گرمای اضافی کارخانه های تولید خمیر کرافت: تجارت بین مصرف داخلی و خارجی در مورد سوئد قسمت 2: نتایج برای سناریوهای بازار انرژی آینده

In this paper the trade-off between internal and external use of excess heat from a kraft pulp mill is investigated for four different future energy market scenarios. The work follows the methodology described in Svensson et al. [2008. Excess heat from kraft pulp mills: trade-offs between internal and external use in the case of Sweden—Part 1: methodology. Energy Policy, submitted for publication], where a systematic approach is proposed for investigating the potential for profitable excess heat cooperation. The trade-off is analyzed by economic optimization of an energy system model consisting of a pulp mill and an energy company (ECO). In the model, investments can be made, which increase the system's energy efficiency by utilization of the mill's excess heat, as well as investments that increase the electricity production. The results show that the trade-off depends on energy market prices, the district heating demand and the type of existing heat production. From an economic point of view, external use of the excess heat is preferred for all investigated energy market scenarios if the mill is studied together with an ECO with a small heat load. For the cases with medium or large district heating loads, the optimal use of excess heat varies with the energy market price scenarios. However, from a CO2 emissions perspective, external use is preferred, giving the largest reduction of global emissions in most cases.

With the growing concern for climate change and increasing energy prices, the need for energy efficiency measures has become more urgent. The Swedish pulp and paper industry, being one of the major Swedish energy users, has a potential for significant energy savings through investments in energy efficiency, such as process integration and new technology (Axelsson et al., 2006; IEA, 2004; Wising, 2003). One way of saving energy is to use industrial excess heat in a more efficient way. Industrial excess heat from kraft pulp mills can be utilized both internally, for example in process-integrated evaporation (PIvap)1 and/or drying of pulp, and/or externally for production of district heating. Previous research using model mills and simulations has shown that, with improved heat integration and installation of new equipment, substantial process steam savings (20–30%) can be made (Algehed, 2002; Axelsson et al., 2006; Wising, 2003). This potential has been verified by a case study (Laaksometsä et al., 2007). The energy efficiency investments that reduce the steam demand also lead to increased amounts of excess heat. The temperature and amount of excess heat depend on the combination of energy efficiency investments chosen (Axelsson et al., 2006; FRAM, 2005). Previously the economic profitability among different internal utilization options such as PIvap, using high-temperature excess heat, and new conventional evaporation (Convap) (no utilization of the excess heat) has been studied (Olsson et al., 2006).

The methodology presented in Svensson et al. (2008) for a systematic analysis of trade-off between internal and external use of excess heat and potential for profitable excess heat cooperation has in this study been used for analysis under different future energy market scenarios. From the results, bearing in mind the comments from the discussion, the following conclusions can be drawn: • Small systems show a larger potential for profitable excess heat cooperation and a larger robustness for the obtained solutions than medium and large systems (which have greater potential for large-scale biomass CHP). • For the scenarios with low CO2 charge and a low biomass price (1 and 3) the potential for profitable cooperation regarding the mill's excess heat is lower than for the scenarios with high CO2 charge and a high biomass price (Scenarios 2 and 4). • For the cases where internal use of the excess heat (no cooperation), together with investment in new biomass CHP, is preferred, both the electricity production and the biomass usage are larger than that for the cases where external use is preferred. • External use of the excess heat gives lower CO2 emissions than internal use for most of the studied scenarios. • The cases with internal use of the excess heat are somewhat more robust to changes in energy prices and distance between the mill and the district heating system.