Advances in Carbon Capture through Thermochemical Conversion of Biomass

Introduction

Climate change and energy crisis are amongst the alarming issues of the world that are inter-related to each other. In 2015, world leaders had pledged at the UN climate change conference in Paris to reduce the greenhouse gas (GHG) emissions by 40-70% in the next four decades compared to year 2010 data (Aalbers and Bollen, 2017). The use of renewable energy sources, especially biomass, is expected to play a significant role in attaining this objective. With a share of 10%, biomass is the fourth most important energy source, after oil. coal and natural gas. Owing to the climate change policies, this share is expected to increase further, as biomass-based technology is an option that can often work as carbon neutral or sometimes carbon negative. The process of photosynthesis captures about 140 TW of energy, which is approximately 0.08% of the total incident solar energy on earth (Goldemberg and Johansson. 2004). In spite of such small percentage, the total volume of biomass produced is about 10 times more than our present energy demand. Nearly 100 billion tonnes of carbon are converted to biomass every year (Abbasi and Abbasi, 2010). Though these figures look attractive, there are some serious limitations on the extent of utilization of biomass for producing energy.

The burning, combustion or decomposition of biomass releases that carbon to the atmosphere which it had recently captured from the atmosphere duiing photosynthesis. Hence, there is no net addition of carbon or CO,. However, the burning of fossil fuels results in net addition of CO, in the atmosphere as they are denied from biological matter that is millions of years old. This carbon neutral nature of biomass as an energy source has created great interest to utilize it in different ways as a substitute for fossil fuels. Figure 1 shows the net carbon balance for different energy alternatives, including fossil fuels and renewables (Quader and Ahmed, 2017; Thengane and Baudyopadliyay, 2020). Recent studies have used integrated assessment models to develop mitigation strategies in order to achieve the Paris Agreement’s targets. The default pathways show an early peak in emissions, followed by rapid emission reductions and, finally, a period of net negative emissions (Fuss et al., 2014). These net negative emissions refer to active removal of carbon dioxide from the atmosphere, achieved by introducing new carbon sinks on a large scale (Anderson and Peters, 2016). The advantages of using negative emission technologies as part of a mitigation strategy are that they can: (1) somewhat dampen the need for quick near-term emission reductions and (2) compensate emissions from hard to moderate sectors (Vaughan et al., 2018).

Department of Mechanical Engineenng, Massachusetts Institute of Technology, 77 Massachusetts Ave, Room 3-339F, Cambridge, MA 02139, USA.

Net carbon balance for different energy alternatives (Thengane and Bandyopadhyay, 2020)

Figure 1. Net carbon balance for different energy alternatives (Thengane and Bandyopadhyay, 2020)

Carbon capture and storage (CCS) is considered as a key technology amongst the greenhouse gas (GHG) emission reduction options, in addition to energy savings and renewable energy technologies, to attain the stringent climate targets. The captured CO, can be stored in depleted oil and gas wells, inaccessible coal seams, saline aquifers, and other geological structures that can act as reservoirs (Thengane et al., 2019). CCS, often linked with fossil fuel-based processes, can be combined with biomass-based processes too, sometimes referred to as bio-CCS (Koomneef et ah, 2012) or bioenergy with CCS (BECCS) (Azar et ah, 2013). BECCS consists of multiple components and stages: biomass feedstock and collection, conversion of the biomass feedstock into energy, production of heat, electricity, or fuels, and capture and sequestration of the carbon resulting from using that energy (NAS, 2018). Figure 2 shows the schematic of a BECCS supply chain, starting from CO, in the atmosphere to the end applications and storage of captured CO,. Bui et ah (2018) recently evaluated a 500 MW pulverized fuel BECCS plant in terms of energy efficiency, carbon intensity and pollutant emissions. They observed the strong dependence of carbon negativity of the technology on plant efficiency, and predicted the energy efficiency of 38% with heat recovery.

In general, biomass can be glass, plants, trees, wood, and several residues, as well as purposely grown food and energy crops. The use of biomass as fuel may lead to competition with other uses of biomass, such as food, paper, fibreboard, furniture, and as a feedstock for some other industries. Hence, it is important to define the kind of biomass that is being targeted for conversion and utilization. The waste biomass or residual biomass or simply ‘biomass’ that we would refer to in this chapter includes various agricultural residues, crop wastes, forestry waste, leaf litter, sawmill waste, food waste, some components

Schematic of BECCS supply chain (Kemper, 2015)

Figure 2. Schematic of BECCS supply chain (Kemper, 2015).

of municipal solid waste, and any other unused biomass which would otherwise decompose. The two major pathways of biomass conversion are thermo-chemical (e.g., combustion, pyrolysis, torrefaction, gasification) and biochemical (fermentation, bio-methauation). However, thermochemical methods have much lower conversion time, can convert the entire biomass without rejecting any component, and are less sensitive to feedstock, unlike biochemical methods (Bhaskar and Steele, 2015).

This chapter discusses the advances in different thermochemical conversion processes for various biomass wastes, with respect to carbon capture, and the environmental impacts associated with each process. Combustion of biomass being the primary mode of utilization has been the main focus in understanding the applicable carbon capture and storage (CCS) approaches. Most of the CCS approaches for biomass combustion are also applicable to gasification of biomass. The new approaches based on chemical looping for carbon capture in biomass combustion and gasification processes are subsequently discussed. The other modes of biomass conversion and utilization, such as pyrolysis and torrefaction, are also discussed and compared with respect to their carbon capture potential.

Thermochemical Conversion of Biomass

Thermochemical processing of biomass involves heating of biomass to different temperatures in the presence of differing concentrations of oxygen in order to produce a range of products, such as heat, fuel or chemicals. Any kind of biomass may be used to extract energy or derive one or other fuel/chemical from it. However, one biomass may provide better quality product at lesser costs than others, depending on the process and operating conditions. Hence, it is important to study the performance of different kinds of biomass in different processes and a varied set of operating conditions. This justifies the high number of research and review articles being published in the areas of combustion, gasification and pyrolysis (Aklitar et al., 2018). Torrefaction, or low-temperature pyrolysis, is a relatively new approach, yet the number of publications in this area is also rising steeply (Ribeiro et al., 2018). The following sub-sections discuss the processes of combustion, gasification, pyrolysis and torrefaction, with respect to their carbon cap true potential and approaches.

Combustion

The use of biomass in earlier times was based on open fires having low efficiencies between 5-10%, but this has significantly improved over the last century. The combustion of biomass and biomass-derived fuels has the potential to partially or substantially substitute fossil fuels, such as coal, oil and natural gas. Industrialized economies are moving towards employing biomass combustion as one of the major options for heat and electricity production. Table 1 shows the broad applications of biomass primarily in four sectors, with the highest usage in heating and cooking, and the lowest usage in transportation. Though biomass is a renewable source, there are environmental emissions happening when it is used as a fuel, especially for combustion. WHO report (2014) estimated about 1.5 million human deaths per annum as a result of this type of pollution.

The three stages in general biomass combustion are diving, volatiles release and burning, and char combustion as shown in Figure 3. Drying requires heat to evaporate moisture, and the rate of diying depends on the particle size and temperature. Next, devolatilization takes place and pyrolysis gases are released, for which oxygen is needed for combustion. The char combustion stage requires oxygen, releases

Table 1. Sector-wise applications of biomass (Vakkilamen et al., 2013).

Description

Energy unit (Gtoe)

Domestic cooking and heating; Heating of commercial premises and large buildings

1

Industrial steam raising-waste biomass, medium size power generation/CHP/district heating units

0.19

Large electrical power plants

0.17

Transportation

0.08

Simple combustion mechanism for biomass

Figure 3. Simple combustion mechanism for biomass.

heat, reduces particle size, and leaves residual ash. It can be seen that the combustion of carbonaceous fuel, such as biomass, results in the formation of carbon dioxide and water as the major products. The increase in CO, concentrations in the atmosphere is related to climate change and the considerable efforts have been devoted to mitigating these emissions.

For large scale solid biomass utilization, most of the combustion technologies are based on those developed for coal (Spliethoff, 2010). Common technologies involve combustors of different types, such as fixed bed (< 5 MWJ, moving or travelling grate (up to 100 MW[h), fluidized bed (up to 500 MV), in addition to suspension firing combustion and co-firing with fossil fuels (up to 900 MW.) (Jones et al., 2014). Then, there are several small scale processes involving combustion, such as open fires, cookstoves, and small boilers (< 10 kWth) which do not have the flue gas treatment unit present in larger scale processes and, hence, emit lot of particulates and imbumt volatiles into the atmosphere. Hence, several design improvements, such as improved air/fuel mixing and secondary air circulation, have been implemented in order to improve the thermal efficiency of the smaller systems used for cooking and boiling water. The common industrial applications (< 100 MWJ employ either travelling bed combustion or fluidized bed combustion. Travelling grate combustors involve a continuously moving grate and are widely used in incinerators and for biomass combustion of various feedstock sizes. Fluidized bed combustors involve a combustion chamber into which air introduced via a perforated plate keeps the char in fluidized state. Biomass typically has higher percentage of volatiles and, hence, most of the combustion takes place above the bed. For sustained fluidization, biomass is usually used in the form of pellets of fine particle sizes, depending on the fluidization requirements.

 
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