Emerging Biodrying Technology for the Drying of Pulp and Paper Mixed Sludges

Abstract

Effective sludge management is increasingly critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks for options such as sludge landspreading and composting. Sludge dewatering challenges are exacerbated at many mills due to improved in-plant fiber recovery coupled with increased production of secondary sludge, leading to a mixed sludge with a high proportion of biological matter that is difficult to dewater. Various drying technologies have emerged to address this challenge of sludge management, whose objective is to increase the dryness of mixed sludge to above critical levels (%42% dryness) for efficient and economic combustion in the boiler for steam generation. The advantages and disadvantages of these technologies are reviewed in this article, and it is found that many have significant technical uncertainties and/or questionable economics. A biodrying process, enhanced by biological heat generation under forced aeration, is introduced that has significant promise. A techno-economic analysis of the batch biodrying process at a case study mill showed an annual operating cost savings of about $2 million, including the elimination of landfilling practices and supplemental fuel requirements in the boiler. It was shown that if a biodrying residence time of less than 4 days can be achieved, payback periods of 2 years or less can result in many mills. The potential for the development of a continuous biodrying reactor and the fundamentals of its mathematical modeling are thus presented. Compared to the batch reactor configuration, it is expected that the continuous process would result in improved process flexibility and controllability, lower investment and operating costs due to shorter residence times, and an improved potential to fit into the crowed pulp and paper mill site.

Effect of airflow on biodrying of gardening wastes in reactors

“…Many works have been published about biodrying waste with an external source of airflow (Sugni et al, 2005;Navaee-Ardeh et al, 2006Zhang et al, 2008Zhang et al, , 2009Velis et al, 2009) and with different initial moisture contents, for instance MSW (65.5%--75%) (Adani et al, 2002;Zhang et al, 2008Zhang et al, , 2009Rada et al, 2009a;He et al, 2010;Ragazzi et al, 2011;Tambone et al, 2011) or more (80%) (Zawadzka et al, 2010), pulp and paper mixed sludges (75%) (Navaee-Ardeh et al, 2006), or sewage sludge (78.4%--82.6%) (Zhao et al, 2011;Xu et al, 2011).…”

Biodrying as a biological process to diminish moisture in gardening and harvest wastes

“…In particular, according to some authors, biodrying produces an increase in the calorific value of about 30-40% or more by reducing the water content in the waste. Subsequent mechanical treatments, used to remove low calorie material (metals) and the fine fraction (size <2 mm), make it possible to produce high-calorie fuel (16,000-17,500 kJ/kg), which is comparable to the calorie content of brown coal (Adani et al 2002;Sugni et al 2005;Navaee-Ardeh et al 2006). The degree of drying, biostability and hygienization during an aerobic biological process can be varied by correct management of the process parameters, the most important of which are temperature, oxygen, airflow rate and moisture content.…”

“…This method is different than other works of biodrying because they uses an external airflow to drag moisture (Sugni et al 2005;Navaee-Ardeh et al 2006;Zhang et al 2008;Zhang et al 2009;Velis et al 2009; NavaeeArdeh 2010).…”

“…Biodrying represents an important opportunity for the treatment of mixed waste to consistently raise the dry solids content so that the waste can be disposed of economically in boilers by generating steam and/or power (Choi et al 2001;Navaee-Ardeh et al 2006). Biodrying dries the biomass while preserving its calorific value.…”

“…The presence of a bulking agent, aeration, the remaining time, and temperature and moisture variation are taken into account to achieve this goal (Adani et al 2002;Sugni et al 2005;Navaee-Ardeh et al 2006;Collick et al 2007;Robles et al 2010). However, unlike previous mentioned experiments, in the present work is not necessary to provide air flow, but aeration is achieved by means of the chimney effect generated in a greenhouse.…”