under the influence of demand variations, considering part-load efficiencies and operating load limits. Application of the model to an illustrative example showed that lignin extraction can compete with electricity production already at a lignin price of 22 €/MWh in the presence of variations due to poor off-design performance for the turbines. This demonstrates the usefulness of the proposed modelling approach.

Operational Flexibility in Pulp Mill Steam Production at Off-design Heat Loads

This paper focuses on the steam production in a chemical pulp mill that is retrofitted to reduce its process heating demand. A multi-period optimization model for design decisions is proposed that takes into account the operational limits of the steam production units as well as the heat load variations over the year. Large variations in combination with the retrofit that causes off-design loads in the steam production system will influence the flexibility of the steam system. Minimum boiler load limits will then be a greater constraint on operation since the average load of boilers is moved closer to the minimum for longer periods of time. A conventional approach that considers fixed annual averages of process parameters therefore risks leading to sub-optimal solutions because of neglecting the variations in heat demand and the operational limits. The multi-period approach suggested in this paper considers this operational flexibility associated with different design choices. A case study based on a Kraft pulp mill with a recovery boiler and a bark boiler shows the benefit of properly modelling the varying heat demand. Numerical results are presented that compares the results of the multi-period model with that of a conventional annual average approach. Differences in design decisions, energy balances and economic performance are demonstrated and discussed.