PIONEERING WASTE TO ENERGY

About RCBC

Rotary Cascading Bed Combustor

How the System Works

RCBC Global Inc and RCBC Technologies Inc have developed a proprietary technology which is applicable to many situations where intimate contact between gases and solids is required. The mechanical unit incorporating this technology is known as the Rotary Cascading Bed Combustor (RCBC). We have designed RCBCs for use as incinerators, hot gas generators, oil shale retorts, and cas­cading bed boilers capable of cleanly utilizing high-sulfur coal. All these devices are based on the same fundamental principals, although details differ.

 

The RCBC utilizes a combination of proven concepts. Similar rotary units have been in use by 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 consists of a hollow cylinder, which may be single- or multi-compartmental, depending upon the application, rotating at 8 to 20 revolutions per minute. This is considerably faster than the conventional rotating kilns to which it bears a superficial resemblance. The latter generally operate at less than 1 revolution per minute.

The RCBC 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 Solids can be readily recycled or transferred from one compartment to another and gases can be introduced or withdrawn as desired.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, desired solid/gas reactions, such as combustion and SO2 absorption, are enhanced by the cascading.

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The drawings to the left show the cascading action of the particles when the unit is operated at various speeds. When the unit operates of speeds producing a centrifugal force of less than 0.05 G, no cascading occurs and particle contact is limited. Cascading increases with speed of rotation. Maximum cascading, producing greatest solid/gas contacting, occurs at approximately 0.5 G. When the centrifugal force approaches 1.0 G, cascading is suppressed as particles are forced against the walls of the cylinder.

The RCBC’s internal design is varied to provide a variety of features specific to each application. Material movement, retention, cascading, backflow, gas/solids contact, temperature control, compartmenting, 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 sulfur-containing feedstock such as high-sulfur coal, the addition of limestone to the combusting solid lowers the sulfur 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.

Two corporate entities have been charged with responsibility for the RCBC development program. The first, RCBC Technologies, is charged with engineering of the RCBC for each application. The second, RCBC Global Inc., is responsible for market development and sale of the RCBC.

Combustion & Steam Applications

Conversion of waste streams to combustion/steam has been a common method of disposal for many years and new federal regulations on hazardous wastes are making combustion/steam conversion options more attractive for a wide range of materials. Rotary kilns, multi-hearth furnaces, fluidized beds, and turbulent combustion chambers are being used for this purpose.

 

Conventional rotary kilns and rotary hearth furnaces have relatively low capacities since the solids are tumbled rather than cascaded and thermal efficiencies are low. Consequently, the capital cost per unit incinerated is high. Conventional fluidized bed incinerators have greater capacity, but the pressure drop across them is high and they are not ideal for incinerating hazardous materials because the gases and particulates are being carried out by high gas velocities before being completely burned. Turbulent combustion chambers are likely to require a great deal of auxiliary fuel and good atomization of the waste stream. With the high gas movement and vapor carryover, afterburners are almost always required.

 

Keeping these harmful effects in mind, RCBC Technologies and Quality Recycling Equipment, Inc. have developed rotary reactor technology that has great potential for the combustion of a wide range of materials including solids, gases, solid-laden gases, sludge, and liquids to steam. Combustion is completed within the rotary, eliminating the requirement for an after­burner. Units have been designed for use in de-oiling of mill scale from sintering plants, combustion/steam conversion of hazardous wastes, and disposal of sewage sludge.

 

A spiral chute recycles heat transfer solids from the end of the combustion zone to the front to bring the feed up to ignition temperature. Following the combustion zone there is a solids reheat zone with lifters and a disengaging zone without lifters. A spiral chute conducts the hot solids from the front of the solids reheat zone to the air preheat zone. At the end of the combustion zone, heat is recovered from the hot combustion gases by passing them countercurrent to cascading solids in a solids reheat zone.

 

Incineration standards set by the Environmental Protection Agency require a Destruction Removal Efficiency (DRE) of 99.99 percent for all toxic and hazardous wastes and the extremely toxic materials such as the PCB’s and dioxins require a DRE of 99.999 percent. To achieve these standards, most incinerators must operate at 2,000°F or higher with a minimum residence time of 2 seconds. The improved contacting of solids with the oxidizing combustion gases in the RCBC will accomplish required DRE’s at lower temperatures in the range of 1,400°F to 1,600°F.

 

The lower temperature range for combustion has significant advantages. The fusion temperature of the ash is usually higher than 1,600°F and below 2,000°F which produces a free-flowing residue at 1,600°F and a problem at 2,000°F. Refractory wear is reduced at the lower temperature and the application of high temperature alloys is expanded.

 

RCBC Technologies and Quality Recycling Equipment, Inc. have signed an agreement for joint development of the RCBC for disposal of “commercial hazardous waste.” The agreement includes the design and construction of a pilot combustor to qualify the process for toxic waste disposal. Patents have been obtained for the key features of the combustor system, with new technology patents pending.

Fuel Preparation

We know that waste management and recycling are national problems.  Studies show that in this country, each person creates three (3) to six (6) pounds of waste per day. At that rate, annual per person totals can exceed 2,100 pounds!  As a national average, we are currently recycling less than 10% of our solid waste products.  With population growth projections and with a growing resistance to new disposal facilities and methods, an innovative approach to drastically reduce downstream waste is desperately needed.

 

A Material Recovery Facility is designed to meet that need, enabling municipalities to separate commingled recyclables and process the separated materials into marketable commodities.  Most MRF vendors have developed a basic design or concept that they market, modifying that design depending on the requirements of the project’s sponsor and the available markets.  Quality Recycling will custom design a system flexible enough to process any of the common recyclables, and tailored to fit the unique market specifications of the area in which it is to be operated.

 

Although typical MRF’s utilize technical set-ups, if someone is looking for a system that provides a magic “Black Box” – where recyclables go in one end and products come out the other without ever being touched by humans —  it isn’t available and probably won’t be for some time.  Most vendors agree that some jobs are best left to humans.  For instance, all of the systems hand separate glass.  It is the most reliable way to insure quality.

 

Given that recycling has been able to function quite well without MRFs all these years, why the sudden interest in this approach to processing material?  The increased cost of solid waste disposal has increased the demand for recycling in general, and spurred the development of MRFs in particular.  When landfill costs were $5 – $10/ton, recycling most of the waste stream was not economically attractive to the waste industry.  With tipping fees in some areas moving toward and beyond $100/ton, waste managers are willing to spend more time and money on recycling.

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