(373af) System Analysis for Energy Efficiency and Process Debottlenecking for a Sustainable Pulp and Paper Industry

Increasing the competitiveness of the pulp and paper industry requires an effective optimization of its existing assets in line with a long-term vision for process transformation, production upgrade and product diversification. Currently, pulp production increase is one of the main sources of additional revenue for this industry. Likewise, energy efficiency is often employed as a cost-effective approach to reduce operating costs, enhancing the possibilities to lower fossil fuel consumption and developing a low-carbon economy. On the other hand, reaching higher production targets and facilitating process transformation such as biorefinery depends heavily on the status and performance of a mill’s current infrastructure. Therefore, a system analysis is needed to assess the new production requirements and the interactions across departments. In order to obtain practical improvement solutions, direct and indirect impacts on process performance and resource utilisation should be considered. This work provides an overview of the key challenges that need to be addressed for production increase and energy efficiency improvement. The methodology starts by the scope analysis for debottlenecking and screening capacity limitations, followed by their ranking (bottleneck ranking diagram). Benchmark, gap analysis and root-cause techniques are applied to diagnose system inefficiencies. This debottlenecking mill-wide assessment is then used to guide the selection of a long-term sustainable portfolio of improvement projects. A case study of a Kraft pulp mill is used to illustrate the proposed methodology.

Introduction

Significant efforts have been taken to improve the competitiveness of the pulp and paper industry while building a new business model in line with the current trends for product diversification, industry digitalization and effective decarbonisation. The main revenue stream for this industry continues to rely on the production capacities and performance of existing assets.

Pulp and paper plants are constantly changing their complex operating strategies to accommodate market demands for a diverse product range and specifications. This context affects a mill’s global performance. Therefore, it requires optimizing key operating parameters and upgrading limiting departments to maximize profit. Moreover, as one of the high energy and water intensive sectors, the pulp and paper industry could benefit from improving energy efficiency and lowering fuel consumption. Consequently, different site perspectives should be evaluated to define the trade-offs that support this complex decision-making process. Such perspectives include process capacity and departmental efficiencies, steam generation and power production strategies, energy use and distribution through the heat exchange network (including production and distribution of hot water).

Process Debottlenecking

Maximizing profits relies heavily on the pulp production capacity and on the system interactions within the process and with the utility systems. In terms of bottlenecks, most Kraft mills in North America are constrained by their recovery boiler, digesters, pulp machine dryer and/or lime kiln, among others. Operating beyond the design capacity, as may occur in some departments (e.g. brown stock washing and bleaching stages), can result in operation instability, increased production cost (due to higher chemicals and energy usage), and overall process reliability issues. Therefore, to debottleneck existing equipment, a global vision encompassing pulp production and utility systems is required. This will enable plants to achieve high capacity utilisation and support the implementation of transformative technologies, while optimally managing resource utilisation.

Energy Efficiency

The overall energy efficiency of a pulp and paper mill is directly influenced by the operation and design of the process heat recovery system, water network as well as heating and cooling utility systems. In this sense, numerous opportunities for saving steam through water reduction, process improvements, and heat recovery have been identified and are already implemented in many mills. Despite such efforts, untapped waste heat recovery potential still remains in most sites. This potential could be valorized through new and underutilized existing heat exchangers taking into account specific process constraints. Quick short-term opportunities for steam savings should, however, be fitted within a long-term sustainable solution.

Utility systems for steam and power generation play a determinant role for the pulp and paper industry. As a key unit operation, the recovery boiler creates a strong interaction between process and utility operation. It is at the core of the chemical recovery cycle for the pulping process and its operation influences the overall pulp production performance as well as the mill energy efficiency. When it produces less steam, the load on power boilers increases, requiring more biomass and fossil fuel.

Several benchmarks, process integration and optimization approaches are employed to evaluate the current stage of energy efficiency, identify energy saving projects and to define the trade-off between power generation, fuel costs and process performance.

Methodology

A debottlenecking methodology is proposed to develop optimal strategies for improving mill performance by focussing on the main capacity issues and their limiting causes. The aim of the methodology is to identify where and how low cost improvements can be applied before significant changes or process transformations are required. To respond to the debottlenecking challenges, several analysis tools have been considered together with process knowledge.

Step 1: Scope analysis for debottlenecking and energy efficiency

In this first step, insights on mill-encountered bottlenecks (at the level of equipment, resources and reliability) are established together with new production targets based on the input from mill upper management and personnel. A mill-wide questionnaire has been prepared and used to facilitate this information gathering. The scope for process transformation and utility system is likewise assessed with a top-level analysis, i.e. via a study of marginal steam and power generation costs.

Step 2: Screen and rank bottlenecks

A systematic screening approach was integrated to identify and quantify process bottlenecks at the departmental and mill-wide levels. A bottleneck ranking diagram maps the individual department production capacities versus the global targets and distinguishes the departmental classification and relevance for debottlenecking.

Step 3: Departmental process performance diagnostic

Focusing next on the critical department(s), the process performance diagnostic is then deepened via an analysis of plant data and a revision of key performance indicators. More precisely, using gap analysis and benchmarking, main inefficiencies are pinpointed for further consideration and improvement. A root-cause diagram is used to navigate through the processing steps from these apparent inefficiencies or departmental bottlenecks to their relevant sources. This approach takes the form of a diagnostic tree built on process knowledge and data-driven analysis. The performance of the heat exchanger network is also addressed at this step together with the utility system operating strategies.

Step 4: Establish improvement solutions

Different debottlenecking strategies for each mill department have been analysed and specific project improvements have been identified to facilitate the application of the proposed approach. A decision tree is developed for each department, including opportunities for capacity increase and energy efficiency improvements. The main pathways considered for overcoming production limitations are departmental load reduction, capacity optimization, equipment availability increase (reliability and maintainability) and size increase by adding new units. Three-level changes based on capital investment requirements are proposed: 1) operational improvements at low cost, 2) retrofit design changes for medium cost and 3) process modifications with large capital expenditure. In regards to utility system and mill water-energy network, a simulation-optimization model is used to develop energy efficient operating strategies.

Step 5: Implementation roadmap

The selection of the projects and prioritisation depends on the potential of each opportunity for production capacity increase, contribution to the operating cost (consumption of energy, water, chemicals), environmental impacts (water, GHG emissions) and capital costs. In order to characterise each opportunity for decision-support, the cross-effects between opportunities and the direct and indirect impacts on the process performance and resource utilisation are also addressed through process modelling and simulation.

A case study illustrating the application of the proposed methodology in a kraft pulp mill will be presented, highlighting several opportunities for debottlenecking black liquor evaporators, pulp machine and brown stock washing processes. Even before any high investment changes are made, this case study shows that significant production increase could be achieved at low cost through effective debottlenecking using a system approach.

Conclusions

This paper presents a site-wide systems approach to assess the current process performance, identify inefficiencies and propose projects to increase mill production and profitability in the pulp and paper industry. Insights from the process knowledge and system interaction analysis are integrated to identify the source of inefficiencies. This enhances the selection and sequencing of improvement projects in a cost-effective manner. To further improve the energy efficiency of the global solution, strategies for increasing waste heat recovery and optimizing utility system are embedded in the methodology.

The approach follows both process-based and utility-based strategies in order to produce a comprehensive understanding of the trade-offs involved in the design and operation of an efficient mill.