Abstract:
More and more effort is being put in the utilization of bio-solids. Bio-solids can be an important source for useful products like fertilizers and biogas. DMT has been developing biogas treatment plants for over 20 years following the market developments closely.
Biogas was first seen as a nuisance at e.g. landfills, creating odor problems and methane emissions. A flare has always been a cheap and simple solution. But with time more and more biogas was produced intentional from bio-solids to generate energy.
In the current market the choice for utilization of the biogas is usually a choice for a single technology (e.g. boiler, CHP or upgrading to natural gas/ fuel quality), which is than fixed for the next 10-20 years. At the same time economical feasibility, especially for biogas upgrading, but also the other technologies, is based on “the bigger the better” principle, which is relying on the benefit of scale. This is in contrast to the reality where the biggest potential for biogas production is on small-scale sites.
DMT has developed a biogas upgrading plant with the focus on small scale utilization of bio-solids. In this article it is shown that, also with a biogas production of only 50-200Nm3/hr, it is feasible to utilize biogas as a combination of energy sources. The CarbO2rex® is a modular plant built in a sea container(s). The biogas upgrading is performed with gas selective membranes. The upgraded gas with a methane concentration of 96-98% CH4 can be used in the local gas grid, or can be further compressed to 220 bar and used as vehicle fuel (CBG). To optimize the energy balance and to prevent methane emissions, the off-gas from the membranes is burned in a micro-turbine, to produce electricity and heat.
Keywords: CarbOrex, CHP, Biogas, Bio-methane, CBG, Car fuel, Gas separation, Green gas, Gas Membrane, Small Scale Upgrading
Introduction:
The transition of fossil to renewable fuels is on its way! Biogas produced at landfills and/ or digesters can be considered as renewable fuel since it is produced from organic waste. Most commonly the biogas is converted to electrical energy by gas engines with an efficiency of around 40%. Increasing efficiency to levels near 100% will require upgrading of the biogas. This can be done with various processes. Upgraded biogas can be used as vehicle fuel or injected into the gas grid. Biogas used as vehicle fuel is one of the cleanest possible fuels, with hardly any CO2 emissions and very low local pollutants. This is shown in figure 1.
Upgrading of biogas mainly evolves the reduction of CO2, H2S and H2O from the raw gas. The CO2 is removed to increase the energy content of the gas. For vehicle fuel this is important, because it increase the action radius of the vehicles. When injecting the biogas in the gas grid a similar energy content will be required as compared to the gas already present. The CO2 concentration is also important to ensure flame stability and energetic value for the end users. H2S needs to be removed to prolong the life time of the all use equipment, piping and burners since it is a very corrosive gas. When H2O is present in a gas stream condensation can occur which is highly undesired, therefore, it should be completely removed. Table 1 shows the composition of raw biogas and the demands of upgraded gas.
Table 1: Raw biogas versus biogas at natural gas quality (The Netherlands/ UK/ German) and biogas at maximum quality for vehicle fuel use (Directie Toezicht Energie 2006; DVGWDeutsche Vereinigung des Gas- und Wasserfaches e.V 2008).
| Component |
Unit |
Biogas |
Natural gas
Dutch |
Natural gas
German/ UK) |
Vehicle fuel |
| CH4 |
v/v % |
45-70 |
90-95 |
>95 |
>97 |
| CO2 |
v/v % |
30-45 |
<8% |
<5% |
<1 |
| N2 |
v/v % |
1-10 |
<10% |
<5% |
<3 |
| O2 |
v/v % |
0.2-1 |
<0.1% |
<0.2-0.5 |
<0.5 |
| H2S |
mg/Nm3 |
10-15.000 |
<5 |
<5 |
<5 |
| CF |
mg/Nm3 |
0-3000 |
<dew point |
<dew point |
<dew point |
| H2O (dew point) |
ºC@8 bar |
Saturated |
<-8 |
< -8 |
< -169 |
| Caloric value |
kWh/Nm3 |
5-7.7 |
8.8-10.8 |
8.4-13.1 |
10.7-11.6 |
| Wobbe index |
kWh/Nm3 |
4.8-8.4 |
12.0-12.3 |
12.8-15.7 |
14.1-14.8 |
Many companies that provide upgrading plants design systems which are suitable and optimized for large scale biogas upgrading. The high investment costs of the installations show that with the present grants and gas prices, approximately 500-1000 Nm3/h of raw gas is needed to make the plant economically viable. However, most of the biogas is produced at farm scale digesters and waste water treatment plants on a much smaller scale. This causes a loss in potential of upgradable biogas. Therefore, DMT developed a system which can upgrade biogas on a small scale in an economical viable way.
