Sheffield upgrades CHP plant to meet emissions standards

incineration plants which provide power for the city’s extensive district heating scheme.

Energy created by burning Sheffield’s waste is converted and delivered through 36 km of pipeline to highrise housing, retail commercial, public, leisure, educational ~nd health facilities – supplying most major buildings in the city centre.

On average, some 34MW of heat is generated by Sheffield Heat and Power’s (SHP) two municipal and two clinical waste incinerators. Hot water is pumped at about 105(C through an underground network of pipes to the users’ buildings, where it is delivered for use in existing heating and hot water systems. Some 120,000MW hours of heat are delivered annually through a connected capacity of over 130MW.

In order to meet stringent new emissions standards for incineration under PR 5/3 the SHP management decided to upgrade the plant in a multi-million pound investment programme on behalf of Sheffield City Council, with the aim also of extracting more energy in the process.

SHP has appointed NNC as prime turnkey contractor for the retrofitting works, which encompass design and

installation of a new flue gas treatment (FGT) system, a steam turbine and additional district heating equipment.

The new systems had to cope with wide fluctuations in demand from the district heating system. The fact that the district heating scheme is divided into two completely separate networks also led to important design

considerations.

New equipment had to be retrofitted on a constricted existing site. Unlike many power suppliers, Sheffield does not benefit from a plentiful, low cost water supply to use for cooling turbine exhaust steam. It has therefore been necessary to provide an air-cooled condenser.

Another aim was to keep the shut down period for the incinerators to the minimum. The NNC Booths Hall design team has now substantially completed its work for the FGT project, and NNC staff have been at work alongside sub-contractors at the Bernard Road site since April 1996. Major civil engineering and

construction work is well under way.

Integration of processes

The solution proposed by NNC achieves the integration of several processes, covering combustion control

modifications, energy recovery, reagent injection and particulate removal. The flue gas treatment plant is

designed to reduce emissions of acid gases, particulates, dioxins, furans and heavy metals to within statutory limits.

In addition to emissions control systems based around the FGT plant, NNC is also planning to install a steam turbine, capable of converting heat from the incinerators into a maximum 6.8MW of electricity. The turbine is a pass-out unit which can provide high and low pressure bled steam to two new sets of district heating heat exchangers, if the demand requires it. When demand for district heating is at its peak, the turbine will produce approximately 3.8MWe and a total of approx 26MW of heat will be transferred to the district heating system.

An air-cooled turbine condenser will be mounted above the turbine hall roof. NNC will also supply water treatment plant to ensure feedwater for the steam circuits remains of a high quality and new dosing equipment will be installed to maintain the feedwater chemistry within the required limits.

FGT system

The scope of the work involves removal of existing electrostatic precipitators on the municipal waste incinerators and a wet scrubber on the clinical waste incinerators, and installation of a dry injection and filtration FGT system. The FGT will treat the combined gases from the municipal and clinical waste incinerators.

Powdered lime, carbon and recovered particulates will be stored in new silos and injected into the waste gases. Combustion modifications will improve combustion and auxiliary burners fitted in each incinerator will ensure that exhaust gas temperature in the incinerator always remains above 850(C to be compliant with current legislation.

In order to prevent the formation of corrosive acids and to ensure that the reagents and the bag filters work effectively, the temperature of the gas entering them for cleaning must remain at approximately 140(C. A heat removal system is therefore being installed in the exhaust gas stream of each of the municipal waste incinerators.

The heat is removed by an economiser installed in each of the two incinerator exhaust streams. The hot water from the economisers is delivered to two new platetype heat exchangers, which allow the hea be extracted and transferred into the district heating system.

With the new heat recovery process, approximately 7MW of additional heat can supplied for district heating. On the limited occasions when there is insufficient demand from the district heating network, excess heat is discharged to the atmosphere through new air blast coolers which will be sited on the incinerator building roof.

The FGT plant provided incorporates necessary supporting plant and equipment in new electrical, air, water and effluent systems.