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Comments of Engineering Landfill Gas

Wednesday, March 11, 2009

Technical article about the use of landill gas as a source of energy for the production of electric power.

Introduction

The Group of Companies of Energía Sur de Europa, S.L; which comprises CLP Envirogas, CLP Organogas, Envirogas (Portugal) y Envirogas (UK), has, for more that 19 years, been involved in teh production of electrical energy using landfill gas as the unique fuel source.

Due to the specialist nature of this working within a landfill; a continually evolving environment, all designs and installations for degassing the site must be as simple as possible; this to ensure ease of replacment and maintenance.

Extensive experience tells us that the main points to bear in mind when designing and installing a gas collection system for power generation and environmental control are as follows:

Long term environmental control
Optimisation of the gas collection in order to maximise electrical output.

A landfill is a highly aggressive environment for equipment of any kind. This is particularly true if the equipment has to last for any length of time; just such a case as is necessary in long-term enviromental control and, no less importantly, the commercial success of the project.

During active operation over many years of landfill gas power generation schemes, we have noted that a landfill gas extraction well, and by inference, an integral part of the gas collection system, has a useful life that can oscillate between 6 months and 7 years. For this reason, whatever material or equipment is used in the construction of a landfill gas collection system must be robust, easy to maintain and of low cost. Our ongoing philosophy is to invest in the largest number of extraction wells possible rather than expensive equipment to monitor and control a limited number of wells. the sad reality is that such equipment will probably not work over the long-term and will require constant re-engineering due to addition or subtractin of wells.

Landfill Gas Pumping Trials

Of critical importance in the decision process for the installation of a power generation system is to prove that the landfill is producing enough gas to sustain power generation over the long term. Usually, theoretical calculations of gas availability are carried out, and these are supported by a field-based pumping trial; this in order to simulate as closely as possible the conditions under which a power generation plant would have to operate.

Once this has been completed an estimate of the capital cost of the project can be made with greater confidence.

There are various methods to determine that quantity and quality of gas that is being generated by any given landfill site. These include:

The rule of thumb method
Computer-based predictions of gas production
Field-based physical pumping trials in the landfill site itself

All of these methods should be used as part of an integral investigation of the landfill site in order to provide as clear a picture of all factors involved in the development of any potential project. Apart from anything else, the fiancial bakers of the project, will require an informed technical opinion that the project will be viable over the term of amortization.

Design Engineering.

A landfill site is a highly dynamic system. The evolution of the site over the long-term inevitably produces a significant loss of mass that results in settlement that can cause damage to a gas collection system. This, in turn, causes losses in the efficiency of the sytem to capture and transport the gas to the control plant.

Extraction wells are usually installed using large-diameter rotational drilling equipment as this tends to guarantee that any obstructions in the landfill (of which there will be many) can be easliy dealt with. The depth and type of drilling equipment finally decided on will depend, to a large extent, on a detailled study of the site maps and technical requirements.

Installation of the Gas Collection System.

Once the site has been drilled, construction of a landfill gas collection system will include:

Installation of vertical well lining. Our experience has tended to lean towards a slotted pipe of 160mm as this allows for back-installation of a pump to remove leachate from the well. 75% of this pipe should be slotted, while the rest, located at the head of the well, should be plain; this in order to avoid the ingress of air into the sytem. The slotted pipe usually terminates around 3 metres from the surface of the site and protected with a layer of clean, carbon-free gravel to act as a filter.
Each well should be sealed with a suitable material in order to prevent air ingress into the system. We normally install bentonite pellets that have an expansion ration of arond 300% once water is added.
Well completion is usually terminated with clay; this in order to protect the bentonite barrier below

Power Generation.

Given the characteristics of biogas that is generated in a landfill site; the main feature being that it is composed of a high percentage of the combustible gas methane, it can be used as a fuel in gas engines that produce electrical energy. Engines that are normally used for this application include internal spark ignition engines that drive a generator to produce a current at 400V. This, in turn, is transformed to a current that can be fed into the national electricity grid, in many cases, between 15 or 20kV.

Operation and Maintenance.

Operation and maintenance of a landfill gas power generation plant requires a highly trained team of engineers who can respond around the clock. In order to optimise the system, these engineers must have a sound knowledge of all aspects of the power generation process from installation and maintenance of a landfill gas collection system, to routine and programmed maintenance of the gas engines, through to a knowledge of the electrical system.

This can often be a rare animal.

Landfill gas is a highly corrosive medium to all parts of the system and the landfill site itself is an entity that is continually in movement; a feature that, if it is not constantly monitored, can result catestrophic disruption of the system without warning.

Energía Sur de Europa carries out all its own operation and maintenance and the team of engineers who maintain our fleet is without doubt, a highly valued company asset.