Cost estimation methods for CO2 capture processes

Abstract

Cost engineering and economic assessment play a crucial role in evaluation of CO2 capture technologies and energy systems. Cost is one of the key decisive factors when considering industrial deployment of a technology. Economic analysis is very important when a selection is to be made from different options. Estimates of CO2 capture and storage processes are essential for making policies, and for making important decisions like funding of research and projects, as well as investment in industrial implementations.

Capital cost estimates made by engineering and procurement contractors (EPC) are usually accurate. Nevertheless, their methodologies are usually not open and transparent for others to adopt due to commercial policies. The technical and economic underlying assumptions utilised are normally not disclosed. They are also difficult and expensive to access by researchers, students and others that are not in the commercial and governmental sectors. The common practice for capital cost estimation in the open literature is that a single overall installation factor is applied uniformly on all equipment. The results from this study propose that it may likely lead to over-estimation of very expensive equipment and under-estimation of least expensive equipment. At best, it limits such methods suitability to only cost estimation of new and large plants.

It has been stated in literature that the accuracy of capital cost estimates can be improved by applying detailed factors and sub-factors as provided by the Enhanced Detailed Factor (EDF) method. The EDF method is robust, especially with the introduction of the plant construction characteristic factors (PCCF). They account for different situations that may be encountered in different plant construction projects. In the EDF method, installation factors are assigned to each piece of equipment based on their costs. A very expensive equipment unit is assigned a lower installation factor while a less expensive equipment unit will have a high installation factor. Therefore, the EDF method is suitable and robust for capital cost estimation of new plants, modification projects and retrofit plants, and large and small plants.

The EDF method’s installation factors are more sensitive to differences in equipment costs compared to the Lang Factor method, Hand Factor method, percentage of delivered equipment (PDE) cost and the Bare Erected Cost (BEC) method. All the seven methods studied in this project estimated the same cost optimum minimum temperature approach (∆𝑇𝑚𝑖𝑛) based on CO2 capture cost. Nevertheless, the capture costs were different, ranging from €66/tCO2 to €79/tCO2. The total plant cost estimates of the BEC method, the Lang Factor method, and the percentage of delivered equipment cost method which are purely based on application of a single installation factor uniformly on all equipment were 31 – 54 % higher than the result of the EDF method.

Due to the details involved in the EDF method, it is relatively time intensive, and it requires more work to implement. This becomes challenging when there is a need for several iterative calculations. For example, iterative cost estimation with each iteration involving process simulations, equipment dimensioning, capital cost, operating cost and other economic analysis. This is the case for sensitivity analysis and cost optimisation studies which are very important in techno-economic analysis. Therefore, the Iterative Detailed Factor (IDF) scheme was proposed as a simple tool for cost estimation and optimisation tool for fast and accurate cost estimation based on the EDF method. The IDF scheme was implemented by means of the spreadsheets incorporated in Aspen HYSYS. The models for equipment dimensioning, capital cost and operating cost, as well as other key performance indicators were created inside the Aspen HYSYS spreadsheets. It is based on estimating new equipment costs using the Power Law when subsequent simulations iterations are performed after the initial one. When a process parameter is varied, immediately after the simulation has converged, all cost estimates can be automatically obtained. For the columns, a cost exponent of 1.1 for new sizes above the original size and 0.85 below the initial size achieved the most accurate estimates in this study. A cost exponent of 0.65 was utilised for estimation of the costs of all equipment that is affected by the change in the process parameter. Other equipment not affected was assigned a cost exponent of 1. The error with the IDF scheme was 0 – 0.4 % in estimation of total plant cost compared to the EDF method.

Different specific types of heat exchangers for CO2 absorption plant were studied. This was to evaluate their cost reduction and emissions reduction potentials. They are the fixed tubesheet shell and tube heat exchanger, floating head shell and tube heat exchanger, U-tube shell and tube heat exchanger, gasketed plate heat exchanger and welded plate heat exchanger. The gasketed plate heat exchanger outperformed all the other heat exchanger types in capital cost, CO2 capture cost, CO2 avoided cost and CO2 actual emissions reduction. Their limitations are not very important in a solvent based CO2 capture system. This project recommends the use of plate heat exchangers for the cross-heat exchanger with a minimum temperature approach of 4 – 7 ℃. It is also recommended for the lean amine cooler and for the direct contact unit water cooler in a CO2 absorption and desorption process.

Cost estimation and optimisation were performed for a standard monoethanolamine based process and for several other alternative processes. For example, the EDF method based on the IDF scheme was also applied to study a combined rich and lean vapour compression configuration for CO2 capture. The combined configuration achieved the best energy and economic performance compared to the simple rich vapour compression and the simple and lean vapour compression configurations.

The EDF method was mainly implemented in the IDF Scheme (automatic) approach in this PhD study and in master students’ projects as well as master’s theses. Most of the studies focused on automatization of cost estimation and process parameters cost optimisation. The studies demonstrated that the EDF method implemented in the IDF scheme approach is fast and robust to optimize process parameters like minimum temperature approach of the lean/rich heat exchanger, columns packing heights and others. Therefore, this work recommends the EDF/IDF method for cost estimation of CO2 absorption processes and process parameters optimisation.