By Steve Hodgson
Natural gas is by far the most-favoured fuel for new CHP/cogeneration schemes around the world, where gas is available locally. Even though most of the earliest CHP schemes built on both sides of the Atlantic were coal or oil-fired, recent years have seen the cleaner gaseous fuel take over in much the same way as gas has grown into a major fuel for conventional, power-only plants. And gas fits the green image of CHP better than coal or oil. Gas turbines and reciprocating engines designed to run on natural gas have become the two main prime movers for modern CHP schemes, accordingly.
In modern Europe, for example, while solid fuels (including peat) were burned at 35% of the total CHP generating capacity in 2005, this figure had fallen to 21% by 2011; the last year for which Eurostat data is available. During the same six-year period, the proportion of CHP capacity fuelled by natural gas rose from 39% to 48% – and there’s every reason to suggest that this overall coal-to-gas trend has been extended since then. Incidentally, the use of oil-fired CHP has been remarkably steady – some 6% of CHP capacity in Europe in 2011 was oil-fired, little-changed over the preceding six years.
Aside from gas, the other fuel to have increased its use in CHP schemes is, of course, renewables; biomass, biofuels and biogases. Eurostat data suggest that renewable fuels were responsible for just 9% of CHP capacity in 2005; though this figure rose to 15% by 2011 and is likely to be quite a bit higher by now. How far can this move to renewable fuels go?
Being a family of technologies rather than just one, CHP schemes can use all three types of renewable fuel: wood and other organic wastes in mass-burn incinerators and biomass boilers, sometimes alongside municipal and trade wastes; and biofuels and biogases in schemes based on turbines and engines. Solid and liquid wastes are sometimes turned into liquid or gaseous fuels first, in anaerobic digestion or gasification plants, before being fed to CHP schemes.
Renewables and CHP fit together well. CHP depends on the existence of locally-available heat loads to achieve its high efficiencies, giving CHP schemes a distinctly local identity. And renewable fuels also tend to be of the locality – neither heat nor most renewable fuels tend to travel long distances. In one of the most elegant examples of its use, CHP schemes fuelled with biogases at a sewage treatment works are used both to provide power, for pumping and general loads at the treatment works, and heat to operate the digestor that produces the biogas in the first place.
Already, less than a month into 2014, Cogeneration & On-Site Power production’s website contains several articles illustrating the breadth of renewables-fuelled CHP schemes being either planned or opened. An Austrian manufacturer of wood panel products is burning its own waste wood to produce heat and power for its factory in Romania. An aerospace company in Northern Ireland, is building an energy-from-waste gasification plant for use in its on-site CHP plant. And developers plan a solid biomass-fired CHP plant for an industrial site in the UK city of Plymouth.
There’s no doubt that natural gas will continue to be natural fuel for most new CHP schemes for quite some time to come – and a new wave of shale gas, particularly in the US, will be part of that. But schemes based on renewable fuels are increasing in number.
How far can this trend eventually go? The answer to that depends partly on how long we tolerate the wastage involved in burning of renewable fuels in inherently low-efficiency, power-only generators – rather than liberating 75–90% of the energy contained through the use of combined heat and power technology.
Steve Hodgson Contributing Editor