Waste pressurised gasification - State of knowledge and perspectives

Synthesis

The production of synthetic methane from biomass and waste is generating considerable interest in France. Stakeholders, and gas network operators in particular, see it as a local solution for decarbonising and recovering value from waste, complementing methanisation and serving local communities. The three main technologies currently in development are power-to-gas, pyrogasification and hydrothermal gasification. This study's analysis of the state of the art of these technologies shows the constraints and opportunities for their development in France.

Thermal gasification under pressure is a technology that is still little known, yet it has the advantage of producing a syngas that is richer in methane than a syngas produced by gasification at atmospheric pressure. The aim of this study is to analyse the changes brought by the use of a pressure greater than 5 bar in order to assess its relevance and potential for development in response to the production of low-carbon gas.

State of the art in pressurised gasification

The European gasification market, and in particular pressurised gasification, is still in emergence. 
Few pressurised technologies are available and, with the exception of very high-power coal gasification in China, South Africa and the United States, none of them has been implemented in many industrial installations. Therefore, difficulties are expected in future projects, particularly those involving biomass and waste, especially during the development phase.

Very high-power coal gasification uses reactor pressurisation to improve profitability. The end-use of syngas is the main parameter in the choice of pressure, with a tendency to align the pressure in the reactor with that required in fine.
The level of pressure also depends on the technology, fluidised beds generally using lower pressure than entrained beds for technical reasons.
All the operating plants (coal-fired for the production of ammonium, methane, polypropylene, etc.) operate above 5 bar and the vast majority between 25 and 40 bar (and even up to 60 bar), demonstrating the industrial interest of pressurised gasification at high power for the syngas chemical valorisation.

As far as we know, coal is no longer used for gasification in Europe, so the main fuels are biomass (wasted wood) and some waste such as SRF (Solid Recovered Fuel). These fuels have a different C/O ratio, ash content and fusibility than coal. They are also more heterogeneous and more difficult to grind (for use in entrained beds). In addition, these fuels are less geographically concentrated, which tends to lead to the development of smaller plants and makes the use of some technologies more difficult.

The current production costs are still too high in relation to European market conditions. However, the increasing maturity of the technologies under development will limit risks and increase productivity. Incentives to decarbonise industry and transport will improve the syngas profitability.

Impact of pressurisation

Positive points:

- Increased volumetric productivity and therefore smaller gasifiers. The kinetics of gasification reactions, which are limiting factors in the process, are greatly accelerated by increasing the pressure. In addition, the density of the oxidising agent is higher, allowing more of it to be injected at a given surface area and speed.

- Energy gain on compression: electricity consumption is greatly reduced by compressing the oxidising agent and fuels rather than syngas, as the volume of gas generated is greater than the volume of input (oxidising agent and fuel). The gain depends on the air factor used (the higher it is, the more oxygen there is and the smaller the difference between the two cases), the type of oxidiser (air, oxygen, oxy-steam in ascending order of gains when the reactor is pressurised), the oxygen production pressure and the possibility of recovering free CO2 already pressurised (after final syngas purification, for example) to avoid the cost of inerting gas. 
Nevertheless, the total electricity consumption of the plant is only slightly reduced, as the largest electricity consumption item is the oxygen production.
In addition, by compressing the oxidising agent rather than the syngas, the gas compressor is moved from downstream (in an ATEX zone) to upstream of the reactor (in a non-explosion zone). This improves the safety and reliability of the installation and slightly reduces CAPEX (smaller, non-ATEX compressor).

- Efficiency gains in downstream processes: WGS, methanation and purification. These gains are moderate and logarithmic (most of the gain is obtained at 10 bar). For example, a gain of +6.9% [0.9%] in CH4 selectivity during methanation at 350°C by increasing pressure from 1 [10] to 20 bar. The use of high pressures will require R&D on downstream processes for adapting them to higher pressure.

- Increase in the CH4 content produced. Pressurisation shifts the thermodynamic equilibrium towards an increase in methane production. This increase remains limited (max CH4 volumetric content: 21% for the EXXON reactor), both theoretically (residence time and temperature limit methane production) and in practice (frequent addition of a catalytic and/or high-temperature cracking stage, which significantly reduces the CH4 content).

- Impact on tars: the literature on this subject is sparse and controversial. It seems that pressurised pyrolysis produces slightly less tars. Their cracking is also modified, slightly accelerated, with a tendency to produce larger molecules such as naphthalene. However, an inhibiting effect on some catalysts has been observed.

Critical points:

- Increased safety constraints: increased risk of explosiveness and self-ignition and risk of leakage. The self-ignition temperature (gaseous medium) decreases, as do the concentration of particles (dusty medium) and the power of the ignition source required to trigger an explosion. The rate of pressure rise and the maximum pressure of explosions are increased. Pressurising the reactor increases the risk of leakage. Unlike a depressurised reactor, a leakage problem results in the emission of syngas into the outside environment, requiring the installation of appropriate safety devices and modifying the ATEX (gas) zoning.

- Complexification of fuel feeding and ash extraction systems, leading to an increase in CAPEX and OPEX (inerting gas consumption). There are no particular technical problems up to 30-40 bar, thanks to several technologies: feed hoppers, screws, pneumatic feeding, piston feeding. Beyond 30-40 bars, the available solutions are limited. The feeding systems availability and CAPEX cannot be precisely evaluated because of the lack of industrial feedback. Fluidised bed gasifiers are more affected than fixed or entrained flow gasifiers.

- Developing new technologies and bringing them to market is more complex: regulatory constraints, technical tuning, choice of subcontractors. This will eventually disappear once the market has matured.

- Increased stress on construction materials: increased reactor wall thickness (offset by the reduction in reactor size in terms of the total metal weight), increased stress on sealing systems. Refractories are generally porous and are therefore little affected by the pressure rise.

Available technologies

Many technologies are available on the market. Mainly developed for coal, there is limited feedback on other fuels (biomass, waste).

The technologies listed in this study that apply to fuels other than coal are :

• Prenflo® (ThyssenKrupp Uhde, entrained flow, solid fuel)
• MPG® (Lurgi Air Liquide , entrained flow, liquid fuel) 
• KEW technology (fluidised bed) 
• EQTec (fluidised bed)
• AFB® (ICC/CAS; fluidised bed) 
• TRIG® (KBR Transport, fluidised bed) 
• High Temperature Winkler (Gidara, fluidised bed) 
• SHI (Foster Wheeler, fluidised bed) 
• TarFreeGas® (Frontline Bioenergy, fluidised bed) 
• Enerkem (fluidised bed) 
• Renugas® (GTI / ANDRITZ Carbona, fluidised bed) 
• EUP process (Ebara Ube JGC, staged, plastics) 
• BioTfuel® / BioTJet (staged, biomass)
• Bioliq® (Air Liquide, staged, biomass)
• Carbo-V® (CHOREN, stepped, biomass)

Each type of gasifier has its own advantages and disadvantages when increasing the pressure.

- Fluidised bed

Commentary: 
Fluidised beds are the most widely used gasifier technology in Europe. They are well suited to the size of fuel deposits and can accept a wide range of fuels, provided they are sufficiently well prepared. However, particular attention needs to be paid to tars, which are produced in large quantities and entail significant CAPEX/OPEX costs for their treatment. This technology is the most sensitive to pressure increases (modification of the injection speed of the oxidising agent - which fluidises the bed - and a fuel feeding system in the lower part of the reactor that is sensitive to pressure increases).

- Entrained flow

Entrained flow is the most mature gasifier technology, but only with coal. They are designed for high power levels (capacity of the Lurgi MPG of 35,000 Nm3/h of hydrogen). The biomass use is still at the laboratory/demonstration stage (TRL 6-8) and involves a high degree of pre-treatment of the fuels to convert them into powder (~15-20% efficiency loss on wood). Given the current state of the technology, this grinding cost is too high for the deployment of entrained flow gasifiers in Europe. 

However, staged technologies (pyrolysis or torrefaction followed by entrained flow gasification) could allow a wider fuel range to be used with an acceptable efficiency loss (~3%), although this would make the installations more complex.

- Staged gasification

Commentary:

It is difficult to analyse this gasifier category 'generically', given its wide disparity.

Each technology has its own characteristics and is designed for a specific type of fuel.

Using two reactors for gasification makes the process more complex. This opens up the field of usable fuels and potentially reduces syngas purification costs.

These are therefore interesting technologies, but a reduced maturity. 

- Counter-current fixed bed

Commentary:

The pressurised counter-current fixed-bed technologies currently on the market are mature on a large scale, but only with coal. They are designed for high output (capacity of the Lurgi Mark IV: 12,600 Nm3/h of methane). 

The high tar content implies the need for a gas treatment system, which seems expensive to implement on a small scale. The use of biomass remains at the laboratory and customer test stage (TRL 6-8). 

This technology does not therefore seem suitable for this study.

- Fixed co-current bed

Commentary:

Generally dedicated to small biomass installations, co-current fixed-bed gasifier technologies are only available on the market at atmospheric pressure.

They could be used to produce syngas at different operating sites (close to the fuel deposit) and then transporting the syngas for processing at a plant where the equipment is pooled. However, this would require a twofold R&D effort (on pressurising fixed co-current bed gasifiers and transporting syngas).

This technology does not therefore seem suitable for this study.

Case studies

Two cases were chosen to illustrate this study: one at atmospheric pressure (GobiGas, as a reference) and the other under pressure (KEW Technology). These technologies were chosen because of their relevance to the study topic and the data availability.

Gobigas

The Gobigas (Gothenburg Biomass Gasification) gasification facility produced 20 MWCH4 HHV from biomass (pellets, forestry chips, recovery wood) in Sweden. 

The gasifier, designed by Repotec, is a double fluidised bed at atmospheric pressure.

This demonstration project was supported by the municipality of Göteborg. It was initially planned as a first step followed by an 80-100 MW plant. 

The plant was in operation from 2014 to 2018. It injected biomethane into the regional network at 35 bar. It was dismantled for economic reasons (low profitability) after operating for more than 12,000 hours.

The project Heat & Mass balance is shown in the figure below: 

The biomethane efficiency (η_CH4=E_CH4/E_combustible ) was between 50% and 63% (pellets) and between 40% and 55% (other fuels). The Heat & Mass balance in Figure 1 assumes an efficiency of 63%.

Regarding a methane production of 150,000 MWhPCS, electricity consumption is estimated between 95 and 110 kWhél / MWh PCS CH4.

CAPEX was €122m (€143m in €2014) and OPEX €7.5m/year, excluding fuel. The cost of methane production (excluding site development and input costs) is around €123/MWhPCS.(€2014 , or €137/MWhPCS  in €2023).

KEW Technology
KEW technology offers 10 MW XTE modules (PCI fuels) in a bubbling fluidised bed. A reformer treats the syngas cracking all the tars into non-condensable gases. The result is a stable, tar-free final syngas under pressure (~7 bar).

A simulation was carried out with 3 XTE modules, giving a power output of 30 MW (PCI fuel input) for a methane production of 20 MWPCS.

Figure 2: Detailed material and energy balance for the KEW 3 XTE modules installation (RECORD, 2024)

The biomethane efficiency (η_CH4=E_CH4/E_combustible) is 59.1%.

Table 1 : Item-by-item electricity consumption of the modelled facility (RECORD, 2024)

Regarding a methane production of 150,000 MWhPCS, electricity consumption is 290 kWhél /MWhCH4 HHV.
The demonstrator produces syngas at around 7 bar. Developments are underway to increase this pressure. As a result, the syngas compressor will no longer be needed between the XTE module and the WGS reactor, and electricity consumption will fall to 278 kWhél / MWh PCS CH4.

CAPEX is estimated at €95m (±30%), excluding site development, and OPEX at €10m/year, excluding fuel.
The cost of methane production (excluding site development and input costs) is around €130/MWhHHV.

Technical conclusion

Up to 30-40 bar, pressurised gasification is industrially feasible without any particular technical problems, with a high level of experience with coal as a fuel, a moderate level of experience with biomass and a low level of experience with waste. Many industrial references attest to this. Feeding systems are available for all fuels.
Above 30-40 bar, the stresses on the fuel feeding system become very great, particularly for screw systems. In addition, the use of oxygen poses safety problems if the pressure is too high.

Three uncertainties remain on the impact of pressurising gasifiers:

- Economic: pressurisation makes the fuel feeding system more complex, requires compression of the oxidising agent, imposes constraints on the strength of the reactors and gas proofing, versus reducing the reactors size (and therefore the materials and construction cost), reducing the electricity consumption associated with syngas compression (if the end-use pressure of the syngas is greater than or equal to that of the gasifier) and, in some cases, avoiding the need for a syngas compressor between the gasifier outlet and the syngas upgrading stages.
The impact on CAPEX will vary depending on the gasification technology and the syngas end-use. However, it should not exceed ± 5-10% of CAPEX regarding the modified items. For an equivalent gasification technology and if the end-use pressure of the syngas is greater than or equal to that of the gasifier, electricity consumption will be reduced by pressurising the reactor, and OPEX will therefore fall slightly (by around 1 to 10% depending on the electricity price and the syngas end-use). 

- Availability: the gasifiers pressurisation imposes technical constraints, particularly on the fuel feeding system and gas proofing, that may have a negative impact on the plant availability. The lack of industrial feedback on pressurised gasification (except for coal) prevents to determine if the availability of a pressurised gasifier will be the same as the availability of an atmospheric gasifier.

- Efficiency: pressurisation modifies the production of tars, the oxidising agent density and the chemical reactions equilibrium in the reactor. The operating conditions and the kind technology have a so strong impact on gasification efficiency (η=(débit syngas ×PCI syngas)/(débit combustible ×PCI combustible)) that it is difficult to assess the impact of pressurisation on the efficiency. At the same residence time and temperature, pressurisation should slightly increase the efficiency (better diffusion of the oxidising agent, fewer tars, better carbon conversion).

Gasifier pressurising reactors up to 30 bar seems to have almost no effect on the acceptability of different fuels. Inside the reactors, there is no change in the reaction mechanisms that would favour or disfavour a fuel compared with its use at atmospheric pressure (at constant temperature). Fuel feeding systems exist for all types of granulometry. Above 30 bar, liquid or powdered fuels feeding is only slightly impacted, unlike high-granulometry solid fuels, which become difficult to feed into the reactor.

Gasification technologies are not affected in the same way by reactor pressurisation.
- The vast majority of entrained flow gasifiers are designed to be pressurised. The fuel is in powder or liquid form, which makes it easier to pressurise. In addition, these gasifiers require high temperatures. Increasing the pressure and therefore the volumetric productivity of the reactors help to reach the desired temperature level.

- Pressurisation of counter-current fixed-bed reactors increases their diameter. The oxidising agent penetrates better and further into the bed. However, care must be taken to avoid the creation of hot spots where the ashes could fuse, causing problems with their removal. The fuel introduction system, which is generally gravity-fed from the top of the reactor, requires the addition of intermediate hoppers to pressurise the fuel. No particular technical problems up to 40 bar are reported in the literature.

- Pressure has a greater influence on fluidised bed gasifiers. Pressure modifies the density of the oxidising agent and therefore its introduction velocity (at a constant mass flow rate). As it is this rate of introduction that fluidises the bed, the reactor must be correctly sized for each pressure. In addition, the fuel introduction system, which is generally located in the lower part of the reactor thanks to a screw, is more difficult to pressurise than other introduction systems. Proven and reliable solutions currently exist up to 20 bar. It should be possible to increase pressure up to 30 or even 40 bar if industrial demand justifies R&D programmes in this field.

- There is no recent feedback on co-current fixed bed gasifier pressurisation.

General conclusion

The European market for the non-fossil fuels gasification, and more specifically for pressurised gasification, is still emerging. For the time being, the ecosystem of manufacturers and subcontractors is limited. It is therefore difficult to compare technologies that have not yet been deployed on an industrial scale. 
In addition, pressurising the gasifier generates changes that remain relatively limited. According to the estimation of this study, the major impact on a wasted wood gasification plant for methane production (a case that is very favourable to pressurised gasification) would be a reduction in electricity consumption of 23.5% (i.e. around 10% of OPEX).
At present, therefore, it is not possible to decide between pressurised and atmospheric gasification. 

In the short term, atmospheric gasification is benefiting from better feedback in Europe.
Pressurised gasification entails numerous constraints (on reactor materials, fuel introduction and ash extraction systems, safety systems, etc.), offset by energy and surface gains.
There seems to be a capacity threshold beyond which the advantages of pressurisation outweigh its disadvantages. This threshold depends not only on the size of the installation, but also on the end use of the syngas, i.e. whether it needs to be compressed (biomethane, ammonia, etc.) or not (cogeneration engine, heat, etc.). 

However, the biomethane production market seems particularly well suited to pressurised gasification, given the output pressure required and the substantial size of the plants needed to make them economically profitable (around 1,000 Nm3/h of methane, according to GRTGaz's AMI). The processes needed to convert syngas into methane (WGS, methanation, CO2 purification) become more efficient as pressure increases. Furthermore, the CO2 produced can also be used as an inerting gas and reused in the methanation loops (to cool the reactor). Substantial savings (>20%) will be made on electricity consumption due to gas compression.

The use of pressurised gasifiers is therefore a promising way of reducing the biomethane production cost by gasification. It should develop further if economic conditions allow a profitable market and a complete industrial ecosystem to emerge.

However, there are still a number of technical hurdles specific to pressurised gasification (fuel and ash management, development, regulations, safety) and others in common with atmospheric gasification (tar treatment, filtration and syngas cooling) before large-scale commercial deployment.