A simple thermodynamic tool for assessing energy requirements for carbon capture using solid or liquid sorbents

Highlights

• Equivalent work required for CO₂ capture is better than heat of reaction to measure power lost due to capture.

• Minimum work of separation using sorption is simply evaluated from room temperature isotherm data.

• It is independent of the heat of reaction and, for most liquid sorbents, independent of the amines used.

• Solid gas reactions require a higher minimum work.

• For TSA, high sorbent capacity and heat recovery are most important and yield good estimates of overall work requirements.

Abstract

Carbon capture and sequestration is known to be energy intensive and will result in 20–30 % reduction in net output of a power plant. However, a simple thermodynamic tool is currently unavailable for assessing the work of CO₂ separation using a given solid or liquid sorbent.

This paper provides rigorous yet simple framework of equivalent work to assess the energy requirement for CO₂ capture using liquid amines or solid adsorbents. First, the theoretical minimum work is determined by assuming that each step in the sorption - desorption cycle is thermodynamically reversible. Then, irreversible heat transfer losses are added to calculate total work for the actual process.

The model provides useful insights into the sorbent and process selection. The minimum work for reversible separation can be calculated merely from CO₂ sorption equilibria at ambient temperature without requiring laborious data or complex models. A sorbent with low ab/adsorption heat does require less thermal energy, but this thermal energy is required at a higher temperature. Thus, contrary to conventional thinking, the equivalent work is not reduced. The irreversible heat transfer losses for the amines are mostly dictated by the amine’s circulation rate which will be minimized by using amines with the highest CO₂ capacity. On an energy requirement basis, the solid adsorbent based processes cannot compete with amines because practical methods of heat recuperation from the hot regenerated adsorbent are unavailable. Without heat recuperation, the solid adsorbent processes will be attractive only if their capital advantage outweighs their higher energy use.

Introduction

Carbon capture from combustion gases is likely to be an important part of the overall effort to mitigate climate change (Boot-Handford et al., 2014; iea, 2013) from greenhouse gases. Most of the proposed CO₂ capture processes use a CO₂ selective liquid or solid sorbent to remove dilute (4–14 %) CO₂ from the flue gases of fossil fuel combustion. The sorbed CO₂ is then stripped from the sorbent using some combination of heat, steam or vacuum to produce a ∼90 % concentrated CO₂ stream. The concentrated CO₂ is compressed to 140–150 bar pressure for sequestration, and the regenerated sorbent is cycled back for sorption of additional CO_2.

The design and selection of proper CO₂ sorbents has received much attention over the years, and involves considerations of their sorption capacity and the energy requirements for use. Other considerations may also include sorption /desorption kinetics, toxicity, resistance to contaminants, and cost. Historically, only liquid sorbents, such as amines (Mumford et al., 2015; Yu et al., 2012; D’Alessandro et al., 2010), have been used. However, their high energy requirement for CO₂ desorption has been an area of concern. This has motivated the search for non-aqueous solvents, such as ionic liquids (Mumford et al., 2015; Koronaki et al., 2015; Muldoon et al., 2007), but not without controversy (Meldon, 2011; Oexmann and Kather, 2010). Solid adsorbents have been investigated as an alternative to liquids in the hope of reducing the energy requirement (Gray et al., 2009).

Desorption from different sorbents requires thermal energy at different temperature levels. However, previous researchers have mostly focused on the thermal energy requirements of the process without any regard to the temperature at which the thermal energy is used, i.e., the “quality” of thermal energy. In a few exceptions (Calbry-Muzyka and Edwards, 2014), large scale simulations have been carried out using the thermodynamic availability function which incorporates the quality of thermal energy. However, the role of the key sorbent properties is not easily discerned because of the complexity of these simulations.

This paper describes how the parasitic energy required for CO₂ capture may be estimated using a simple tool that merely requires sorption data at ambient temperature. This tool can be used for evaluating new sorbents and processes without requiring extensive pilot plant data and process simulations. Yuan and Rochelle (2019) have previously used a different approach to calculate the work lost in liquid amine processes by adding the work loss in each process step. Whereas their work was limited to only amines, the methodology described in this paper is very general and encompasses both liquid amines and solid adsorbents.