Process Integration in Model Kraft Pulp Mills. Technical, Economic and Environmental Implications

Doctoral thesis, 2003

The technical and economic conditions for reducing the energy consumption cost-efficiently and thereby releasing a surplus of biomass fuel in the pulp and paper industry are changing. For example, new environmental restrictions increase the need for closed water loops and other system modifications, which can lead to increased opportunities for energy savings. Closing the water loops can however require that new processes be developed to remove non-process elements, and those processes are likely to increase the energy consumption. There has also been a deregulation of the power market, which creates new incentives and opportunities for cogeneration of electric power in the pulp and paper industry. Finally, energy policy instruments such as CO2 taxes, CO2 trading and long-term agreements between government and industry are intended to increase the incentive for industry to invest in energy saving projects. As a result of these new conditions, the degree of energy efficiency is likely to become a more strategic issue in the future for pulp mill operations. In order to achieve optimal cost-competitive operation, there is a need for a systematic approach to the system consequences when making the above-mentioned changes, also taking into account external factors, such as cost of fuel, power, etc. Given this background, the objective of this thesis has been to perform overall energy system analysis for several green field mills and scenarios. The emphasis has been on understanding how the energy saving potential changes under different circumstances in the mill, i.e. the amount of warm and hot water produced, type of digester, etc. The results show that there is between 1.0 and 2.2 GJ/air-dried metric tonne (GJ/t) of excess heat available in the model mills evaluated. Depending on the mill configuration, different amounts of live steam saving can be achieved, varying between 12 and 21% of the total live steam demand.