New ambitious targets for reduced greenhouse gas emissions and increased energy efficiency in industry and in the stationary energy sector provide incentives for industrial plants to investigate opportunities for substantially increasing recovery and use of excess heat from their operations. This work investigates the economic feasibility of recovering industrial excess heat at a Swedish chemical complex site for increased site internal heat recovery or export to a regional district heating (DH) network. The work is based on investment cost data estimated in previous work by the authors. A site-wide heat collection and distribution system based on circulating hot water was envisioned, which is also connected to a regional DH network. With the help of multiobjective optimization, the optimal heat contributions from the individual plant sites were identified that minimize the total system cost for a large range of options involving different quantities of internally recovered heat and heat export to the DH system. A payback period analysis was conducted together with a risk assessment to take into account uncertainty regarding utility steam production cost and heat sale price. The results of the study indicate that a payback period of around 3 years can be achieved for a number of cases in which 30% to 50% of the total excess heat produced by the site plants is recovered. Although it seems more profitable to recover heat at the site rather than exporting heat to the DH system only, profitability appears to be maximized by hybrid solutions that allow a share of the excess heat to be sold to the DH system and some heat to be recovered at the site simultaneously.

In Sweden, over 50% of building heating requirements are covered by district heating. Approximately 8% of the heat supply to district heating systems comes from excess heat from industrial processes. Many studies indicate that there is a potential to substantially increase this share, and policies promoting energy efficiency and greenhouse gas emissions reduction provide incentives to do this. Quantifying the medium and long-term economic and carbon footprint benefits of such investments is difficult because the background energy system against which new investments should be assessed is also expected to undergo significant change as a result of the aforementioned policies. Furthermore, in many cases, the district heating system has already invested or is planning to invest in non-fossil heat sources such as biomass-fueled boilers or CHP units. This paper proposes a holistic methodological framework based on energy market scenarios for assessing the long-term carbon footprint and economic benefits of recovering excess heat from industrial processes for use in district heating systems. In many studies of industrial excess heat, it is assumed that all emissions from the process plant are allocated to the main products, and none to the excess heat. The proposed methodology makes a distinction between unavoidable excess heat and excess heat that could be avoided by increased heat recovery at the plant site, in which case it is assumed that a fraction of the plant emissions should be allocated to the exported heat. The methodology is illustrated through a case study of a chemical complex located approximately 50 km from the city of Gothenburg on the West coast of Sweden, from which substantial amounts of excess heat could be recovered and delivered to heat to the city's district heating network which aims to be completely fossil-free by 2030.

This study compares value chains based on domestic forest biomass for the production of bio-synthetic natural gas (SNG) with respect to economic performance, GHG emissions, and energy efficiency. Value chains in which raw material is upgraded to intermediate products before transportation to an SNG plant integrated with a district heating system for further upgrading are compared with a chain in which the raw material is transported directly to the SNG plant. The intermediates considered are either dried biomass from forest residues, or bark, upgraded at pulp mills, or pellets from sawdust upgraded at sawmills. The findings show that the difference in performance between the studied value chains is generally small. The highest cost and significantly lowest energy efficiency are associated with the value chain with pellets, which leads to the conclusion that more pretreatment than what is required by the SNG process, to lower transport costs, is not profitable. Drying forest residues at pulp mills before further transportation to and upgrading at an SNG plant leads to somewhat higher transportation costs because of the relatively high fixed costs associated with transportation. However, the benefit of drying the biomass using excess heat at pulp mills is that heat is "moved" from a location, where it can be hard to find profitable ways to use it, to the SNG plant, where the excess heat can be used for district heating. With these two factors working in opposition, the total cost is similar if forest residues are transported directly to the SNG plant or via a pulp mill. The lowest cost is achieved when falling bark from pulp mills is used because the first transportation step is avoided and no additional investment for biomass handling at the mill is required. However, there is a technical uncertainty regarding how much bark can be used in the SNG process.