Production of biohydrogen from gasification of waste fuels: Pilot plant results and deployment prospects.
Summary (4 min read)
- In recent years hydrogen has received increasing attention as a potential fuel that could be produced from non-fossil fuel sources (Hart et al., 2015; Barisano et al., 2017; Ogden, 2018), both because it can be generated with low greenhouse-gas (GHG) emissions, and because it generates no emissions at the point of use.
- There are currently two prominent pathways for the production of low carbon hydrogen: one is through reformation of natural gas (by Steam methane reforming (SMR) or Auto-Thermal Reforming (ATR)), combined with carbon capture and storage (CCS).
- Waste and second-generation biomass materials, although readily available and abundant, have limited uses in terms of chemical feedstocks, due to the need for pre-treatment and presence of many contaminants which add complexity and costs (Materazzi and Lettieri, 2017a).
- Future process improvements are also identified, along with an early stage assessment of carbon emissions.
- This section provides an overview of the main transformation steps for production of bioH2 from wastes.
2.1 Hydrogen use and distribution
- Hydrogen has been viewed as a potential vector to decarbonise transport for the last two decades.
- While electrification looks set for a large role in decarbonising cars, there are fewer low carbon solutions for heavy goods vehicles (HGVs) and buses (Hua et al., 2014).
- A site-specific exemption has been granted by the Health and Safety Executive (HSE) to blend hydrogen at 20% into the gas network at Keele University, and it is anticipated that blending of hydrogen from a variety of sources into the gas distribution could be feasible as early as 2020 (Hodges et al., 2015).
- Injecting hydrogen into the network to create a blend for use by consumers provides an opportunity for the early roll out of biohydrogen production, since there is significant demand for low carbon replacements for natural gas that are compatible with existing appliances.
- The longest pipelines are operated in the USA, followed by Belgium and Germany (HyARC, 2017).
2.2 Scale definition and feedstock preparation
- Compared to biomass, MSW materials introduce a greater concentration and diversity of contaminants, due the large number and variability of sourcing points.
- This presents a major challenge, compounded by the fact that more sophisticated applications (including catalytic processes for bioH2 production) have low tolerances.
- The untreated municipal or commercial waste is first mechanically processed in a materials recycling facility (MRF).
- A good potential reference WtH2 plant size could treat around 100,000 tonnes per annum of RDF, this being supplied from a reasonably sized town, accounting for residual domestic, commercial and industrial waste arisings.
- A typical bus will consume around 5 tonnes per annum of hydrogen.
2.3 Syngas generation
- The key element for a consistent quantity of bioH2 is the production of a high quality syngas very rich in hydrogen, and suitable for catalytic processing.
- In principle, these offer a significant thermochemical efficiency advantage for bioSNG production, providing a syngas with as much as 15% methane.
- The use of plasma has increasingly been applied with waste treatment for its ability to completely decompose the input material into a tar-free synthetic gas and an inert, environmentally stable, vitreous material known as slag.
- This include various hazardous wastes, such as polychlorinated biphenyls (PCBs), medical waste, and low-level radioactive (Gomez et al., 2009; Wang et al., 2009; Byun et al., 2010; Lombardi et al., 2015; Sanlisoy and Carpinlioglu, 2017).
2.4 Syngas clean up
- The fuel gas exits the gasification stage at temperature usually higher than 800 ˚C and comprises mostly H2, CO, H2O and CO2 and a number of minor contaminants.
- After heat recovery, the syngas has to go through a gas cleaning system to remove tars, particulates, vapour phase metals, acid gases (mostly, Cl- and S- based), and a myriad of trace species released from RDF which could hinder the effective syngas utilization downstream.
- The cleaning from these problematic species down to values that are acceptable for different downstream catalysts are of crucial importance for successful implementation of waste gasification technology, and in particular bioH2 applications.
- Secondary poisons are the metal hydrides (arsine/phosphine) and acid gas components (HF/HCl).
- For economic reasons at small scale, the organic sulphur, unsaturated hydrocarbons (e.g. ethylene, acetylene, etc.) and light aromatics arising from lower intensity gasification cannot be removed using physical solvent scrubbing such as Rectisol, and must be dealt with thermal or hydrothermal technologies.
2.5 Water Gas Shift reactors
- Hydrogen produced from gasification is not sufficient to sustain a WtH2 plant on its own.
- This is an exothermic equilibrium reaction with hydrogen production favoured by low temperature, often obtained with two sequential reactors with intercooling stage.
- The most common LTS catalysts are varieties of copper/zinc oxides supported on alumina substrates (Cu/ZnO/Al2O3) (Gokhale et al., 2008).
- Cobalt/molybdenum (CoMo)-based shift catalysts are instead widely used for syngas streams that contain very high levels of sulphur.
- The aim in the bioH2 case is, therefore, to push the WGS reaction to a practical limit, while providing a clean and good quality syngas to ensure high catalyst longevity.
2.6 CO2 removal and upgrading
- The product from WGS is normally a mixture of H2 and CO2, with some other minor components (N2, CH4, CO) depending on the upstream processes.
- There are many commercially deployed techniques for separating CO2 from process streams with the optimal solution depending on factors, such as the required specification of the product stream, required CO2 purity, and the temperature and pressure conditions of the inlet and outlet streams.
- There are two relevant specifications for hydrogen produced by a WtH2 plant presented in this work, namely: bioH2 for use in fuel cells: 99.95% purity, with additional restrictions on certain contaminants (Shabani and Andrews, 2015); bioH2 for use in the gas network, in industry, or blended into the natural gas network (“grid-quality”): <2% inerts, CO content below 100ppm, some methane can be accommodated (up to 10% vol) (De Santoli et al., 2017; Kouchachvili and Entchev, 2018).
- Several studies have shown that PSA results in significant slip of H2 into the tail gas, reducing both product yield and purity of the CO2 stream (Olajire, 2010).
- Figure 2 500 kW pilot gasification plant for clean syngas production Figure 2: 500 kW pilot gasification plant for clean syngas production (top), 50 kW bioH2 production from stored syngas RDF Fluidised bed gasifier Plasma converter Dry filter Waste heat Boiler Vitrified slag Steam O2 Raw syngas O2 Steam Wet scrubber unit Fan FeO Guard bed Syngas storage Dry scrubbing residue Scrubber treatment chemicals NaHCO3 +.
3.1. Syngas generation plant
- The gasification process is a combination of two distinct thermal process steps.
- The plasma converter completely degrades complex hydrocarbons and tars reducing them to a clean syngas stream along with simple inorganic contaminants such as hydrogen sulphide and hydrogen chloride, which are readily removed with conventional gas scrubbing techniques.
- This is a particularly important aspect in waste treatment facilities because it ensures that only minimum residues (mostly for air pollution control and gas cleaning media) are sent to treatment and disposal (Materazzi et al., 2015a).
- This includes a dry filter (incorporating a ceramic filter unit with sodium bicarbonate and activated carbon dosing), and an oxidative alkaline wet scrubber.
- The syngas is generated at approximately 0.05 barg pressure and is compressed to 50 barg through a four-stage reciprocating compressor, featuring interstage cooling with condensate removal.
3.2. Water Gas Shift stages
- The steam-laden syngas provides the feed gas to the water gas shift reactor.
- Because of the small-scale of the plant, heat losses necessitated the use of electric blankets around reactors to ensure components were maintained at sufficient temperature.
- The HTS comprises a tubular reaction vessel containing a suspended canister containing undiluted catalyst beads, supplied by Johnson Matthey (KATALCO 71-5).
- Some or all of the cleaned, shifted syngas from the Guard Bed passes through a water-cooled heat exchanger (HX-2) so as to cool the gas to a temperature appropriate for that required for downstream second water gas shift (LTS) stage.
- During the course of the test programme it became clear that shift reactions alone would not be sufficient to remove CO to the levels required and so the programme was expanded to investigate methanation as a solution to this problem.
3.3. Product Gas Conditioning
- From the final methanation reactor (MTH) the methanation product gas is cooled through a water-cooled heat exchange unit (HX-4) and thence to a knock-out pot (KOP) where any condensed water droplet are separated and removed from the gas stream.
- This gas mixture is then passed to a pressure PSA unit where the gases are separated from one another to yield a pure H2 product stream and a carbon dioxide-rich tail gas stream.
- The gas composition is continuously monitored using an IR Xentra 4210 analyser in the gasification facility, a Gasmet Fourier Transform Infrared (FTIR) Continuous Emissions Measuring System (CEMS) and Gas Data Click gas analyser in the bioH2 facility, and in the PSA unit a Siemens Ultramat 23 for CO/CO2 and a Siemens Calormat for H2.
- Three gas sampling locations SP-1, SP-2, SP-3 were used to measure gas composition using the FTIR, i.e. downstream the HTS, LTS and MTH respectively.
4.1. Plant start-up and System tuning
- The 50 kW bioH2 pilot plant was operated for approximately 4 days, with the first 12 hours used for warm up, catalyst reduction and system tuning.
- Symmetrical trends of CO and CO2 were observed in the first HTS reactor reflecting the occurrence of water gas shift as the dominating reaction from temperatures above 250°C .
- A broad range of conditions were tried but the lowest product gas CO content achieved in the HTS was around 8 vol%, with conversions of CO of 50-60% at 340 °C.
- Excessive steam feeding the WGS reaction has two beneficial effects: it increases equilibrium conversion and disfavours coke formation.
- Gas compositions at different locations Figure 5 contains a snapshot of the gas compositions generated throughout the process.
4.3. Full scale Plant considerations
- This section summarises the process design for the proposed WtH2 plant on the basis of pilot plant results, and developed to meet the functional specification described in Section 2.
- Taking this as a counterfactual, CO2 emissions associated with fossil carbon in the feedstock should be discounted because they would have been emitted in any case.
- Emissions from each process step for the capture and non-capture cases, are given in Table 2, per megawatt hour of bioH2 on a higher heating value basis.
- According to a recent report, as the technology matures, a commercial waste bioH2 plant should produce a gas with a similar levelised cost to hydrogen produced by SMR with CCS, and significantly lower than the cost of hydrogen produced by electrolysis using off peak electricity (Manson-Whitton, 2017).
- The key disadvantage for WtH2 is that its potential is limited by the availability of sustainable feedstock.
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