System Aspects of Black Liquor Gasification – Consequences for both Industry and Society

Licentiate thesis, 2001

The pulp and paper industry consumes large quantities of biofuels (mainly bark and black liquor) to satisfy process requirements. Biomass is however a limited resource, and biofuel usage should therefore be as effective as possible. Modern pulp mills and integrated pulp and paper mills have excess fuel compared to the amounts needed to satisfy the process steam demand. The excess fuel is often used for cogeneration of electric power in CHP units. For integrated pulp and paper mills, this usually requires import of supplementary fuel to the plant. For market pulp mills, the excess internal biofuel quantities are sufficient to also allow electric power generation in condensing power plant units. If biofuel availability at a reasonable price is limited, import/export to/from a mill changes the amount of such biofuel available to alternative users. The goal of this thesis is to compare different mill powerhouse technologies and CHP plant configurations (including conventional recovery boiler technology and black liquor gasification technology) in order to identify the technology and CHP plant configuration that can produce the most electric power output from a given fuel resource for a given process steam demand. Different process steam demand levels for different representative mill types are considered. The comparison accounts for increased/decreased electricity production in an alternative energy system when biofuel is imported/exported to/from the mill. The alternative energy system considered includes a district heating system with CHP capacity and natural gas fired combined cycle power plant capacity. The results show that black liquor gasification is in all cases considered an attractive powerhouse recovery cycle technology compared to conventional recovery boiler technology. If the marginal electric power generation efficiency for biofuel exported to the reference alternative energy system is 49%, excess mill internal biofuel should be used on mill-site for gas turbine based CHP power generation. The remaining excess biofuels in market pulp mills should be exported and used in the reference alternative energy system in this case. For integrated pulp and paper mills, biofuel should be imported, but only for cogeneration usage (i.e. condensing power units should be avoided). If biofuel can be used elsewhere for high efficiency CHP power generation (i.e. a marginal electric power generation efficiency close to 100%), mill internal biofuel should be used exclusively for process heating, and the remainder should be exported for use elsewhere. The thesis also includes an assessment of the potential for increased electricity generation in Sweden if black liquor gasification is systematically implemented in the pulp and paper industry. The assessment accounts for biofuel import/export to/from the mill from/to an alternative energy system consisting of a district heating system with CHP capacity and natural gas combined cycle condensing power plant capacity. Biofuel usage in the reference alternative energy system must also aim to maximise total electric power generation from a given fuel resource. Under these conditions, it is shown that the annual increase in total electricity production resulting from implementation of high performance black liquor gasification technology in the Swedish pulp and paper industry and adapting the reference alternative energy system accordingly could amount at the most to 21 TWh/year. This can be compared to the current on-site production of 3 TWh/year in the pulp and paper industry. Current pulp mills and pulp and paper mills both import biofuel. If decreased electricity production elsewhere as a result of biofuel import is accounted for, the current total electricity production is in fact negative.