Exploiting the Energy Potential of Medical Waste

by R Franklin, NREL, USA

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Introduction

In Pennsylvania, USA a number of regulations restrict the transportation and disposal of medical waste. Recent legislation promotes on-site waste destruction, as well as control and disposal of ash generated in the medical waste destruction process.
 Donlee Technologies Inc of York, Pennsylvania has responded by devising a technology that exploits the energy potential of medical waste being destroyed through combustion. The company demonstrated a method of co-firing coal with shredded hospital waste in a circulating fluidised bed combustor. This method not only eliminates the infectious waste, but generates steam for use in a laundry and to heat hospital buildings.

The Technology

Most waste incinerators are based on hearth or rotary kiln designs. Circulating fluidised bed combustion (CFBC) technology uses limestone in the process together with higher levels of turbulence to reduce gaseous emissions. CFBCs can efficiently destroy infectious medical waste while meeting state and federal pollutant emission requirements. The use of coal-fired CFBCs in medical waste applications was investigated by a team originally led by the Good Samaritan Hospital in Lebanon, Pennsylvania.
 Donlee's pilot facility in York, Pennsylvania used a 2.9 MWth circulating fluidised bed and experience with this was used to design a larger CFBC facility at the Veterans Affairs Medical Center in Lebanon, Pennsylvania. Engineers and technicians analysed three years of data from the pilot unit which showed positive results for steam generation and reduced emissions.
 The Veterans Affairs facility is currently permitted to run at 136 kg/hour waste destruction with a steam flow of 9.1 tonnes/hour. The facility has an actual capacity for combusting 250 kg/hour of waste. The facility, which provides steam for use in the laundry and to heat 73,000 m2 of space in hospital buildings, will undergo at least six months of environmental monitoring following commissioning.

Design Features

Beneficial features of the CFBC design include high levels of turbulence, excellent combustion temperatures and a residence time about 50% longer than required by Pennsylvania standards for medical waste destruction. As a result, dioxin and furan emission levels are one to two orders of magnitude below those of conventional medical waste incinerators.
 The CFBC system shredder reduces medical waste to strips 25 mm wide by 75 mm long. This increases the surface area of the material, making combustion easier. The shredding system can process carbon steel plate, metal and glass items such as syringes. Shredded metal and glass are removed by the bed cleaning system.
 Waste falls from the shredder onto a feed auger. Coal, limestone and waste are fed into the main bed portion of the combustor. Particulates are carried away in the gas stream and are separated from the gas in a cyclone. The solids flow downward into a sluice and then back into the combustor. Gaseous emissions leave the top of the cyclone, flow through the boiler, economiser and baghouse, and are discharged from a stack.
 Reduced maintenance and improved reliability are provided by an adjacent bed heat exchanger which allows for precise temperature control and more effective sulphur capture. The solids flow rate is controlled by the gas velocity in a lift channel, a feature which helps to determine the combustor temperature. Such dynamic control ensures stable temperatures that are independent of fuel composition or feed rates. This is a crucial benefit, given the importance of temperature in coal combustion, waste combustion and pollutant reduction.
 The normal combustion temperature for anthracite coal will be 940 C. The Pennsylvania Department of Environmental Resources requested that the facility be fired with anthracite coal because it contains a lower sulphur content than bituminous coal and burns at a higher temperature.

 Performance

The heat input at full load is about 25,000 MJ/hour. Non-infectious waste, such as plastic bags and hoses, glass and metal, is considered to have the same heating value and chlorine content as infectious waste. At a rate of 135 kg/hour waste destruction, the unit has a saturated steam flow capacity of 9,000 kg/hour at 8.5 bar. A turndown ratio of 4:1 allows for a range of steam production between 2,300-9,000 kg/hour.
 The Veterans Affairs facility emissions requirements include both New Source Performance Standards regulations and the new Pennsylvania Best Available Technology Standards. The results of the shredder tests and their associated stack tests demonstrated that the emissions from the pilot CFBC meet and exceed all required emissions standards. Use of limestone was effective in capturing sulphur dioxide and hydrochloric acid.

Economics

The project is funded by the Department of Energy and the Department of Veteran Affairs. Donlee has developed the circulating fluidised bed and is the prime contractor for the post-testing expansion phase of the project.
 Annual operating and maintenance costs for the CFBC system are significantly lower than for a gas-fired boiler system. This is attributed to the combination of the excellent control system, the high reliability of the equipment and the resulting low maintenance required for the CFBC system.
 The operating cost for the existing waste disposal and the steam generation system is $910,311 (where $ is the US dollar). This compares with $477,829 for succeeding years for the CFBC. A comparison of annual fuel and operating costs for the two systems shows that a 42% cost reduction can be achieved with CFBC technology.
 The life of the facility is 15-20 years. Calculated using a saving of $432,400 for the first year and $382,400 for the second and succeeding years, a CFBC with a $2.45 million capital cost should pay for itself in simple terms in about 6.3 years. This assumes no maintenance costs during the first year of warranted coverage and $50,000 in maintenance costs each year thereafter.

 

Table: Economics First Year

Fuel saving resulting from change to coal

$387,000

Income from waste disposal*

$45,400

Total

$432,000

For further information contact the CADDET US National Team in Golden, Colorado.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

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