Abstract
Sustainability and energy efficiency have become important factors for many industrial processes, including chemical pulping. Recently complex back-end heat recovery solutions have been applied to biomass-fired boilers, lowering stack temperatures and recovering some of the latent heat of the moisture by condensation. Modern kraft recovery boiler flue gas offers still unutilized heat recovery possibilities. Scrubbers have been used, but the focus has been on gas cleaning; heat recovery implementations remain simple. The goal of this study is to evaluate the potential to increase the power generation and efficiency of chemical pulping by improved back-end heat recovery from the recovery boiler. Different configurations of heat recovery schemes and different heat sink options are considered, including heat pumps. IPSEpro simulation software is used to model the boiler and steam cycle of a modern Nordic pulp mill. When heat pumps are used to upgrade some of the recovered low-grade heat, up to +23 MW gross and +16.7 MW net power generation increase was observed when the whole pulp mill in addition to the boiler and steam cycle is considered as heat consumer. Combustion air humidification proved to yield a benefit only when assuming the largest heat sink scenario for the pulp mill.
Highlights
The pulp and paper industry is one of the largest global industrial sectors in terms of energy use [1]
The authors of this paper considered a heat recovery scheme based on scrubbers as part of a carbon capture and storage (CCS) scheme based on biomass chemical looping combustion with oxygen uncoupling (CLOU) [25]
Condensing heat recovery schemes including the use of heat pumps and combustion air humidification are becoming increasingly common in modern biomass boiler plants for producing low-temperature heat, and recently in co-generation plants used, e.g., for district heating
Summary
The pulp and paper industry is one of the largest global industrial sectors in terms of energy use [1]. Most of the energy demand is fulfilled by burning black liquor, a liquid byproduct from extracting the fibers from wood in a kraft pulping process, in recovery boilers [2]. The liquor contains both the used pulping chemicals to be recovered and about half of the organic matter in wood, lignin. Combustion of this liquor in a kraft recovery boiler achieves two goals—regenerating the pulping chemicals and recovering a significant amount of energy by burning the organics. The pulping process is energy-intensive and consumes considerable amounts of both electricity and heat, the energy from black liquor combustion is enough to make a kraft pulp mill, typically a net energy producer, able to sell electricity to the national grid
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