27 Apr 2013 05:04:30
Power generation from landfill gas – art or science?
Ian Cooper, business development director for ENER-G Natural Power, discusses landfill gas generation.
The practice of landfill gas generation to produce renewable energy is one that has developed over many years, particularly in the UK and North America. It is evolving worldwide at an increasing pace, spurred by an increasing need for cleaner, cheaper energy and carbon reduction policy and regulations, such as the Kyoto Protocol. Assessing the potential of a landfill gas management project both in terms of economic and quantitative viability poses a question with far-reaching implications: is it a scientific or artistic process?
Landfill gas monitoring
The scientific aspect employs a straightforward process. Using a standard questionnaire, the relevant data about a power generation opportunity can be collated – if the client has reliable data.
With a landfill gas opportunity, we look for data about when the site started taking waste and when operations will cease? Additionally, we need to ascertain what tonnages and types of waste have been disposed of to date and the likely figures for the future – municipal solid waste, industrial, commercial or inert? With the advent of recycling targets, composting and alternative technologies, a decline in the organic content of the waste is likely to be the biggest single influencing factor in the viability of a landfill gas project going forward.
From the data received, a desk study is carried out for each project. At this stage very little visual knowledge of the site is evident. Using assumptions for a ‘typical’ landfill, the collection efficiency is usually set at between 65% and 75% and a gas curve is generated with the data provided by the client. This also assumes good operation of the site, with low volumes of leachate, reasonable daily cover and areas of temporary and permanent capping systems to keep rain fall out and landfill gas in the site.
To confirm the draft gas curve, a site visit is carried out, and this is where the project moves from science to the art of experience and knowledge.
Interpreting the results
The site visit generally shows that the data from the potential client varies both in quality and factual correctness, mainly because it has a commercial implication. In this context, the experience of the landfill gas technician is drawn on to read between the lines and make an interpretation based on empirical information and professional insight.
It is easy to see, at the being that the process, that a scientific approach is key. However, as a project is explored in greater depth, the balance begins to shift and the art of interpretation and many years of experience becomes increasingly central to the outcome. In particular, adjusting the effects of the collection efficiency as a result of seeing leachate weeping from the sides of the landfill, lack of daily inert covering material, poor compaction on site, etc.
Exploiting the gas
The process of exploitation involves installing a network of deep wells into the waste mass and abstracting the gas using a pump and blower unit, which directs it through a surface laid system of connection pipework to a generation compound. In the compound, spark ignition reciprocating engines utilise the fuel to generate renewable electricity, which can be sold to the local network operator.
The process sounds simple. However, should oxygen greater than 5%v/v enter the system, it can have the catastrophic effect of destroying the generator units in an instant. This is where ENER-G's decades of skill and experience of working on landfills comes to the forefront by balancing the gas from each well to optimise the quality being delivered to the compound.