Techno-economic evaluation of hydrochar via hydrothermal carbonisation of organicresidues

Abstract: This thesis has investigated the techno-economic feasibility of upgrading the sludge from a chemical pulp mill to hydrochar via hydrothermal carbonization (HTC). The intended use of the hydrochar was to replace fossil coal within metallurgical applications in the iron and steel industry. Process models were developed in order to obtain mass and energy balances of the HTC process for different technical configurations. The balances were used to evaluate the economic performance, in terms of hydrochar production cost as well as different profitability parameters. Two main scenarios were investigated: Scenario-1: HTC process integrated with the pulp millScenario-2: Stand alone HTC process.To see the effect of having one or two HTC reactors, two cases were developed for each scenario, where the first case used only mixed sludge from the pulp mill as feedstock for the HTC process (case 1, one reactor), while the second case used both mixed sludge and bark as feedstock (case 2, two reactors). In scenario 1, the effects on the pulp mill’s mass and energy balances of integrating the HTC process were investigated. The results showed only very small impacts on the pulp mill, due to that the HTC process is significantly smaller than the mill. The total amount of steam to the steam turbine increased by 0.8 % and 0.9 %, for case 1 and 2, respectively. In combination with the removed sludge, which is otherwise combusted in the mill’s socalled power boiler, this entailed a total increase of the wood fuel consumption in the boiler by 3.2 % and 3.6 %, respectively. By implementing a second HTC reactor, the production cost of hydrochar could in the integrated scenario (scenario 1) be decreased from 4 600 SEK/ton (case 1) to 3 700 SEK/ton (case 2). The corresponding production costs in the stand alone scenario (scenario 2) amounted to 5 400 SEK/ton (case 1) and 4 200 SEK/ton (case 2), respectively. Both integration with the pulp mill and increasing the HTC production scale were thus found to be strategies that can lead to decreased hydrochar production cost. However, even the lowest production cost noted in this report is significantly higher than the corresponding price of coal. This indicates that other measures are required in order for hydrochar to become cost competitive to fossil coal in the metallurgical industry. Examples are the possibility to use even lower-cost feedstocks, as well as policy tools targeting, e.g., the CO2 emissions from using fossil materials and energy carriers in the iron and steel industry. Based on the results from the investment calculation, it is concluded that the HTC process integrated with a pulp mill is preferable compared to a stand alone HTC process. The reason why integrated HTC is preferred is that it gives higher NPV and correspondingly lower payback time, as well as lower hydrochar production costs.