Analysis of a biorefinery integration: application to a sulfite wood pulping process
The pulp and paper (P&P;) industry in industrially mature countries is facing a long-term crisis. Large and modern manufacturing facilities in tropical regions emerge as new competitors. The constant increase of energy costs has also contributed to the financial difficulties encountered in the industry. To stay viable and sustainable, P&P; mills need to elaborate new strategies to increase their revenues. In this context, converting a P&P; mill into an integrated forest biorefinery could be the solution to restore the health of the industry and overcome the crisis. Diversifying the industry's product mix by producing bio-energy and bio-materials could be a way to penetrate new markets while maintaining its core production of pulp and paper. Producing bio-materials implies energy consumption and waste heat. The energy management cannot therefore be dissociate from the biorefinery integration in a P&P; mill: both energy and mass integration must be considered. The objective of this work is to illustrate the trade-off between conversion of materials and of energy and shows the importance of considering simultaneously the heat recovery through heat exchangers and the combined heat and power production. Process integration techniques have been applied to study a bisulfite mill which produces pulp and three additional by-products: bioethanol, lignosulfonate and yeast. Particular attention was devoted to the integration of the chemical recycling loops which have a significant impact on the process energy balances and therefore influence the choice of biorefinery integration strategies. A key element of the process integration pathway was to consider the heat cascade as a model of the heat exchanger network by decoupling the installed heat exchangers network when analysing the potential effects of the recycling and production strategies at the plant level. Using this model, the optimal integration of a combined heat and power production unit was determined taking into account the simultaneous maximisation of process internal heat recovery. To facilitate the computational formulation of the problem, the overall process was subdivided into several production sub-systems with flow diagrams and flow rates dependant on the main product being manufactured in each sub-system. Mass balance constraints were introduced to model the flow distribution between sub-systems and the hot and cold streams were computed by means of a conventional flowsheeting software. The corresponding hot and cold streams were subsequently integrated at the mill scale level. The pulp production line and energy conversion equipment such as the sulphur boiler, lignin boiler and the biomass boilers were simultaneously optimized.