RockRoadRecycle staff writer
The reuse of materials and energy from waste is becoming a central facet of modern waste management. The foundation for this is laid by mechanical preparation in the form of waste splitting. In it, recyclables are recovered from the waste stream, with high yield combustibles being separated out for further processing as refuse derived fuels (RDF). The remaining residue fraction can then be stabilized and landfilled. In order to provide a recycled waste material that can be used as an energy source a series of steps must be followed, with service providers and manufacturers now delivering solutions purpose-developed for these tasks.
Efficient solutions for complex tasks
The process technology developed for turning waste into a material that produces energy revolves around shredding, screening and separating. Low speed shredders with an adjustable degree of shredding generate a homogeneous material stream at the desired particle size. Following shredding, a range of technologies can then be deployed, depending on the specific material and conditions. These include all-purpose drum screens, ballistic separators with rotating screen elements to separate flat from three dimensional fractions or disc screens that work efficiently even with material that tends to wrap and tangle.
Drum screens, star screens and flower disc separators detach contraries and non-compostable fractions. Windrow composting the material sees it mixed with structure material and screen oversize during the layered build-up with a wheel loader and homogenized in subsequent mixing with a turner. The input material and the associated limiting factors also decide the rotting process and whether it is conducted closed (in an indoor part of the system) or open as windrow composting. The first phase of the process often takes place in a closed system with ventilation and outlet air treatment.
Using the right shredding and separation technology, fuel mature enough for the marketplace can be generated from green waste (from gardening and landscaping), root stocks, logs and untreated waste wood. This way, companies previously operating in the disposal business can transform themselves into energy suppliers. A fuel with a coarse structure can be generated from woody green waste, clean waste wood and forest wood residue by shredding with a general purpose wood shredder. With green waste having a high leaf or grass content, adding a further rotting process with triangular windrows has proven to be a sensible option. After around two weeks with frequent turning, green parts are largely decomposed and the material moisture is reduced.
Getting the right equipment and advice is key to getting the best from waste material and enabling it to be used as a source of energy. Fortunately, there are now companies in North America who are focused on supplying the right equipment and solutions, enabling waste processors to turn their businesses into highly focused suppliers of recycled materials for energy.
Impact Bioenergy (IBE)
Impact Bioenergy™ was formed in 2013 with a mission to construct and deliver the best bio-conversion technologies and services available. These have been developed to empower communities by making renewable energy and soil products locally through organic materials recycling. The company now manufacturers equipment in the Pacific Northwest and considers simplicity, local equipment sourcing and local empowerment to be the foundation of its success. What has particularly driven the company is the realization that the old mindset of exporting waste to regional processors and large facilities wins by economy of scale — but is not the ideal model anymore. IBE is focused on proving that a new model of local energy, recycling and food production can be connected in a real and powerful way. Its simple approach is to convert waste to resource with onsite bioenergy solutions. Minimizing the export of waste makes sense, especially when this enables the generation of clean renewable energy and the recovery of water, nutrients, organic matter, carbon and beneficial microbes.
Providing the tools needed
IBE believes that the primary barrier preventing more waste from becoming energy has been the lack of scalable technology and the belief that economy of scale at distant facilities trumps onsite material recycling. The company now provides a wide range of technologies for food and beverage waste, landscape waste, wood waste, packaged unsalable food and fats. All are designed to serve food and beverage sectors, commercial, industrial, institutional and residential communities of between 100 and 50,000 people. They are designed to make the very best use of materials and to return as much carbon to the soil as possible. This propels the cycle of organic and renewable resource generation locally.
As part of its extensive offering, Impact Bioenergy now manufactures commercial scale, portable anaerobic digesters that convert food waste into renewable energy and probiotic plant food with zero waste. Within the massive existing problem of commercial food waste in urban and suburban settings, the company is optimized for corporate, educational and government campuses and islands. Instead of paying $20,000 per year to have food waste trucked away to the landfill, IBE customers now pay the same per year for use of one of the company’s on-site units. In addition — providing real benefit — the unit will create about $7,000 worth of electricity and $10,000 worth in organic fertilizer.
Impact Bioenergy’s solutions therefore reduce energy and disposal costs by generating energy from adjacent organic waste streams. Its prefabricated bioenergy units are uniquely engineered and sized to allow organizations and communities to practice onsite bioenergy food waste diversion, thus substantially reducing multiple costs and generating a faster ROI than wind or solar. The company’s portable bioenergy systems are able to convert 1,000 to 35,500 pounds per week of organic waste into renewable energy and biofertilizer with zero waste. These micro digester systems can be used in urban, campus or farm environments and have odor control and semi-automatic or automatic operation.
RCBC Global Inc.
Another North American company at the forefront of energy from waste is RCBC (Rotary Cascading Bed Combustor Global), which produces steam and energy from municipal, industrial and sewer sludge waste. It also proudly states to have the know-how and expertise to have the ability to burn coal safely with no harmful emissions. Furthermore, the by-product of RCBC’s system can be used for fertilizer and land reclamation, and having been established for 26 years, it is one of the pioneers of the waste to energy movement.
The company states that one challenge in implementing a waste to energy program has been the high capital cost of proposed systems. The value of the energy produced simply has not enabled a payback on the capital required. Thus, governments have been required to subsidize the systems. Unlike other proposed methods, the RCBC system’s significantly lower capital cost enables a sufficient coverage ratio for the payment of loans related to the initial construction of the facilities. Although the program economics of building and operating an RCBC facility are substantial, communities avoid the costs and risks of landfills, electric power can be produced to complement the electrical grid or supply high use private companies, and sustainable jobs are created to increase the economy and stability of the local community. At the same time, the facility owner can generate profits sufficient to pay back loans and provide a strong return to investors or government sponsors.
A new solution
At the heart of RCBC’s solution is new boiler technology. The Rotary Cascading Bed Combustion (RCBC) system cleanly and efficiently converts high sulphur coals, waste coals, refuse derived fuels (RFD), shredded tires and industrial and municipal wastes to energy. This has been extensively tested at North American Rayon Corporation in Elizabethton, TN, with the technology being the mechanical equivalent of large fluidized bed combustors being developed for utilities. In operation, fuel is fed into a cylindrical combustion chamber and is mixed with limestone, which limits the formation of acidic sulphur dioxide (SO2) emissions. Due to combustion temperatures that range between 1,600 – 1,650 degrees Fahrenheit, far lower than conventional steam generating systems, nitrogen oxide (NOx) emissions are also reduced.
The RCBC system utilizes a combination of proven concepts. Similar rotary units have been in use by the industry for several decades, particularly in the phosphate and fertilizer industries. Large units such as rotary kilns, rotary dryers and ball and rod mills have been utilized and developed to satisfactory levels of mechanical reliability through many years of operation. The RCBC solution consists of a hollow cylinder, which may be single or multi compartmental (depending upon the application) rotating at 8 to 20 rpm. This is considerably faster than the conventional rotating kilns, to which it bears a superficial resemblance. The latter generally operate at less than 1 rpm.
The system provides excellent contact between solids and gases by cascading the solids through gases and thereby effecting high rates of heat transfer. Solids can be readily recycled or transferred from one compartment to another and gases can be introduced or withdrawn as desired. This provides great flexibility and allows designs to be optimized for a particular processing operation. Cascading solids through the surrounding media ensures complex mixing and intimate contact. This provides the mechanism to transfer heat between the substances; for example — when desired —solid/gas reactions, such as combustion and SO2 absorption, are enhanced by the cascading.
The RCBC’s internal design is flexible providing a variety of features specific to each application. Material movement, retention, cascading, backflow, gas/solids contact, temperature control, compartments, reaction kinetics and a variety of combustion conditions can be controlled to desired levels by varying the design features. The RCBC technology can provide high thermal efficiency by recovering much of the heat generated within the system. In most cases, the hot products of combustion (both gas and solids) are used to preheat the feed gases and/or solids. Due to its high contact efficiency and ease of adjustment, the RCBC can be readily controlled. Combustion conditions such as temperature and flue gas composition can be closely controlled. Feed variations can readily be compensated for and turndown can be accomplished without loss of control.
When burning sulphur-containing feedstock such as high sulphur content coal, the addition of limestone to the combusting solid lowers the sulphur dioxide content of the exhaust gases, eliminating the necessity for external scrubbing equipment. A similar approach can be employed to neutralize acidic vapors or remove reactants from the flue gases. This results in an environmentally acceptable, economically attractive installation.
Doppstadt transforming green waste
Doppstadt has a hard won reputation in the market for producing shredders, trommels and other material processing equipment. This equipment is also able to transform green waste and untreated wood into energy sources following recycling. Highlighting this, the Japanese prefecture of Gunma is now using Doppstadt equipment to provide an efficient technical solution for the process of thermic recycling. This is in the form of an integrated, individual plant concept which includes pre- and fine shredding systems as well as components for the deposition of metals and screening.
In the Japanese context, input material is processed and transformed in four stages. Following the pre- and fine shredding steps, metallic contaminants are removed and the material is separated into fine, finished and oversized fractions. The oversized grain will be automatically circulated back into the fine shredding process until it meets the requirements of the finished product. If there is no need to run through the entire process chain; the Doppstadt plant can be flexibly used for partial processes as well.
A shredder receives the input material for the coarse shredding procedure in which the roller teeth break the material through a shredding comb. The mixed fraction is then moved on for fine shredding, with an overhead magnetic system separating ferriferous contaminants from the wood. The procedure is repeated right before the material enters the screening process.
A trommel screen finally takes care of the proper granularity of the material, screening out the fine fraction (grain sizes of less than 10mm). The targeted product size will be between 10 and 50mm, depending on specific requirements, with oversized particles being automatically transported back to the fine shredding process to be processed one more time. This recirculation process only ends when the entire material fits the size requirements of the finished product. The screening procedure allows for coarser shredding with a substantially reduced fine grain portion, thereby affecting the Gaussian distribution in favor of less tool wear, energy consumption and fine grain.
Similar to Doppstadt, Austrian company Komptech is better known as the manufacturer of shredding and material processing equipment. However, it also possesses expertise in turning waste biomass into energy, with solid biomass being intended to make a major contribution to the transition from fossil fuels to renewable energies. However, to reach the ambitious goals for heating and energy generation from woody biomass, it will need to be made available in large amounts. Using the right shredding and separation technology, fuel mature enough for the marketplace can be generated from green waste, root stock, logs and untreated waste wood. This way, companies previously operating in the disposal business can transform themselves into energy suppliers.
A fuel with a coarse structure can be generated from woody green waste, clean waste wood and forest wood residue by shredding with Komptech’s general purpose Axtor wood shredder. Simply exchanging the free swinging tools for fixed tools converts the Axtor into a powerful chipping machine for logs. With green waste having a high leaf or grass content, adding a further rotting process with triangular windrows has proven to be a sensible option. After approximately two weeks with frequent turning, green parts are largely decomposed and the material moisture is reduced. A fine fraction, a medium fraction and a coarse fraction are produced in one screen operation. The medium fraction generally represents the usable fuel sample. The fines are processed to compost and the coarse particles are brought back to shredding.
If required, plastic is separated by the company’s Hurrikan windsifter, with metals being removed by FE separation and stones are eliminated by a Stonefex stone separator. Komptech states that its new Hurrifex accomplishes both these tasks in one step