This article was published in the June/July 2020 issue of BIC Magazine.
Many times the solution is directly in front of you, like with scrap metal and cardboard. Those are easy commodities to have segregated because metal and cardboard are easy to identify. There are many suppliers to provide solutions – scrap metal yards are found in every municipality. E-waste, light bulbs, and used oil are common wastes that have many recycling outlets. All are treated as 100% recycled. This is the easy part of sustainability.
Your manufacturing process generates waste, though, and you have done as much as possible with systems and controls to reduce and minimize waste generation. So once the commodities have been solved, there is the sustainability problem for non-hazardous waste that does not have an easy solution. The most commonly thought of solution is waste-to-energy.
Most of the time, however, there is the problem of non-hazardous waste that you are not sure whether it can go into waste-to-energy or not. You want a sustainable solution, but you don’t know if someone can handle it. This is where you have to start digging for answers.
One of the most common questions for thermal destruction of waste is whether the waste qualifies in terms of heating value or not. High Btu material such as oil is thought of as a no-brainer because it has a high Btu and is easily thought of as a candidate for thermal destruction.
But there is the problem of other non-hazardous wastes such as water that can’t go to the POTW, isocyanates that are DOT hazardous, or plant trash that seemingly has no value at all. None of these have heating value. So the question presents itself about how waste with no heating value can go into a thermal destruction program.
Another common problem is figuring out how to package and ship waste. Waste-to-energy is typically thought of as bulk waste that is dumped into a pit that burns waste for steam and generates electricity. But most of the time, waste is not in bulk and it is not thought of as easy to manage. Pallets of paint pails do not seem like a good fit. Totes of water do not seem like a good fit.
Sustainable solutions should be sustainable, so environmental controls with thermal destruction is very important.
Gasifiers have a broader range of acceptance. However, lower temperatures give off higher emissions and generate more ash. The issue is in scrubbing the gases and dealing with high landfill rates. Cement kilns are pickier but more environmentally sound than gasifiers.
Environmental controls vary by the method with which waste is converted to energy. Gasifiers and cement kilns use waste as a fuel in its process. Gasifiers use waste in a fluidized bed reactor to boil water, capture the steam to power a turbine, and ultimately convert waste into electricity that goes back on the grid. Temperatures are around 750oF to 1000oF. Scrubbers collect the uncombusted emissions and scrubber material goes out with ash to the landfill. Gasifiers generally have ash about 20% to 30% of total mass, so of 100 tons input gets 20-30 tons ash going to a landfill.
Cement kilns are unique in its processing of emissions since 70% to 75% of the raw material to make cement is limestone (which is the primary material used in gasifiers’ emissions scrubbers). When the combustion process is complete the emissions must travel through a few hundred tons of limestone to exit and remain in a high temperature environment for several seconds. This is referred to as residence time. The input of waste into a cement kiln is in an area much hotter than typical gasifiers, up to 1800°F. So, with high combustion temperatures, long residence time, and a natural scrubber, cement kilns burn at a 99.99% DRE (destruction ratio efficiency) of contaminants. Ash becomes “part” of the cement in the process at the molecular level and therefore cannot leach.
You are reducing waste and finding local solutions for commodities. The “last mile” of waste you want to stay out of a landfill. Waste-to-energy solutions are out there to satisfy your sustainability needs.