Landfill Gas Powers Brick Production

by the CADDET UK National Team

INTRODUCTION

Marshalls plc produce high-quality facing bricks using tunnel kilns at the company's Stairfoot Brickworks site, in the UK. The company extracts clay from the adjacent quarries, which are subsequently filled with domestic waste. In 1981 Marshalls decided to exploit the landfill gas (LFG) resource 'on its doorstep'.

GAS EXTRACTION

The scheme involves burying a grid of perforated pipes within the domestic waste landfill and then sealing the quarry top with clay. Originally vertical wells were constructed; however, horizontal wells were found to be more practical because the gas can be used as soon as it becomes available.

Picture

Careful landscaping provides a local amenity

There are around 65 gas collection wells in operation, spaced 30 m apart and designed to capture gas from four 2 m-thick layers. The gas is extracted by a blower with variable speed to a maximum of 11.3 m3/minute. Pre-treatment is limited to one water knock-out pot, and a 50 micron filter. Methane levels are maintained at 40-50% at 72.4 mbar pressure at the burner.

Gas is transported from the quarry through a 230 mm pipe, carefully laid to have a constant slope and avoid U-bends where moisture can collect and inhibit gas flow. Marshalls now use their own brick rubble to fill-in around the pipe, rather than the expensive gravel originally used. The maximum distance travelled by the gas is 0.75 km. Condensate is removed at a low point in the main supply pipe immediately prior to the gas pump. Controls on each well and pipe measure gas pressure, volume and flow.

USING THE GAS

The gas is burned in the brick kilns, and LFG accounts for 20-30% of all gas used on the site. LFG is better suited to the medium temperature parts of the kiln, 870-960 oC; natural gas is used for the higher temperature processes (up to 1,050 oC). However, the burners can be switched from one type of gas to the other, with pre-set valves controlling the flow to ensure a constant heat input is maintained.

Landfill gas has been used constantly in certain sections of the kilns for the past 12 years and, in practice, there is little or no changing of gas supply to a particular burner. This conforms to the control philosophy which is to find and maintain stable settings for the system.

The kilns have three types of burner. Bricesco burners are used for LFG, and are set to run at 66 kW. Eclipse burners (used in the kiln roof as 'crown burners') and North American burners use natural gas, and are set at 88 kW. The number of burners which can operate on LFG is limited. At least 12 burners/kiln must operate at the higher rate to provide the required temperatures.

The kilns operate 24 hours/day and 50 weeks/year. During the two-week Christmas shutdown, the LFG is flared on-site at 350 m3/hour. Methane levels are 44-48% and there are no problems with contaminants or condensation. The only servicing necessary is to blow the burners out with compressed air occasionally.

Gas Consumption at Marshalls' Stairfoot Brickworks in 1995 and 1996

Kiln 1
(cubic metres)

Kiln 1
(cubic metres)

Kiln 2
(cubic metres)

Kiln 2
(cubic metres)

1995

Average Daily

Total Annual

Average Daily

Total Annual

Consumptio n

Landfill gas

3,048

1,030,208

3,990

1,368,763

Natural gas

4,352

1,501,790

4,129

1,416,081

Total

7,400

2,531,996

8,119

2,784,844

1996 to
31 October

Landfill gas

3,226

922,433

3,911

1,110,568

Natural gas

4,104

1,148,640

3,987

1,156,494

Total

7,330

2,071,073

7,898

2,267,062

ENVIRONMENTAL BENEFITS

A brickworks and a waste site are not generally regarded by the local population as amenities! However, at Stairfoot, the quarry site has been developed to provide grazing for cows, tree plantations and a willow coppice. The local junior school has been heavily involved, particularly participating in the oak tree plantation.

The site also offers a permanent geological display on the Mansfield Marine Band (an important layer of sediment and a rich source of fossils), developed in co-operation with the National Geological Survey and the local community. A purpose-built bat cave has also been designed and constructed on land close to the outdoor classroom facility in the yew tree plantation.

When Marshalls first began extracting LFG there were problems with crop damage (identified by aerial photography) and smell; however, these were quickly overcome by retrofitting an extraction system.

ECONOMICS

Marshalls' initial investment of 330,000 (where is the UK pound) for landfill gas equipment compares with savings in natural gas costs of 1.33 million over 12 years. Operating costs are restricted to a few staff hours/month for monitoring and maintenance, and the charge for the electricity supply to the booster (typically 4-5 kW). The site is expected to produce usable quantities of methane for several decades.

For more information contact the CADDET UK National Team in Oxfordshire.

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

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.