So the gasification market has finally kicked off, but what is the best use of syngas and how can we recover the most energy?
Recovering energy from refuse derived fuels and similar feedstocks is growing and to date the most bankable and robust solution has been to oxidise the syngas – synthesised hydrogen and carbon monoxide emissions sometimes combined with carbon dioxide (CO2) emissions – raise steam and drive a conventional turbine giving an efficiency of around 24%(electrical). Not brilliant but very reliable, hence bankable. Efficiency could be boosted by 5% to 10% by heat recovery for example from the stack, but at best the likely overall efficiency would still only be 35%. Of course, putting syngas into engines will increase efficiency to maybe 40% electrical plus 20% thermal. But you then face the thorny issue of effective gas clean up to ensure reasonable availability can be achieved on the engines. This issue has sunk many a gasification project. However, there has been some real progress in this area, with the Air Products Tees Valley plasma based facilities due on stream in 2015, and the eagerly awaited outcome of the Energy Technology Institutes competition to design the most economically and commercially viable, efficient energy from waste gasification demonstrator plant possible.
What other options are available now and in the future? Aviation fuels have hit the headlines recently. Solena Fuels and Lanzatec are greening our blue skies by harnessing syngas to produce fuel. The vast expansion of air travel in recent years through low cost flights now accounts for 6.3% of UK CO2 emissions. These are predicted to increase by between 40% and 60% by 2050. So while current legislation and fiscal drivers are driving the levels of CO2 emissions for road fuel, electrical and heat production generally downward, aviation emissions are increasing in quantity and as a percentage of the total. Details of the actual gasification fuel preparation process are not yet public – we await details with interest.
There are also options around methanation. Converting syngas to methane for injection into the gas grid seems an obvious target which would be supported by the Renewable Heat Incentive mechanism in the same way as biogas (bio methane) to grid. This should be an ideal way to capture the majority of the useable energy in the original feedstock. More exotic options would be the production of other platform chemicals, altough lack of real scale is likely to be an issue.
As always when analysing the efficiency of a process, it’s important to set process boundaries to compare net efficiency and account for internal or parasitic energy requirements. The overall cost of a project has to be commercially viable, so expensive, power-hungry gas clean up processes, however efficient, may not make the grade.
So where does this leave us? In the short to medium term, certainly for developer led/project financed energy facilities, I think the focus will stay with the “good old steam cycle”. It’s boring but safe, until some of the syngas to engine schemes have some operating hours on the clock. Some of the large corporates will no doubt make progress on fuel types as part of their green agenda, but for me, I like the look of syngas to methane to grid and use existing infrastructure to raise energy. Then of course there are fuel cells…
- Mike Crane is a technical director at MWH Treatment - Waste and Energy