Negative Emissions Technologies (NETS): Feasibility Study - Technical Appendices

Appendix 5. Biomass conversion techniques

Thermal Treatment (Combustion)

Thermal treatment, more often referred to as incineration or combustion, involves the combustion of biomass to produce useful heat. Combustion is a complex interaction of chemical and physical processes, highly dependent on the quality of fuel source and quantity of air. Biomass feedstocks are rich in biogenic carbon, hydrogen, and oxygen, which are all essential in the combustion process. There are several systems available, however, the principle of biomass combustion is essentially the same for each.

The three main stages of combustion, in order, are heating and drying, pyrolysis, and char combustion. Biomass contains moisture, which must be released prior to the latter stages of the process. This is achieved through supplying heat produced by radiation from the flames, as well as the stored heat in the combustion unit. Once the biomass has been dried, it is then able to undergo pyrolysis, where it is subject to temperatures between 200degC and 350degC. At these temperatures volatile gases are released, including CO, CO₂ and CH₄, that react with the O₂ present in the incinerator, thus resulting in a self-sustaining process, ceasing when all the O₂ has been consumed or all volatile gases released. Pyrolysis results in the deposition of a residue known as biochar in the incinerator, once all the volatiles have been combusted. Biochar is a material consisting of carbon and ash. The final stage of the combustion process involves the injection of O₂ and subjecting the biochar to temperatures more than 800degC, allowing the biochar to oxidise and thus fully combust, i.e., for all the energy present in the biomass to be fully extracted. Longer residence times and sufficient air in the chamber allows for complete combustion to occur, and results in a flue gas containing lower concentrations of CO. It should be noted that all the above stages can occur subsequently as well as simultaneously^[223].

The heat generated from combustion is used in a boiler to produce high pressure steam that is passed through a turbine, which in turn, generates electricity.

Gasification

Biomass can be gasified to produce syngas and eventually hydrogen using a range of different feedstocks: woody biomass, energy crops and waste^[106]. During gasification, biomass is partially oxidised at 800degC to produce syngas, which is subsequently cooled down to enable the removal of particulates, heavy metals, tars, and acidic gases. The cleaned syngas is then processed in a similar manner to the SMR/ATR configurations; however, the process efficiency is lower (46-60% for bio gasification versus 74% for SMR/ATR)^[106]. The keys methods of biomass gasification are autothermal and allothermal gasification, where fluidised bed gasifiers are favoured due to their flexibility and robustness^[106]. Hydrogen can also be produced via pyrolysis or hydrothermal carbonisation^[35].

Autothermal gasification

In order to maximise hydrogen production via autothermal gasification, pure oxygen and steam are used as the gasifying agents. This is because they increase the syngas H₂:CO ratio, which in turn encourages the conversion of CO to H₂ and improves hydrogen yields. The moisture content of the feedstock should be limited to 15% wt, which leads to feedstocks having to be pre-treated and dried beforehand^[110].

Allothermal gasification

During allothermal gasification, steam is typically used as the gasifying agent and fluidised beds are the favoured choice of reactor. Fluidised bed technology is typically split into the categories of externally heated reactors, fast internally circulated fluidised beds (FICFB), and dual fluidised beds (DFB). More recently attention has been focussed on plasma allothermal gasification^[128].

“The process functions in a similar way to coal gasification, but there are additional requirements for pre-processing the feedstock (e.g., drying), and more effort is required to clean the syngas to remove contaminants before upgrading it to hydrogen. Therefore, there remains some uncertainty around whether biomass gasification can be deployed at scale in a commercially viable way.^[224]” Efficiencies of 46-60% are achievable and the production of biohydrogen is seen as a high value use of sustainable bioenergy; however, the feasibility of technology is uncertain at present due to the lack of demonstration plants.

Advanced Thermal Treatment (Pyrolysis)

Advanced thermal treatment technologies are those that employ pyrolysis and/or gasification to recover energy present in biomass. Both are thermochemical reactions, the difference being that pyrolysis is carried out in the absence of oxygen, i.e., anaerobically, whereas gasification utilises a controlled amount of oxygen.

The biomass feedstock the pyrolysis unit where it undergoes thermal degradation at temperatures between 250 – 900degC. Three main products are formed during this process, namely, char (solid residue rich in carbon), pyrolysis oil, and syngas. Of the three outputs, syngas, which is a combustible gas, is utilised for power generation. Exact chemical composition of syngas is highly dependent on that of the biomass inputs, nevertheless, the composition of syngas produced from pyrolysis is composed primarily of carbon dioxide, carbon monoxide, methane, and hydrogen. The main application of syngas is typically the generation of power and heat; this can be realised either in stand-alone CHP plants, or through co-firing of the product gas in large-scale power plants. This combustible gas can be used for production of power in several types of equipment, such as gas engines and turbines, both of which employ steam cycles. Syngas does not typically require extensive gas treatment before use in the previously mentioned steam cycles, however, when it is utilised in gas engines, it requires a higher degree of purification and preparation^[225].

Biological Treatment (Anaerobic Digestion)

The two main biological treatment techniques of biomass are anaerobic digestion and aerobic digestion. Biological treatment is based on the decomposition of biodegradable waste by living microbes, namely bacteria and fungi, which use the feedstock as a food source for growth and proliferation. The process occurs either aerobically or anaerobically, although anaerobic digestion is more prevalent in the biomass context as this method produces useable, combustible gases.

The biomass is mixed and macerated, with water added to create the required moisture and flow conditions. As the process is anaerobic, the digestors are sealed, and mechanical stirring devices continuously mix the contents of the unit. Biodegradable material present in the digestor under these conditions is ultimately converted into a biogas containing high concentrations of methane (50 - 75%) and carbon dioxide. Additionally, water is produced due to the fermentation that occurs within the vessel, resulting in a wet organic mixture also being present. Anaerobic digestion takes approximately three to six weeks to complete depending on the exact biomass feedstock and is carried out at temperatures of 30 – 40degC^[226].

There are several methods in which the energy present in the biogas can be recovered, either in a CHP generator unit to produce electricity and heat, or in boiler where it is combusted to produce hot water and/or steam. In some cases, treatment of the biogas is required to remove contaminants such as hydrogen sulphide, or moisture.

Biomethane reforming

Steam methane reforming (SMR)

Methane reforming is the conventional method of hydrogen production, where natural gas is the typical feedstock source. However, a low carbon alternative is to use upgraded biogas (‘biomethane’), produced from anaerobically digested waste/residues, as a direct substitute to natural gas.

SMR is the reaction of methane with high temperature steam, at 912degC and 28.5 bar, to produce syngas (a mixture of H₂, CO and CO₂). This syngas is subsequently passed through a reverse water gas shift (RWGS) reactor, where excess CO is converted to CO₂ and H₂ to improve hydrogen yields. Typically, either a sequence of one or two RWGS reactors are used; a single high temperature reactor (HT) or both a high temperature and low temperature reactor (HTLT). The HT reactor operates at temperatures in excess of 300degC, whilst the LT reactor operates at circa 180degC. The gas stream exiting the RWGS reactor(s) is passed through a CO₂ capture unit and a pressure swing adsorption (PSA) column, where CO₂ and H₂ are recovered at purities of 99.97%. The CO₂ and H₂ are then compressed to 110 bar and 200 bar receptively, ready for transport. A combustible by-product exiting the PSA unit, known as ‘tail gas’, is recovered and burnt within a fired heater to partially meet onsite energy demands. This maximises process efficiencies to 74%, with the remaining onsite demands being met through supplementary firing of natural gas. To ensure that all process emissions are captured, post-combustion capture must be used^{[120], [127]}.

The choice over the HT or HTLT configuration does not have an impact on NETs process efficiency. In the case of HTLT, the higher H₂ yields lead to greater upstream efficiencies; however, these are counteracted by the tail gas exhibiting a lower heating value, and hence more supplementary natural gas must be burnt onsite to compensate. The opposite is true for the LT configuration. A proportion of the H₂ product can also be recycled back to the reaction vessel to increase H₂ yields; however, this again leads to a reduction in heating value for the tail gas, and hence reduces NETs efficiencies^[127].

Auto thermal reforming (ATR)

Unlike SMR, the ATR configuration partially oxidises methane using pure oxygen sourced from an Air Handling Unit (AHU). The heat from this exothermic reaction is sufficient to meet the reactor’s energy demands, and hence no external heat source is needed. The subsequent processing steps relating to RWGS, CO₂ capture, and PSA are the same as SMR. The tail gas exiting the PSA unit is also burnt in a fired heater to meet onsite energy demands, with no need for supplementary natural gas firing. Therefore, pre-combustion capture can be utilised. Please note that the purity of H₂ is lower than compared to SMR (99.9%)^[127].

Unlike SMR, the HTLT configuration is favoured, as it increases NETs efficiency by 7%. The HTLT configuration does again lead to increased hydrogen yields and a reduction in tail gas heating value; however, as no supplementary natural gas firing is needed onsite, then the reduced heating value does not compromise the gains in upstream process efficiencies. The impact of H₂ recovery is negligible^[127].