Growing demand for energy, feedstock and alternatives to fossil fuels has increased interest in AD and bio. The key to scaling up may well be focusing small, discovers Brian Attwood.
Fatbergs – those unlovely Jabba the Hutt-sized mounds of grease and sanitary items that regularly threaten to suffocate sections of the United Kingdom’s sewerage network – deserve a collective vote of thanks from the anaerobic digestion and biogas sector.
For they have probably done the most to capture the imagination of a sceptical public, whether domestic consumers or industry, and incline them more favourably to the cause of AD and its benefits.
This year’s headlines in the mainstream press bear it out, with copious coverage of the collaborative efforts of Argent Energy and Thames Water to turn the offending items in the latter’s network from a blight on efficiency and a threat to the environment; not merely removing their presence or adding to the country’s landfill burden, but converting them into a positive – a useful fuel and feedstock source.
Plants attached to an industrial process tend to be better at this and look at what they actually need and design the AD process around that
Alison McKinnie, project manager organics, Zero Waste Scotland
Fatbergs – or to give them their proper name, FOGs (fats, oils and grease) – are estimated by Argent to cost the water industry some £15 million per year. Their contaminated product regularly causes flooding and pollution but offers the opportunity for producing clean oil for biodiesel.
However, as those working in the sector know well, they represent only a part of the multiple sources of raw material that can and are harnessed in the cause of environmentally-friendly energy.
The UK Anaerobic Digestion and Bioresources Association (ADBA) points out that British business alone generates 5.8 million tonnes of food waste every year, at a cost of £8 billion. That figure is a third of the country’s overall volume.
Traditionally, landfill has offered the solution, but rising demand, coupled with environmental concerns, has resulted in regular year on year rises in landfill tax rates and a system that is judged unsustainable given the predicted volumes.
Waste disposal
Anaerobic digestion offers the opportunity for eliminating waste, relieving landfill pressures and creating new sources of power – digestate, fuel and the like.
For industry and individual companies, it means a chance to take the moral high ground and also reap the PR and tax benefits. More to the point, it has the potential to increase both process and cost efficiencies.
The process itself involves the gradual breakdown of organic matter (food, waste, etc) via micro-organisms over four stages including hydrolysis, acidogenesis, acetogenesis and methanogenesis, with the end products water, biogas methane and CO2, plus organic material.
The latter (digestate) provides feedstock, notably for ethanol production and good quality fertiliser. Biomethane supplies vehicle fuel and the gas grid, with biogas also contributing to the production of electricity and heat.
There’s an argument that our greater reliance on imports for energy and feedstock makes investment in AD and biofuels more attractive; and a counter one that says a government and industry facing economic retrenchment won’t be inclined to invest the necessary amounts.
Dr Ian Archer, technical director of the Industrial Biotechnology Innovation Centre (IBiolC), acknowledges the argument that the UK’s need for new and cleaner supplies makes investment in AD and biofuels attractive.
Basically the biomass will generate some methane if allowed to decompose naturally (in landfill or elsewhere) so we should avoid releasing that into the environment and generate some value from it
Dr Ian Archer, technical director, Industrial Biotechnology Innovation Centre
“Biomass in landfill degrades anaerobically to methane and carbon dioxide. Burning that methane to carbon dioxide and capturing the energy released provides the double benefit of generating some energy from waste materials and reducing greenhouse gas emissions. The biogas generated forms part of the renewable energy mix while reducing carbon emissions.
“Basically the biomass will generate some methane if allowed to decompose naturally (in landfill or elsewhere) so we should avoid releasing that into the environment and generate some value from it.”
ADBA statistics show 557 operational AD plants with capacity of 730 MWe-equivalent and a total of 28% of sites based in the sewage sector. Predictions for new plant commissions for 2017 to 2019 range between a low of 40 and a high of circa 140.
Yet its credibility as a viable alternative source of power and energy will depend upon an ability to scale up. That in turn will be determined by how cost efficient it proves when measured against the alternatives.
Alison McKinnie [pictured above], project manager organics at Zero Waste Scotland, insists process efficiencies will determine the rate of adoption.
“For example, government incentives are encouraging electricity production, but the conversion from biomethane to electricity is very inefficient and produces a lot of excess heat which can often be wasted. Could more plants be looking to gain income from this heat, or maybe other uses for the gas and not electricity, that are more efficient?
“Plants attached to an industrial process tend to be better at this and look at what they actually need and design the AD process around that.”
The bigger picture
Innovations include cleaner gas, enhanced digestate, capture of high-value products and removal of unwanted ones that point the way to improve plant economics, she adds. Scalability depends too upon the finite quantity of product, reminds Archer: “Food waste alone will not deliver significant benefits although this is by far the best way to deal with that waste...
“Conversion of sewage sludge to biogas would seem to be a more scalable solution. We won’t run out of sewage and should try to extract as much value from it as possible.”
In the UK there is some way to go. Natalie Bachman’s 2015 report for IEA Bioenergy [Sustainable Biogas Production in Municipal Wastewater Treatment Plants] estimated our total biogas production in 2013 at 6.637 Gwh/y.
Yet just 11% of the figure derived from wastewater treatment plant sewage sludge. South Korea and Sweden achieve 38% and 40% respectively while Switzerland manages an efficient 49%.
One very clear theme from my discussions with AD operators, power companies, water companies and also academics is that there is a desire to understand the micro-ecology of AD systems
Dr Ian Archer, technical director, Industrial Biotechnology Innovation Centre
For those seeking increases in process efficiency, focus has inevitably been directed towards nuts and bolts technological processes and the workflow big picture.
Archer and his colleagues at IBiolC suggest the time has come to think small – in this case microscopically small.
“One very clear theme from my discussions with AD operators, power companies, water companies and also academics is that there is a desire to understand the micro- ecology (the microbiome, i.e. what bugs are in there?) of AD systems,” he explains.
Both activated sludge water treatment, which accounts for 1% of all electricity used in the UK, and AD rely on complex cocktails of microbes to break down organic matter to carbon dioxide and methane (AD) or just carbon dioxide (activated sludge).
“No one fully understands what this mixture of microbes is but several companies and academic groups are talking about research to identify what is there and therefore use that information ?to optimise the processes to make them more efficient. From an engineering perspective this is the most likely opportunity for process improvement.”
Water based
Not surprisingly perhaps, it is a water company that is pioneering this work. Dr James Chong, senior lecturer at the University of York, is working with Yorkshire Water to understand the microbiome composition of the company’s anaerobic digesters and has evidence of at least 1,500 microbes in samples taken, says Archer.
What goes for AD also applies for AS and vice versa, he adds, because the technology to understand the systems (sequencing of the metagenome and bioinformatics to build up the picture of the microbiome) is identical.
The potential advances in the next decade are considerable and mirror the advances being made in biotechnology. Better DNA extraction and analysis allow observers to map the makeup of the microbiome over time, determining its response to different feedstocks.
Further ahead, predicts Archer, there is the prospect of simplifying the microbial composition to remove those microbes that scavenge carbon without contributing to the process: “Bioinformatics techniques will allow us to understand the microbiome and, building on that, will be able to model the metabolic pathways within each organism that contribute to the production of biogas. This understanding will allow us to optimise conversion of biomass to biogas more efficiently and more robustly.”
All over algae
As demand for ethylene continues to grow, so does interest in bio-ethylene produced from biomass as a viable, chemically identical alternative that offers the additional benefit of reduced environmental impact.
While the US and Brazil accounted for the vast majority of supply (based on corn and sugarcane) respectively, growing food and power demand will impact on availability of feedstock.
However, help may be on the way, courtesy of some of our simplest life forms: algae.
In June, Honeywell Process Solutions’ annual UniSim Design Challenge was won (for the second consecutive year) by Izmir Institute of Technology chemical engineering students Ozgun Deliismail and Okan Akin [pictured above with associate professor Dr Erol Seker].
Their entry on this occasion was a preliminary conceptual design and simulation of the production of bio- based ethylene from the marine microalgae of Nannochloropsis oculata.
John Roffel, director of Honeywell’s simulation and operator competency product lines explains further: “The purpose of the paper was to determine whether the algae-derived ethylene process was feasible and profitable, which turned out to be the case.”
“Assuming that the processes are optimised for design, have the same capacity, and are built in the same geography, the CAPEX would be higher for this novel process as compared to the conventional ethylene process, while OPEX would be lower for the algae- derived ethylene process.”
Thermochemical processes (gasification, pyrolysis, liquefaction) have been the preferred options used for algae conversion.
However, the process proposed in the Honeywell software simulation has a much lower severity (temperature/pressure).
It is also optimised for heat integration, offering a better approach to energy usage.
For Roffell, the greatest opportunities for advances in AD and bio over the next decade are apparent: “Bioengineering of the marine algae and process control of the growth environment, to improve the growth rate algae and product yields of bio-ethylene.”
