Drives in the biomass industry
28 maggio 2021

Biomass is, on the face of it, an important resource. It represents just under 60% of the renewable energy used in the European Union; and in a country such as Brazil, with its sugarcane production, it accounts for a significant share of the total energy used in industry. For all this, and certainly compared to solar and wind power, biomass is not widely discussed and what discussion there is tends to be controversial in character.

The controversy – which turns on the extent to which biomass may plausibly lay claim to any green credentials at all – may or may not be behind lacklustre levels of industrial investment in related technology. And the mixed fortunes of recent ventures, such as the Abengoa Bioenergy plant in Hugoton, Kansas (which did invest, only to close its doors a year later), may or may not speak of uncertain levels of confidence in the future.

Either way, manufacturers and engineers of industrial control systems may fairly ask: what opportunities exist for upgrades and new builds in the world of biomass projects? Does it represent a legitimate fresh field of green technology or is it a dead end? What, if anything, can drives do for biomass?

Biomass plants and variable speed drive benefits

Biomass-fuelled power plants, as they stand, are full of the kind of motor activity that benefits from variable speed drives. This is because they are essentially old-fashioned direct-fired combustion systems. A feedstock (typically wood pellets) is burned in a furnace to heat water; when the water becomes steam, it is directed through a turbine to generate electricity.

The pumps and fans that figure in this kind of set-up are obvious beneficiaries of drive control. Conveying mechanisms, too, are needed to keep production on the move: feedstock may move about a plant between yard, silo and combustor by crane, stacker and belt. And the more automated operations are – as in any industry – the more work there is for drives to do in terms of keeping motor behaviour linked and synchronised.

The manufacture of the wood pellets is in itself a full-scale industrial procedure. Some of the mechanical work, of the kind that benefits from drive control, is the same as in sawmills: the long, timber-bearing conveyor belts, for example, or the rotating drum debarkers into which the raw wood is fed. Other equipment presents further scope for adjustable speed control: the spinning blades of the wood chipper, the hammer mills that shred the chips down to fibre, and the eventual die-casting of the pellets themselves, performed by the rotating arms of pellet mills.

Converting to biomass

Since 2013, and incentivised by a significant annual government subsidy, the British electricity generation company Drax has been using biomass alongside coal in its flagship power station in the north of England. It has converted three of the plant’s six units for the purpose. The viability of using old coal-burning venues for biomass illustrates how fundamentally similar the two types of operation are – including considerations of motor control. For example, variable speed drives are used for the fans which mix fuel with air in exactly the same way, whether the fuel be coal or wood dust.

If companies like Drax are adding to the remit of the drive engineer, then, it is less because of the existence of any new core technology than it is through the need for such ancillary work as pellet production (a Drax subsidiary has facilities across the Southeastern USA) and handling and storage, much of which can be successfully automated.

Many, though, have questioned the future of this industrial model. After all, the argument runs, how convincing is a green technology that manufactures fuel on one side of the world only to ship it to be burnt like coal on the other?

A credible renewable?

The status of biomass as a credible renewable, some say, exists only thanks to a trick of so-called carbon accounting: that burning organic matter may be considered green because the CO2 thereby put into the atmosphere can be offset by the planting of replacement trees (which take it back out). The extent to which burnt plant matter is in fact replaced in this way – not mention that to which new trees are capable of offsetting the carbon footprint in anything like a meaningful timeframe – are widely questioned.

Any political loss of confidence in biomass’s carbon-neutrality would have shaping consequences for industry – as in fact it has in Germany, whose ongoing transition towards renewable energy now decisively favours solar and wind power schemes, with a corresponding lowering of goals and incentives for bioenergy.

And there seems to be some acceptance on the part of biomass plants themselves that their future depends upon them becoming less carbon-heavy. Drax is currently looking into using chemical solvents to capture and store CO2 from flue gas – a process already developed for coal-burning plants but which for biomass is still at the experimental stage.

Such a technology, should it become established, would be expected to deploy motorised elements familiar to the industrial control systems engineer – such as variable speed drives on the compressors that would ready the CO2 for transportation and removal.

The future

It may be, though, that the longer-term future of biomass lies away from the world of monolithic power plants and more within the realms of decentralised, building-sized energy generation. A number of college campuses, both in North America and in India, have had recent successes with gasifiers (furnaces that create a gas fuel from biomass by heating without combustion). Though relatively domestic, these systems offer scope for motion control in areas such as variable speed feeding hoppers or the rotating grate that keeps the base of the unit clear of debris.

While the future of big bioenergy remains unclear, it is in smaller-scale, locally sourced projects – politically uncontroversial and demonstrably carbon neutral – that developments for biomass and its associated technologies are most likely to thrive.

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