Power generation from landfill gas - The Garraf landfill site
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Enough power to light up the entire city of Barcelona could - in theory - be provided
from one Spanish landfill site, according to modelling of its biogas generation capacity.
But risk assessment and continued government co-operation will be of vital importance if
this capacity is to be fully utilized.
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Christopher Eden |
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The controlled landfill site of Vall d'en Joan - Garraf - is the largest landfill site
in Spain, and one of the largest in Europe. It is located in the centre of the
Natural Park of Garraf on the outskirts of Barcelona. Since its opening in 1974,
more than 22 million tonnes of refuse has been deposited there, and
projections are that there will more than 25 million tonnes in place once it
closes in 2004 or 2005. The landfill site occupies an area of around 64 hectares
and, although the depth varies with the original contours of the landfill site, it is over
100 metres deep in places, filling a complex valley system.
In May 2001, two tenders were issued by the owner of the landfill site, the Entidad
Metropolitana de Serveis Hidràulics i Tractament de Residus (EMSHTR) - the
department of the City of Barcelona responsible for the treatment of solid waste. The
first of these tenders was for the physical restoration of the oldest part of the landfill,
and the second for the control and use of the biogas that is being generated from the
whole of the landfill site. The power generation contract was awarded to a temporary
joint-venture company formed by CLP Envirogas S.L. and ECYR (Endesa Cogeneración
y Renovables). The scope of the project awarded included the theoretical modelling of
gas production from the landfill site, physical pumping trials to prove the models, the
construction of a gas extraction system, and the installation of a power generation
complex to produce and export energy to the Spanish National Grid.
The project has been entirely financed by the developers, which have not only
provided the technological know-how and experience, but also assume much of the risk
for the project. In addition to this, the project pays a royalty to EMSHTR for using the
gas.
Legislative background
The Spanish Government is committed to encouraging the
production of renewable energy. In December 1998, a Royal
Decree, or new legislation, was approved. This legislation
obliges the main electricity supply companies to purchase
part of their baseload supply from renewable energy sources,
which in effect means that they must purchase all the energy
produced by authorized renewable energy generators. In
exchange for this, the renewable energy producer can choose
between a fixed price and a price
that varies according to the
behaviour of the electricity 'pool'.
In the latter case a subsidy is
included that varies according to
the technology employed to generate the renewable power.
The document that governs the production of renewable
energy in Spain is the above-mentioned Royal Decree, Real
Decreto 2818/1998, published on 23 December 1998. The
law is designed to encourage renewable energy production,
and so comply with the various global and European treaties
that attempt to reduce the effects of the emission of
greenhouse gases. The target for the Spanish Government is
to produce 12% of energy demand using renewable sources
by the year 2010, one that is being met by a large push in the
construction of wind energy projects. Landfill gas and other
sources of biogas account for a smaller percentage of total
renewable energy production in Spain, while solar energy
remains largely a demonstration technology.
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View of the Garraf landfill site and power generation plant
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One of the main problems for renewable energy
producers is that the premium paid for the production of
renewable energy is subject to yearly assessment. This
effectively means that the Spanish Government reserves the
right to set the level of the premium paid for renewable
energy. The tendency in the two years since the system has
been operational has been for the premium to reduce
slightly. However, given the fact that the pool price of energy
is governed not only by the price of traditional fossil fuels
but also by the input of energy from sources such as hydro
electric schemes (projects that under normal conditions
reduce their power output during the summer months, thus
leading to an increase in the pool price), the price of
electricity sold to the grid has been maintained at relatively
stable levels, and has ensured a fragile commercial viability
for small-scale renewables projects. It is evident, though, that
until the Spanish Government adopts a system that enables
renewable power generators to establish reliable long-term
cash flow models, these small-scale projects will remain at
the mercy of the changing fashions in the legislative
environment.
Risk assessment
The risks of power generation from landfill gas are not only
commercial - renewable energy production, by its very
nature, is a risky business, as the availability of fuel can be an
unknown quantity. Whether it is wind, sunshine or
availability of organic material, the risk of a reliable source of
fuel has to be carefully assessed before embarking on a
renewable power generation project. This is especially true if
using landfill gas as a fuel source. The business is - so to
speak - littered with examples where an over-assessment of
available fuel resulted in an engine, or several engines,
standing idle, and this is potentially a commercial
'nightmare scenario'. Even if all methods to assess fuel
availability within any landfill site are rigorously followed,
there will still remain a question of doubt until engines are
commissioned and power is exported to the electricity grid.
Reliability of the fuel source has to be assessed before embarking on a project |
There are several methods for assessing fuel risk, ranging
from computer modelling of waste decay rates versus the
amount of waste in place in a landfill site, to physical
methods such as pumping trials. All of these are subject to
many variables, the nature of which is hard to define. Lack
of humidity in the waste, high leachate levels, permeable
underlying strata, and the evolving nature of the gas as waste
degradation progresses are all factors that - although they
have been described and scientifically modelled - remain
unknown and mostly unquantifiable. In the end, a
combination of methods is usually
employed to minimize risk and maximize
the possibility of project success.
Experience, and indeed failure in the
construction of this type of project in many
landfills, helps to ensure that the operators
have a higher chance of correctly defining
the level of capacity to install at the
beginning of a project. The most efficient
method is to estimate below the theoretical
maximum, leaving an 'absorption zone' to
account for unknown factors. Once the
engines are operational, ongoing flaring of
gas will provide more 'live' data as to
additional potential capacity.
View of the engines together with the high-temperature
flare stack, showing the elongated exhaust pipes used
to avoid accumulation of heat above the radiators
Other factors to consider in whether a
project will be viable or not include:
- location of the landfill site
- cost of the electricity export line
- active life of the landfill site
- level of engineering of the site
- the type of restoration
- whether or not there will be disruption to the gas extraction
system during ongoing site operations
In other words, there are a number of factors to consider
when deciding whether or not a landfill gas-powered
generation system is commercially viable. The commercial
risk associated with a possible reduction in subsidy is one
that could and should be removed from the equation. What
is fundamental in any subsidy programme for the promotion
of renewable energy is that it should consider not only the
financial aspects, but also the level of technological risk that
the operators have to assume, given the uncertainty attached
to the availability of the fuel source.
The Garraf landfill site
Data provided by the owner of the landfill were introduced
into a computer model. Since landfilling at the site began in
1974 more than 22 million tonnes of refuse has been
deposited. As indicated in Figure 1, the site is modelled to
produce more than 13,000 Nm3/hour of biogas from 2001
(Nm3 being the 'normalized' volume of gas). This will reduce
to around 6000 Nm3 by the year 2015. Theoretically, the
former figure is sufficient to produce up to 20 MW of power.
However, because of the uncertainties of restoration,
capping and so on, a significantly reduced level of 12 MW
was selected. Should gas levels remain consistent with the
models, it will be possible to install more engines, as
conditions of operation on the site become more stable and
better defined.
FIGURE 1. Model for biogas production from the Garraf landfill site in Barcelona,
with optimistic, pessimistic and average models, estimated with 50% CH4 and
70% collection efficiency. The 10 MW, 12 MW and 15 MW figures relate to
planned capacity, based on the modelling
The financial models for the project were constructed on
the basis of 12 engines with a gross capacity of 1048 kWe
each or 12,576 kWe in total. This level of power generation
is projected to remain the same during the first six years of
the project and reduce thereafter to around 9 MW by the
end of the concession in 2013.
Project details
The project was part of a threefold execution of projects
being carried out on the site. In parallel with the
construction of the gas extraction system, a thoroughgoing
upgrade of part of the site has been carried out. This
upgrading includes the installation of an impermeable liner
and the movement of several thousand tonnes of material
for capping and restoration. As a result of this work, along
with the ongoing site operations and waste input, much of
the site was unavailable for the installation of gas extraction
facilities. Table 1 lists some of the details of the landfill gas
power plant installed.
TABLE 1. Main technical details of the Garraf landfill gas power plant |
Number of extraction wells |
Approximately 240 |
Well dimensions |
500 mm diameter |
20 metres deep on average |
wellscreen of 160 mm external diameter |
Booster and gas collection system |
Three boosters, supplied by Organics Ltd, each with a capacity of 3000 Nm3/hour |
Flaring facilities |
Up to 3000 Nm3/hour designed to ignite automatically in the event of engine failure |
Flow rate to the engines |
Each engine has a flow rate of around 560 Nm3/hour at a methane content of 50% |
Engines |
12 engines mounted in acoustic containers |
acoustic conditioning: 60 dBA |
1048 kWe each |
electrical efficiency: 37.1% |
Sub-station |
transformer power: 18 MVA |
export tension: 66 kV |
Electricity line |
double circuit of 66 kV |
one circuit of 25 kV |
length: 2.1 km, following the trace of an old electricity line from the landfill site |
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The gas extraction system consists of over 240 extraction
wells connected to a network of pipes that range in size from
355 mm, with connections to individ
ual extraction wells
being up to 63 mm diameter. The total length of the gas
extraction system is more than 10 km.
An automatic control system that would have controlled
each of the extraction wells individually according to
pressure and methane content was rejected on the grounds
of additional complexity, cost and difficulty in long-term
operation. Additional gas extraction wells were installed in
its place. These are operated and maintained by fully trained
operators who not only optimize gas extraction from
individual wells but, because gas extraction
systems on landfill sites are high-maintenance
installations, also carry out regular visual
inspection and repair of all the components
of the gas extraction system.
Drilling of the extraction wells
commenced in early 2002, and was carried
out on several artificial terraces that were
constructed as part of the restoration
programme. As gas that is generated within a
landfill site is supersaturated with water, it
tends to condense on the walls of the
connecting pipelines once it is removed from
the humid conditions that operate at depth
within the landfill site. In order to minimize
accumulation of this water (or condensate)
and subsequent pipeline blockage,
connection of the extraction wells was carried
out by ensuring that the pipeline was laid with
a significant gradient. This configuration
takes advantage of the effects of gravity and
diverts condensate to low points in the system
where it can be safely removed. Because of the
way the landfill has been built - following the contours of the
original valley system - the site has very steep slopes, a
feature that assists in dewatering the system.
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View from the booster station platform showing the three boosters each with a capacity of 3000 Nm3 |
Boreholes being drilled in the waste for extraction wells, to a depth of 20-25 metres |
The average depth of the extraction wells is around
20 metres. Given the V-shape of the original surface below
the waste, drilling was deeper in the centre of the site than
on the edges. The final depth was limited due to the
presence of leachate within the landfill at differing horizons.
The gas field is kept under constant pressure by three
booster sets supplied by the company Organics Ltd that are
capable of a maximum flow rate of 3000 Nm3 each. This
means that there is an effective ceiling of gas extraction of
9000 Nm3/hour, enough for around 15 MW capacity. A
high-temperature ground flare, with a capacity to incinerate
3000 Nm3/hour of landfill gas at a temperature of more than
1000ºC, and with a residence time of 0.3 seconds, is installed
before the engine complex to provide emergency back-up.
Under normal circumstances the flare will be switched off,
although as the project progresses, it will be used to confirm
whether there is sufficient gas to increase electrical output.
During the construction phase of
the gas collection system, the flare
was used to confirm the results of
the theoretical calculations that
form the basis of the original
proposal.
The site uses 12 Jenbacher JMS 320 engines, each with a
capacity of 1048 kW. They have been supplied in containers
that, because of the environmental requirements, have a
noise specification of 60 dBA (acoustic decibels) at one
metre. One of the engines has equipment to continually
monitor the rate of carbon monoxide and
oxygen production, ensuring that the
stringent environmental requirements
related to emissions, as stipulated by both
European and Catalonian legislation, are
complied with.
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Inside of one of the 12 containers, showing one of the Jenbacher JMS
320 engines |
View of the engine containers, together with the high-temperature flare stack |
Although the engines produce energy
at 6.3 kV, the power station is equipped
with transformers that supply energy
at 0.4 kV to the on-site facilities and, on
a separate platform located some
100 metres from the engines, the export
transformer 6.3/66 kV has a total power
capacity of 18 MW, which ensures that
there is room for expansion should
ongoing gas trials prove positive.
Environmental statistics
It is well known that uncontrolled
emission of landfill gas contributes significantly to the global
production of greenhouse gases. Methane, one of the main
components of landfill gas, is some 21 times more effective
as a greenhouse gas than carbon dioxide.
Annually, the engines at Garraf will burn 50 million m3 of
methane. This equates to the removal of around 600,000
tonnes of CO2 annually.
Garraf burns 50 million m3 of methane a year, equivalent to 600,000 tonnes CO2 |
The Garraf power generation plant will produce around
100 million kWh of energy per year. This means that there
will be a considerable displacement of CO2 that would
otherwise be produced from conventional fossil fuel energy
plant. Effectively, it amounts to a saving of around
50,000-150,000 tonnes of CO2 per year.
Altogether, the power plant at Garraf will result in a
saving of up to 750,000 tonnes of CO2 being
emitted to our atmosphere. This is equivalent
to the planting of around 17,000 hectares of
new forest. This becomes significant when
one considers that the whole size of the
Natural Park of Garraf covers an area of some
12,820 hectares.
Perhaps a more concrete comparison is that
the 100 million kWh of power that will be
generated by the Garraf power plant is enough
to supply electricity to the entire public
lighting system in the City of Barcelona, which
uses around 97 million kWh a year.
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