Biomass Gasification Integrated into a Reference Canadian Kraft Mill

L'integration de la bioraffinerie forestiere dans l'industrie des pâtes et papiers cree une augmentation de la demande d’energie. Comme l'industrie des pâtes et papiers est un des plus gros consommateurs de biomasse ligneuse au Canada, les dechets (ecorces, liqueur noire, etc.) de cette industrie peuvent etre utilises pour repondre a la demande energetique des usines de pâtes et papiers. Les ressources en biomasse peuvent egalement etre utilisees pour produire des biocarburants, de l'electricite et des produits chimiques de specialite. Le processus de gazeification est la methode la plus efficace pour utiliser le potentiel energetique de la biomasse. Dans cette etude, un bilan de masse et d'energie ainsi qu’une analyse economique et environnementale d’un procede de gazeification integre a une usine Kraft de reference ont ete realise. Pour ce faire, un modele d'un procede de gazeification comprenant une unite de sechage de la biomasse, une unite de gazeification ainsi que des unites de nettoyage des gaz a ete simule sur MATLAB puis sur Aspen Plus. Ce modele a par la suite ete lie a une simulation d’une usine Kraft de reference developpee sur la plateforme CADSIM Plus. Un modele thermodynamique d'equilibres modifies pour le gazeificateur de biomasse a ete developpe afin d’ameliorer la precision de la modelisation en equilibre. Le modele a ete evalue en utilisant aussi bien des donnees commerciales que des donnees provenant de la litterature. Deux etudes de cas pour la gazeification de la biomasse integree a usine Kraft de reference ont ete evaluees. La comparaison des donnees du modele d'equilibre avec les donnees experimentales disponibles a partir de circuler gazogenes de biomasse a lit fluidise a indique que le modele d'equilibre nettement sous-estimer le rendement en methane. Dans le modele modifie, un modele de degazage est utilise pour moderer l'ecart du methane base sur la reconnaissance du fait que le methane qui est initialement genere dans le processus de degazage de la biomasse, ne pas reformer par des reactions de gazeification. Un modele de devolatilisation generale semi-empirique est applique pour predire la composition des gaz volatils. Deux coefficients empiriques sont calcules a partir des donnees empiriques disponibles pour des experiences de degazage de la biomasse. Les atomes correspondants de carbone et d'hydrogene a partir du methane rendement predite par le modele de devolatilisation sont elimines du courant d'entree gazeificateur. Le modele modifie donne une meilleure estimation de la composition du gaz de synthese a partir d'un air souffle gazeifier de biomasse a lit fluidise circulant en comparaison avec les donnees d'equilibre et deux donnees d'equilibre modifies sont tires de la litterature. Dans le IV premier cas, le produit issu de la gazeification de la biomasse est utilise pour substituer le carburant fossile utilise dans le four a chaux. Dans le second cas, la gazeification de la biomasse a ete integree a un cycle combine (cogeneration). La faisabilite technique de ces deux scenarios a ete demontree. Une analyse economique pour les deux scenarios tenant compte du cout total en capital, des couts d'exploitation et des recettes et considerant egalement la reduction des taxes sur les emissions de CO2 a ete realisee. ---------- The integration of forest biorefinery into the pulp and paper industry introduces increased energy demands to the mills. As the pulp and paper industry is the largest woody biomass consumer in Canada, biomass resources such as bark, wood wastes, and black liquor that are used by pulp mills can be processed to supply energy demands of the mills and biorefinery units. Also, pulp and paper biomass resources can generate renewable biofuels, electricity, and chemicals. Gasification process is the most efficient method for utilizing the energy potential of biomass. The principle of this study is to perform mass and energy, economic, and environmental analyses for an integrated gasification process into a reference Canadian Kraft mill. To achieve this, a model of a complete gasification process including biomass drying, gasifier and gas cleaning units was simulated in MATLAB and Aspen Plus and then linked to a reference Kraft process that has been developed on the CADSIM Plus platform. A modified thermodynamic equilibrium model is developed to improve the prediction of syngas composition and heating values. Thermodynamic calculations are implemented based on non-stichiometric formulation to develop an equilibrium gasifier model. Comparison of equilibrium model data with available experimental data from circulating biomass fluidized bed gasifiers indicated that the equilibrium model significantly under-estimate the methane yield. In the modified model, a devolatilization model is employed to moderate the methane deviation based on acknowledgement the fact that methane which is initially generated in the biomass devolatilization process, does not reform through gasification reactions. A general semi-empirical devolatilization model is applied to predict the composition of volatile gases including methane composition. Two empirical coefficients are calculated from empirical data for available biomass devolatilization experiments. The corresponding atoms of carbon and hydrogen from predicted methane yield by devolatilization model are removed from the gasifier input stream. The modified model gives better estimation of the syngas composition from an air-blown circulating biomass fluidized bed gasifier in comparison with equilibrium data and two modified equilibrium data are taken from the literature. Two case studies for integrated biomass gasification with the reference Kraft are evaluated: using wood gasified to fire lime kiln, and integrated biomass gasification combined cycle. In the first scenario, the feasibility of replacing natural gas with syngas from wood residues gasification to fire the Kraft mill lime kiln is assessed. For the second scenario, the feasibility of replacement of the existing bark boiler with the integrated biomass gasification VI combined cycle is analyzed. Technical practicability of both scenarios based on the mass and energy balances derived from gasification process model is investigated. To evaluate the economic viability for both scenarios, total capital costs, operating costs, and revenues which achieved through lime kiln fossil fuel reduction, and also CO2 emissions taxes are considered. The net present worth, NPV for 20 years payback period and 10% interest rate are estimated for the two case studies. It has been shown that gasified biomass is an attractive alternative lime kiln fuel for mills with access of woody biomass and concerns in reduction of fossil greenhouse gas emissions. Moreover, if the necessity of new bark boiler installation considered, the replacement of the existing bark boiler with integrated gasification combined cycle is technically viable. The results also show that the replacement of bark boiler has the potential to contribute to reduce global CO2 emissions. Economic analysis proves the replacement of bark boiler with gasification power unit is economically feasible.