– Introduction
Although an industry with a long history, the energy use of pulp and paper mills remains high and a continuous topic for process engineers, researchers and politicians. This is especially true for thermomechanical pulp (TMP) mills, such as Holmen Paper AB Hallstavik, which are heavily dependent on electricity for pulp production.
– Problem
The aim of this case study is to map the energy use of the Holmen Hallstavik integrated pulp and paper mill β as well as current cleaner production work β to present further cleaner production measures in relation to energy use. Energy and exergy analyses performed show both well-functioning sub-systems of the plant, but also areas with significant losses.
– Solution
– Background
Due to the fact that the processes and flows are the same for the most part in all mills, this means that the efforts of improving the design of the mill and the processes in general could be applied to most mills globally and the cost of innovation can be shared and more likely to be implemented. However, even with these improvements, as of 2016, the pulp and paper industry still places fourth globally for energy consumption (Kong et al., 2016). It also accounts for approximately 15% of the use of energy in the industry sector (Laurijssen, 2013). Equipment modification and improved process control have allowed the paper and pulp industries to become more efficient while also allowing them to be more resourceful. The environmental performance and efficiency of an industry can be improved by adopting Best Available Techniques (BATs) which are technical and economically feasible techniques which have been compiled for various industries.
For a study of this magnitude, specific methodologies need to be used which include:
- A pre-assessment
- An assessment
- A feasibility analysis
– The pre-assessment
A pre-assessment allows an initial understanding and mapping of the mill where literature reviews along with a system flowchart is drawn up. Energy balances are created based on the energy ratings and requirements of the machinery and system boundaries are solidified.
– The assessment
The assessment stage involves more detailed site specific literature reviews including analysing relevant sustainability reports and determining the plausibility of the acquired data. Calculations of the heat content of the pulp, paper, water and all the entities that constitute the mill are made and energy and exergy values are obtained in the assessment stage.
The heat/energy content of a substance is calculated using,
π = π β πΆπ β Ξπ
where, m is the mass of the substance measured in grams, Cp is the specific heat content and ΞT is the change in temperature. Exergy calculations follow the basic formula for exergy which states,
πΈ = πΆπ β (πβ β πβ) β (1 + ππ(πβ/πβ) β¦ (eq. 2)
Where, πΆπ is the heat/content of the flow, Tβ is the ambient or reference temperature and Tβ is the temperature of the flow. While calculating the exergy content of steam, exergy with reference to ambient temperature of 13.5ΒΊC needs to be considered (eq.2) as well as the exergy with reference to the heat of vaporization of steam, which is calculated by,
π = Ξπ―πππ β (π β πβ/πβ) β¦ (eq. 3)
Where, Ξπ―πππ is the heat of vaporization of water at 100ΒΊC and Tβ is the ambient temperature at 100ΒΊC and Tβ is the vaporization temperature of steam. The energy and exergy efficiencies for each sub-system are calculated individually using the same formula as described below,
Energy/exergy efficiency = (Output/Input) * 100
Initial Sankey diagrams are drawn and areas pertaining to high energy requirements are analysed for cleaner production opportunities following which the actual required Sankeys are obtained.
– The energy flows and energy balance
Based on the energy flows in the system, efficiencies of the machinery as well as efficiency of the system is assessed. This gives a more detailed explanation of the areas that require to be evaluated and the relevant CP measures to take in these areas. Opportunities for improving these efficiencies may also rise but may not be feasible but it more so depends on the system.
– The exergy flows and exergy balance
Calculations of exergy efficiency is key in order to evaluate the efficiency of the energy conversion process that takes place in the system in each sub-process (boiler, paper machine, etc). This paints a clear picture of how much energy is destroyed in the system and how much can be saved by using intermediate or βlower qualityβ sources of energy.
– The feasibility analysis
The feasibility analysis establishes relevant and viable CP measures by reviewing the sustainability reports for pre-existing CP measures, research and legal documents from the European Union regarding BATs within the paper and pulp industries. Additionally, upstream and downstream impacts are also assessed while evaluating CP measures. The existing CP measures are compared with the energy and exergy analyses to take and make decisions based on how they are currently functioning.
– The way forward
Based on the feasibility analysis the suggested CP measures include an upstream measure of investing more in renewables for electricity to reduce the dependency on external energy producers while also cutting down on the cost for electricity bought.
Plant level CP measures include the use of Aq-vane forming in order to improve the quality of paper to ensure reduced damages during paper rolling. This is also energy efficient saving up to 25% of the normal process used. Another CP measure includes the use of shoe presses in place of rolled presses which possess flexible belts and withstand high pressures. This also comes with the advantage that it reduces evaporation losses which is already high in paper mills.
The evaporation losses can however be utilised better with industrial symbiosis downstream, these can also be converted to electricity using facilities for converting low value heat. Similarly, bark or chips from losses can be used as fuel for transportation either internal or external to the industry.
As a conclusion, losses in the form of energy conversion from electricity and steam to heat and evaporation could be identified as the major sources of losses, which can be addressed by either looking at specific changes in the mill or by an expanded view by including upstream or downstream activities. A means to achieving these goals are demonstrated by the CP measures.
