What is BECCS?
As already indicated, the term ‘BECCS’ covers a number of different carbon capture technologies. The system most commonly referred to involves the capture of carbon dioxide emitted from the combustion of biomass for heat and power. Potential capture technologies are the same as those beginning to be deployed and under development for conventional fossil fuel plants, and include post-combustion, oxy-combustion (combustion in pure oxygen rather than air), and pre-combustion gasification (high-temperature non-combustive reactions between biomass and oxygen and/or steam to produce synthesis gas, which is a combustible fuel). An alternative approach captures carbon dioxide from fermentation processes, such as those used to produce ethanol from crops such as maize, wheat or sugarcane. This has the advantage of resulting in a purer stream of carbon dioxide that is easier to process. The carbon dioxide captured can then be transported to geological storage at sites such as saline aquifers or abandoned oil wells, just as with conventional CCS projects.
Another approach, depending on the feedstock and process conditions, is to produce a proportion of the carbon as ‘biochar’ (solid carbon). Biochar has various uses, mainly related to increasing agricultural productivity by improving soil conditions. Although questions remain about the long-term stability of biochar, its use in agricultural soils may reduce the need for fertilizer, since it absorbs and slowly releases nutrients such as phosphorus to plants, improves soil moisture retention (increasing the drought tolerance of crops), improves germination rates and reduces methane emissions from rice paddies. Despite various pilot projects, however, biochar has not yet been deployed at a commercial scale.
BECCS projects and models typically involve five main categories of feedstock:
- Wood: either from whole trees harvested specifically for bioenergy or from forestry wastes and residues, such as small branches, bark and thinnings left over from forestry operations.
- Industrial wood wastes and residues from wood-processing industries (e.g. sawmills): this includes black liquor, a waste product of chemical pulp and paper processes.
- Energy crops: crops grown specifically for energy supply, including fast-growing trees (e.g. willow, poplar and eucalyptus) and herbaceous crops such as miscanthus (elephant grass), switchgrass or energy cane (genetically modified sugarcane).
- Agricultural residues: including field residues (materials left in the field after a crop has been harvested, such as stalks, stubble or leaves) and process residues (leftover materials from the processing of crops for food or other products, including husks, seeds, bagasse,4 molasses and roots).
- Other organic wastes: including post-consumer wood waste, the organic fraction of municipal solid waste, livestock manures, sewage sludge, tallow and used cooking oil.
There is currently an extensive industry using modern technologies to convert energy from biomass without CCS into applications for heat, power or transport. The industry is particularly well developed in the EU, mainly as a result of member state renewable energy targets set out in the 2009 Renewable Energy Directive. Heat, power, liquid and gaseous fuel production processes are all amenable to the addition of CCS, though the capture potential is generally higher for heat and power.
Worldwide, the main biomass feedstock for heat and power is currently wood, though municipal wastes, agricultural residues, and palm oil and other vegetable oils are also used in various locations. Wood is more energy-dense (and slower-growing) than most other feedstocks, and tends to be easier to collect, process, store and transport, often as wood chips or pellets. Some types of residues, particularly bagasse from sugarcane processing and black liquor produced from the digestion of pulpwood into paper pulp, are harder to transport and generally used for power generation on site in sugar mills or paper mills.
The main feedstocks for liquid biofuels – which are usually used in transport applications, though sometimes for heat and power – are oil crops such as oil palm, soybean, rapeseed and sunflower (for biodiesel), and starch crops such as wheat, maize (corn), sugarbeet and sugarcane (for bioethanol). A range of other feedstocks, including wastes, agricultural residues and algae (a high-energy-density feedstock with the potential to significantly reduce land and water constraints), are under development or beginning to enter commercial production.
Biogas is less extensively used in modern energy systems, though production has increased in recent years (particularly in the EU), mainly through anaerobic digestion – a collection of processes by which micro-organisms break down biodegradable material in the absence of oxygen. Most existing installations process residual sludge from wastewater treatment plants or agricultural and food waste, though in principle almost any organic waste can be used. Biogas can be used directly for heat, power or transport, or injected into natural gas grids for heating.
4 The dry pulpy residue that remains after sugarcane or sorghum stalks are crushed to extract their juice.