Hydrothermal Carbonization of Chemical and Biological Pulp Mill Sludges

A modern pulp mill generates a variety of different by-products and waste streams, some of these can be recycled, refined, sold, or used on-site for energy production. However, some, such as chemical and biological sludges produced in wastewater treatment cannot be reused or disposed of easily, mainly due to their high moisture content and poor drying characteristics. Tightening legislation regarding waste disposal as well as the growing need to increase the process efficiencies of pulp mills act as driving forces to find environmentally friendly and energy-efficient techniques for pulp mill sludge treatment. This study summarizes the current methods for pulp mill sludge handling and evaluates the potential of hydrothermal carbonization (HTC), a conversion process through which wet organic substrates can be transformed into a carbonaceous material (hydrochar). Depending on the process parameters, the material’s structure is modified, enabling hydrochar use in energy, soil conditioning and adsorption applications. The sludges were hydrothermally carbonized at 180, 200, 220 and 240 °C for 3 h. The hydrochar and liquid products’ main properties were analyzed. Their potential applications were also evaluated. The effective treatment of sludges from the pulp industry with HTC could transform energy-demanding waste into a value-added source of materials

Optimization of a shell-and-tube district heat condenser for a small back pressure combined heat and power plant

Wood-fired combined heat and power (CHP) plants are a proven technology for producing domestic, carbon-neutral heat and power in Nordic countries. Such plants are often of backpressure configuration, the district heat condenser replacing the vacuum condenser of a condensing power plant. The condenser is usually a shell-and-tube heat exchanger with district heating water on the tube side. In this paper, a new approach is presented to optimize the condenser design considering not only the design-point performance, but also variations in the operating conditions. A power plant model is used to determine the plant performance (net power output and boiler fuel consumption) as a function of the main performance parameters of the condenser at each point. Cuckoo search algorithm is used for the optimization. The results show that although electricity price variation has a significant impact on plant net cash flow rate, the effects of electricity price and heat exchanger specific cost on condenser design are low.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Improving Kraft Pulp Mill Energy Efficiency through Low-Temperature Hydrothermal Carbonization of Biological Sludge

Of the various waste and side streams created in a kraft pulp mill, the biological sludges from the wastewater treatment plant are some of the most problematic to handle. Incineration is becoming a common solution as landfilling is no longer permitted by legislation in many countries, but this is also problematic due to the high moisture content, poor drying characteristics, and high ash content in the solids. This study evaluates the technical potential of mild hydrothermal carbonization (HTC) at 160 °C for 3 h to improve the energy efficiency of on-site incineration as a biosludge handling method. HTC treatment transforms wet organic substrates into a hydrophobic carbonaceous material (hydrochar). The heating value and elemental composition of both the sludge and the hydrochar product were analyzed. Based on this, a hydrothermal carbonization model developed earlier was adjusted for the feedstock, and process integration modelling performed to evaluate the performance impact on the power and heat generation at the mill. The results indicate that if the alternative is combustion in the power boiler, HTC pre-treatment could allow a significant increase in power generation. If the sludge is combusted in the recovery boiler, a practice often avoided in order to not introduce non-process elements to the chemical recovery cycle but sometimes necessary due to, e.g., absence of a power boiler, a much smaller increase is obtained. The increase is smallest if the freed evaporator plant capacity cannot be utilized for increasing the firing liquor dry solids content