Screening of site-wide retrofit options for the minimization of CO₂ emissions in process industries

Highlights

• Systematic screening of retrofit options for the minimization of CO₂ emissions.

• A holistic view considering multiple sources of CO₂ and multiple retrofit options.

• Graphical methods to suggest the most cost-effective combinations of retrofits.

Abstract

This paper addresses the systematic screening of possible retrofit options for the minimization of CO₂ emissions from process industries. In retrofit scenarios there are various options for the reduction of CO₂ emitted which can be considered with different costs and capacities for CO₂ reduction/removal. This study considers a holistic view accounting for multiple sources of CO₂ emitted from a site and multiple potential retrofit options which can be implemented individually or in combination to meet CO₂ reduction targets. For a given site the different possible retrofit options are ranked in terms of cost-effectiveness and the most appropriate options are highlighted using graphical methods to suggest the most cost-effective combinations of retrofits. A case study is used to demonstrate the applicability of the proposed design methodology. This case study illustrates how fuel switching and energy-saving projects can be practical and beneficial measures for the implementation of decarbonization in process industries.

Introduction

Industrial plants such as refineries and petrochemical plants etc. emit flue gases from various sources with different CO₂ compositions and flow rates. These sources of CO₂ emissions will typically be found at different locations in the plant unlike conventional power plants which will generally have just a single source of CO₂ emissions (flue gas from the combustion units).

The reduction of CO₂ emissions can be achieved through various different methods. These methods can be divided into three different categories:

• Process modification

• Energy systems modification

• CO₂ capture

Modifications applied to the processes or to the energy systems are generally aimed at reducing the energy consumption (and also costs) which lead to the indirect reduction of CO₂ emissions for example due to reduced fuel consumption. Alternatively CO₂ capture involves the direct extraction and removal of the generated CO₂ (e.g. from emitted flue gases).

The implemented modifications are limited by a number of constraints when considering the retrofit of an existing site. Possibly the most important constraints are the upper limits placed on capital investment required to purchase new equipment but also there may be practical engineering constraints (e.g. limited space for equipment and for gathering/ducting flue gases due to the plant layout).

Hence, CO₂ capture and process/energy system modifications requiring additional equipment may be considered unfavorable due to the investment costs required. The most desirable options are energy-saving retrofit projects without any process modifications or additional capital investment required on the site. One such example of this type of ideal retrofit project is the energy-saving achieved through operational optimization of site utility systems. However, in terms of the total site-wide CO₂ reduction the impact of this single retrofit option may not be significant and so a retrofit including multiple different modifications should be considered to achieve a more substantial reduction of CO₂ emissions.

When considering the modification of energy systems heat integration techniques can be applied to assess theoretical potential energy savings in the plant with the aid of energy targeting tools [1], [2], [3], network pinch methods [4] or superstructure-based optimization methods [5] which can identify options giving the greatest reduction of energy consumption.

In addition studies have also been carried out linking the improvement of energy efficiency to the minimization of CO₂ emissions in the industrial sector. For example the studies of Mahmoud et al. [6] and Tiew et al. [7] investigate methodology based on fuel switching and heat exchanger network (HEN) retrofit for the minimization of atmospheric emissions in flue gases.

Pinch analysis has also been adopted to provide conceptual guidance regarding the top-level management of carbon emissions. This involves the use of graphical representations of carbon emissions which are incorporated with energy demand and supply composite curves. Such methods have been implemented by Tan and Foo [8] for the purpose of planning in the energy sector and by Atkins et al. [9] or the purpose of management in the electricity sector.

Furthermore, attempts have been made to elaborate these graphic-based methods with the aid of mathematical optimization techniques. For example, an automated targeting method has been suggested by Lee at al. [10], with which the minimum consumption of low-carbon or CO₂-neutral energy sources and their optimum configuration are automatically identified.

Considering the large number of possible modifications which can be made to the energy systems of a process site it can be a challenge to identify the most appropriate options in terms of the cost effective reduction of CO₂ emitted. In the methodology of Gharaie et al. [11] a hierarchical approach is taken which sequentially considers retrofit of HEN, utility system and fuel switching options. Their method generates investment cost vs. reduction of CO₂ graphs for each option which can be used to select cost effective solutions combining the three options while satisfying CO₂ reduction targets and constraints placed on capital investment. Similarly Al-Mayyahi et al. [12] proposed a graphical method using CO₂ emissions and cost composite curves which can also be used to identify cost effective solutions satisfying emission reduction targets.

However, in the study of Al-Mayyahi et al. [12] their method uses graphical analysis of only utility system retrofits. While the sequential method of Gharaie et al. [11] considers multiple different retrofit options (HEN, utility and fuel switching), for each retrofit option the emission reductions are plotted on separate graphs and hence comparisons between the different retrofit options and the cumulative reductions obtained from multiple retrofits are not clearly shown.

Additionally, while the methods of Gharaie et al. [11] and Al-Mayyahi et al. [12] both suggest the possibility of generating solutions using different emission limits (Al-Mayyahi et al. [12] suggest the creation of pareto curves), the priori selection of emission targets introduces some bias into the methods which makes “high-level” comparisons (e.g. considering the basic calculation of CO₂ reduction capacity and costs) of different retrofit options more difficult to visualize. For example this would require repeated implementation of the methods with a range of different emission reduction targets to obtain a clear picture showing situations where each retrofit option becomes beneficial.

Hence, in this study a holistic “high-level” screening methodology is proposed which allows the comparison of different retrofit options though the construction of graphs showing the site-wide cumulative retrofit costs plotted against the cumulative CO₂ avoided (including all possible retrofit options under consideration). These graphs are constructed based on the quantification of the costs and capacity for CO₂ reduction of each retrofit option which are ranked in terms of the cost per ton of CO₂ avoided. Hence, this graphical representation allows the simple selection and comparison of different potential retrofit options and combinations of different options which can meet any given reduction target within the range considered.

In this way the proposed screening method allows a quicker and simpler analysis of different retrofit options compared to existing methods which focus on more detailed investigation of small numbers of different retrofit options or which consider different options in sequence. However, it should be noted that this is a “high-level” screening method and following the selection of appropriate retrofit options a more detailed design should be carried out to find the optimal configuration and operating conditions for the selected retrofits.

A case study looking at a refinery plant will be presented to illustrate the developed methodology and to demonstrate the benefits of this approach for the identification of retrofit options giving cost-effective reduction of CO₂ emissions.