New publication: measuring residence time distributions in screw conveyor reactors

Charles Chamberlin, David Carter, and Arne Jacobson recently authored an article on measuring residence time distributions of wood chips in a screw conveyor reactor. A screw conveyor or auger makes use of a rotating helical blade inside of a tube or trough to move wood chips, sawdust, flour, or other granular materials through a reactor — such as a dryer, heater, cooler, gasifier, or torrefier. How much change in the materials takes place in such reactors depends on the average residence time and how variable that residence time is.

Internal view of a screw conveyor.

The screw conveyor in this Norris Thermal Technologies torrefier moves the woods chips from the inlet (on the left) to the outlet (on the right). The rate of rotation controls the residence time within the reactor. The reactor cover has been removed to show the screw.

This paper compares three alternative methods for measuring the residence time distribution of wood chips in a screw conveyor reactor using experimental results from a pilot scale torrefier:

  • addition of material to an empty reactor (step-up),
  • halting addition of material to a reactor under steady flow, (step-down), and
  • addition of a pulse of labelled material (i.e., a tracer) to a reactor under steady flow.

We found that all three methods yield residence time distributions that are approximately symmetrical and bell-shaped, but the distribution estimated from the pulse input of tracer exhibited a long trailing tail that was not detectable in either the step-up or step-down results. Second, we demonstrated that a normal probability plot provided a useful way to display and analyze the distributions obtained in the tracer experiments. Finally, we observed that all three methods yielded mean residence times that consistently differed from the nominal values, with the step-up method averaging 8% shorter, the pulse addition of tracer averaging 7% longer, and the step-down averaging 60% longer.

The article appeared in the August 2018 issue of Fuel Processing Technology and is available to download here in pdf.

Quality Matters: a new report from Lighting Global

The Lighting Global Quality Assurance Program works to ensure that solar products sold around the globe meet established quality standards for product durability, representation of product performance, and warranty. To obtain quality verification, manufacturers may submit products for testing at laboratories in the Lighting Global network.

Pico-solar products include lanterns and simple systems with a peak PV module power up to 10 watts. These small systems encompass 85% of the global cumulative sales of off-grid solar devices. Although more than 30 million quality assured off-grid solar products have been sold globally over the past eight years, the sales numbers for products that do not undergo quality verification (hence are “non-QV”) is even higher. Field observations and customer experiences indicate that non-QV products typically underperform compared to the standards established by Lighting Global.

In order to ascertain the actual performance of these devices, Lighting Global laboratories recently tested 17 pico-solar non-QV products that are top-sellers in Ethiopia, Kenya, Myanmar, Nigeria and Tanzania. Products were purchased direct from market retailers.

Key results:

All 17 evaluated products failed to meet the Lighting Global Quality Standards for pico-PV products.

  • 94% of the tested products fail to meet the Standards due to one or more deficiency that
    affects product durability.
  • 88% of the tested products inaccurately advertise product performance.
  • 88% of the tested products do not include a consumer-facing warranty.
  • 76% of the tested products would require significant changes to product design and
    components to meet the Quality Standards.

The Lighting Global Quality Assurance team issued the report this August as part of the Technical Notes series. Chris Carlsen (a Schatz Center alumnus) led the effort in collaboration with team members from CLASP, the Schatz Center, World Bank Group regional lighting programs, and the Lighting Global network of test labs.

Read the complete report on the Lighting Global website…

Corroded batteries, shown inside and removed from the product

NiMH batteries with leaked electrolyte: When a battery is faulty, of low quality, or stored at a deeply discharged state, the battery cell can rupture and leak electrolyte. The battery pack in this product was not functional, and has leaked corrosive chemicals that damaged adjacent electronic components. – From page 12 of the Quality Matters report

New publication on CA’s Low Carbon Fuel Standard

Kevin Fingerman, Colin Sheppard, and Andrew Harris recently authored an article on California’s Low Carbon Fuel Standard: Modeling financial least-cost pathways to compliance in Northern California. This paper shares the results of a technoeconomic model developed at the Schatz Center to explore cost-effective pathways for replacing gasoline with alternative vehicle fuels, such as electricity, biodiesel, ethanol, and hydrogen.

Our study focused on six regions within Northern California, with the goal of simulating the most effective pathway to reaching the 10% reduction in transportation fuel greenhouse gas emissions that is mandated by California’s Low Carbon Fuel Standard (LCFS). Within the study regions, the analysis found that compliance with the LCFS will be more difficult than expected, and that electric vehicles should be expected to play a critical role in achieving vehicle emissions reduction goals.

The article will be published in the August 2018 (Vol 63) edition of Transportation Research Part D: Transport and Environment and is available to download here in pdf.