Realisation of a carbon negative combustor for gas turbines

Gas turbines can be converted to carbon negative technologies using biomass energy combined with carbon capture and storage processes

Pietro Bartocci and Alberto Abad
Instituto de Carboquímica (ICB-CSIC)

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Article Summary

Gas turbine sector evolution. The power sector is undergoing rapid technological change. It is likely that conventional gas turbines will need to be integrated in systems employing biofuels and/or carbon capture usage and storage (CCUS) at competitive costs. The EU is moving swiftly towards low carbon technologies (such as energy efficiency, smart grids, renewables and CCUS) under the European Energy Union Strategy (European Commission, 2015). The development of carbon sequestration methods was identified by the US National Academy of Engineering ((NAE, 2017) as one of the ‘Grand Challenges for Engineering’. This technology was considered of paramount importance for the UN Sustainable Development Goals, SDG7 (Affordable and Clean Energy), and SDG5 (Climate Action).The Gas Turbine Chemical Looping Combustor for Carbon Negative Power Generation (GTCLC-NEG) project addresses two of the five pillars of the European Energy Union Strategy — renewable energy and CCUS — and couples these with breakthrough technologies, such as chemical looping combustion (CLC).

Biomass absorbs CO2 during growth and releases it when burnt, but if the CO2 is captured after combustion, this will result in a net flow of carbon out of the atmosphere. Thus, when bioenergy is coupled to CCUS, it is called bioenergy with carbon capture and storage (BECCS), which according to the Intergovernmental Panel on Climate Change (IPCC) report of 2014 is a negative emission technology (NET) (IPCC, 2014). This means that BECCS is able to remove CO2 by the atmosphere.

One of the most effective ways to couple biomass energy and CCUS is to burn biomass or biofuels (i.e. pyrolysis oils, biogas, and solid biomass) in a chemical looping combustor, where CO2 can be easily captured after exiting the fuel reactor (see Figure 1). CLC uses an oxygen carrier based on a metal oxide to transfer oxygen to the fuel and obtains CO2 in pure form inherently in the process. As CLC is not burdened with any separation work, it is particularly applicable for carbon capture and can achieve negative emissions when used with biomass. The social and economic impact of substituting current natural gas combined cycle power plants with GTCLC cycles is huge. In a study presented at the SDEWES Conference in 2020, we calculated that this project could be covered in part by the increasing carbon credit price and could generate at least 100,000 new jobs on a European level, solving a key problem for the gas turbine sector.

GTCLC-NEG concept
As well as being awarded a Marie Curie - Individual Fellowship, the GTCLC-NEG project has been funded by the European Commission (EU Horizon 2020 Framework Programme, Grant 101018576), with the Spanish National Research Council (CSIC) as the host institution. The GTCLC-NEG project is being developed at Instituto de Carboquimica (ICB-CSIC, see Figure 2) from July 2021 to June 2023.

The aim of the project is to promote a carbon negative technology capable of burning multiple biofuels derived from biomass and to capture the CO2 emissions at a very low cost. In this way, there will be negative GHG emissions due to the use of BECCS, a technology that is set to be developed by 2050 according to the IPCC.

The proposed plant is based on the coupling of a chemical looping combustor to a gas turbine, as proposed in Figure 3.

As it can be seen in the proposed plant, the compressed air used to oxidise the oxygen carrier is heated in the air reactor and then heated in the air expanded in a gas turbine to produce electricity. In the fuel reactor, biofuels (in this case, pyrolysis oils) are used to reduce the oxygen carrier.
Possible technical barriers include:

 Suitable metal oxides or bimetallic oxygen carriers are needed
 Low attrition rate oxygen carriers that can work in extreme conditions are required
- Kinetics aspects under high pressure and temperature conditions are unknown
 Reactor injection system must be adapted to biofuels  
 The use of the hot air produced from the air reactor (see Figure 3) in a gas turbine has to be optimised; exhausts should be filtered to retain the dust released by oxygen carrier attrition
‘ High electrical efficiency of the power system has to be granted together with high fuel conversion in the combustor.

To summarise, one of the most critical aspects of the technology is the operation of the chemical looping combustor at high pressures. Rarely has this been done on a large scale, and for this reason the modelling of the reactor and the chemical processes that occur during pressurised CLC appear to be of scientific interest.

Effective models have already been developed at zero-dimensional level at the Instituto de Carboquimica, based on the shrinking core model (SCM), which is widely adopted in literature to describe oxygen carrier behaviour. Computational fluid dynamics (CFD) models have also been developed. Nevertheless, the effect of pressure on the CLC process has not yet been fully described.

The GTCLC-NEG project aims to apply different strategies, which can be found in the literature, to model the fuel and air reactors using CFD software, with improved kinetic constants. Once the CFD model has been tested on plants available at the Instituto de Carboquimica, the data available will be used to design the final combustor and couple it to a gas turbine in Aspen Plus software. The mass and energy balances of the complete plant will then be verified by the proprietary software of an important gas turbine production company.

Project objectives
The main objective of the current project is to develop a new combustor capable of burning multiple biofuels derived from biomass (such as pyrolysis oil, biogas, and syngas) and to capture the CO2 emissions at a very low cost.

The development process will be efficient and cheap, and it will be possible to integrate it in a power plant with negative CO2 emissions. This will be done by overcoming current barriers through proper training.

The following objectives will be developed by the project work packages:

 - Production of four innovative bimetallic oxygen carriers and characterisation of their performance at high pressures and high temperatures
Realisation of tests of CLC of three biofuels (pyrolysis oils, syngas, and biogas) in high pressure (1-1.5 MPa) and high temperature (1200-1300°C) batch fluidised bed reactor. Attrition and agglomeration issues will be assessed.

 - Development of:
    -    Particle model, where the main kinetics aspects of redox reactions is analysed in critical conditions
    -    Zero-dimensional model of the CLC reactor
    -    Scaled-up model in Aspen Plus of the GTCLC plant

Realisation of tests with the best-performing oxygen carrier in a recirculated multifuel 1 kW CLC unit fluidised bed reactor, optimising the reactor injection and hot exhaust gas cleaning, to obtaining carbon conversion of >95% and CO2 output of >90%.

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