A matter of efficient particulate removal
With increasingly tougher restrictions on industrial air emissions, pellet manufacturers are looking to “future-proof” particulate matter (PM) compliance, cost-effectively. One Portuguese producer, Glowood Indústria S.A., has recently installed a brand new type of mechanical cyclone. The most recent results show a remarkable reduction of its PM emissions, from 700mg/Nm3 to under 15mg/Nm3.
Located in Cercal Do Alentejo near the Port of Sines in southern Portugal, Glowood Indústria S.A. is a 100 000 tonne-per-annum pellet plant. Commissioned in 2012, the EUR 10 million uses pine sawdust from sawmills as well as eucalyptus and pine roundwood.
Production is 100 percent geared towards export. Like many pellet producers Glowood uses a biomass boiler with the exhaust stream heating a rotary dryer to dry the incoming feedstock. The material is dried and carried over via dryer cyclones to the next stage in the pelleting process.
In addition to separating wet wood particles from the dryer, a much smaller amount of fly ash from the biomass combustion is captured in these cyclones.
The total concentration of wood dust and ash entering dryer cyclones from a rotary dryer is typically in the 200-300g/Nm3 range. Whilst dryer cyclones are capable of capturing the larger particles to a high degree (>99 percent), particulate matter (PM) emissions exiting dryer cyclones are still around 200-350mg/Nm3 exceeding existing regulatory limits in many countries.
As a result, additional end-stage dust removal systems need to be installed if PM emissions are to be brought under 50mg/Nm3, a threshold under discussion for the upcoming EU Medium Combustion Plant (MCP) directive.
However conventional end-stage dust removal systems have limitations. Flue-gas tars essentially rule out the use of bag filters, and Dry Electrostatic Precipitators (ESPs) cannot be used because of dust explosion hazards. Energy consumption, corrosion, and secondary pollutant issues limit the use of Venturi scrubbers.
While able to handle tars and very efficient at PM 10 and 2.5 removal, Wet Electrostatic Precipitators (WESPs) come with high investment costs along with water consumption and wastewater treatment issues. Finally, conventional multi-cyclones have a removal efficiency of less than 50 percent.
An advanced cyclone system
The Glowood plant has a triple-pass Maguin-Promill rotary dryer with partial flow rate recirculation that enables heat recovery, destruction of volatile organic compounds (VOCs) and odour abatement. The original dryer cyclones could reduce emissions to under 700mg/Nm3 in the worst drying conditions.
Though part of the flow rate is circulated back to the dryer for heat recovery, the remaining part (71 839 m3/h at 87 oC) was being discharged resulting in material losses and potential compliance problems. No doubt an issue shared by many pellet manufacturers.
In July 2014, Advanced Cyclone Systems (ACS), a Portuguese compatriot company specialised in the development and manufacturing of particle separators for PM emission control in various applications including biomass boilers, dryers and gasifiers, installed a line of newly developed mechanical cyclones for Glowood.
These were installed downstream of the existing dryer cyclones. The objective for Glowood was to radically reduce PM emissions from its pellet manufacturing process to under 150 mg/Nm3 at all times to ensure regulatory compliance.
To design the most efficient system, an isokinetic dust sample was collected at the stack and measured by ACS in a laser sizer to obtain the Particle Size Distribution (PSD). After confirming what PSD to consider for Glowood, ACS designed a cyclone system comprising of six Hurricane HR numerically-optimised cyclones, with ø1550 mm, disposed in line.
A frequency-driven fan was also installed next to the stack to overcome the additional pressure drop. The system is capable of guaranteeing PM emissions under 150 mg/Nm3 at a pressure drop of 1.2 kPa, on condition that the incoming dryer cyclone emissions are below 700 mg/Nm3.
The secret, Araújo revealed, is the understanding of a phenomenon in cyclones known as particle agglomeration. Several years of research by ACS has helped the company build accurate models of collection efficiency estimation, capable of explaining why sub-micrometer particles are often captured with much higher efficiency than predicted by standard models.
This knowledge has been incorporated into a numerical simulation tool, the PACyc model. By combining stochastic numerical optimisation with the PACyc model, a completely new line of cyclone geometries has been developed and pilot units subsequently tested with typical biomass boiler emissions at the engineering faculty of the University of Porto.
The company has launched a series of mechanical cyclones for a range of industrial applications located in 26 countries. The Glowood installation is the first such application at a pellet facility.
The Hurricane represents a giant leap in terms of efficiency. This line of cyclones can be used whenever emissions have to be very limited after case-by-case fine-tuning. The advantage of these cyclones is that compliance with strict PM emission limits, such as 30mg/Nm3, will be possible with purely mechanical and therefore more economical cyclone systems. This opens a door for compliance not only for pellet producers like Glowood but others like the particleboard industry as well as those with small- to medium-size, 1-10 MWth biomass boilers, said Araújo.
It’s a bold claim, achieving WESP-class PM reduction values with a mechanical cyclone. Yet the most recent PM emission results from the Glowood installation were a mere 12mg/Nm3, less than a tenth of the guaranteed 150mg/Nm3 limit and well below the expected 58mg/Nm3.
Though the investment cost for Glowood remains undisclosed it is “a small fraction of the alternative investment, in a WESP” according to Araújo.
I was confident that the project would be efficient. However the results are excellent and were achieved thanks to a brilliant engineering project, said Álvaro Magalhaes, Founder and Managing Director of Glowood Indústrias.
Apart from the reduced environmental impact of the plant, another benefit is that feedstock material losses have been reduced by over 99 percent, resulting in process optimisation and additional operational savings for the company. Furthermore, the installation process itself required minimum production downtime.
Of course, installation conditions are always site-specific. The Glowood project took approximately three weeks and was done in parallel with the plant working. The actual tie in/tie out installation took just a couple of days, said Araújo.
Certainly, it would seem that Glowood has a “future proof” pellet plant in PM emission compliance terms. And this ought to create a stir amongst other pellet producers.
We are looking at projects in France and Latvia and finding a lot of traction in North America. Here it seems that PM emission issues are taken more seriously than in many European countries. We feel that most companies simply don’t know that they can solve their PM emission problem with high efficiency cyclones instead of a WESP, concluded Pedro Ribas Araújo.
This article was first published in Bioenergy International Pellets Special 2015. Note that as a magazine subscriber you get access to the e-magazine and articles like this before the print edition reaches your desk!