6.1 Capital costs for biogas
Large-scale AD plants using municipal solid waste (MSW), agricultural waste or other industrial organic wastes are proven technologies, but they can be limited in scale by feedstock availability. The use of energy crops increases the opportunities for larger and/or more numerous facilities, albeit with higher feedstock costs. This biogas is then upgraded for use in vehicles. The upgrade removes the high level of CO2 (typically 45% before the upgrade) from the biogas to create biometh-ane. The level of impurities such as carbon dioxide permissible in the biomethane varies by country depending on local regulations.
Total installed costs for an AD biogas plant can depend on the feedstock. Those based on manure and sewage are typically cheaper. This is because the handling and storage of the feedstock is already available, or would have to be constructed even if there was no AD being considered. For AD systems based primarily on energy crops (e.g. maize silage) the total investment costs will typically be higher to take into account feedstock storage and handling (Figure 6.1). The digester system is the most expensive component of an AD biogas plant, although for large-scale systems using energy crops, the cost of storage can also be large.
Figure 6.1: Capital costs per unit of capacity for AD systems by plant size and feedstock
Source: Urban, 2009.
Total installed costs for AD systems are usually expressed in terms of cost per unit of capacity, where capacity is expressed in terms of normal cubic metre (Nm3)/hour.35 Total installed capital costs for an AD system using 90% manure and 10% maize silage vary from USD 7 310 to USD 5 050/Nm3/hour. This is for systems with hourly output capacities of 100 Nm3 and 500 Nm3 respectively (Figure 6.1).36 AD system components have an expected economic life of 15-20 years.
The total installed costs for AD systems using 90% maize silage and just 10% manure vary. They range from USD 5 400/Nm3/hour for capacities of 2 000 Nm3/hour to USD 7 500/Nm3/hour for systems with an hourly output of 250 Nm3/hour respectively (Figure 6.1).
This section discusses the capital costs for upgrading systems to remove the CO2 and other impurities from the biogas. The main upgrading technologies are (Bauer, 2013):
- Amine scrubbing process' use a reagent, typically a water solution of amines, which chemically binds to the CO2 molecule and removes it from the gas.
- Pressure swing adsorption (PSA) is a dry method where the raw biogas is compressed to high pressure and then fed into an adsorption column where the CO2 is retained, but not the methane.
- Membrane separation uses a dense flter to separate the components in the biogas or a liquid at the molecular level. The selective membranes used for biogas upgrading retain most of the methane while most of the CO2 permeates through the membrane for treatment.
- Water scrubbing uses a physical scrubber where the CO2 is dissolved into water in an absorption column in a high pressure environment. The CO2 is then released from the water again in the des-orption column, by addition of air at atmospheric pressure.
- In organic physical scrubbing, the CO2 in the biogas is absorbed in an organic solvent (e.g. a mix of dimethyl ethers of polyethylene glycol) in a process otherwise similar to that of a water scrubber.
Upgrading allows the combustion of biogas in vehicles and its injection into existing natural gas grids. Table 6.2 presents the typical composition of biogas and landfill gas, as well as the natural gas network requirements for the Danish and Dutch networks. Upgrading the biogas typically requires the removal of CO2 and other impurities such as hydrogen sulphide and ammonia.
Source: Petersson and Wellinger, 2009.
As would be expected, capital costs are proportionately higher for small-scale applications with throughputs of 500 Nm3/hour or less (Figure 6.2). Installed costs for these smaller systems are between USD 4 400 and USD 5 950/Nm3/hour of capacity for 250 Nm3/hour systems and between USD 2 600 and USD 3 450/ Nm3/hour of capacity for 500 Nm3/hour systems. For large-scale facilities which process 2 000 Nm3/hour or more, the capital costs for biogas upgrading are around USD 1 950/Nm3/hour of capacity, which adds 36% to the AD biogas plant costs.
For small-scale systems with capacities of 100-500 Nm3/hour of biogas, the total system costs are between USD 8 950 and USD 13 800/Nm3/hour, with the upgrading system accounting for 37-47% of the total installed costs. Large systems with capacities to generate and then upgrade 1 000-2 000 Nm3/hour of raw biogas into biomethane have total installed costs of USD 8 600 and USD 7 350/Nm3/hour respectively. The share of the upgrading system drops to between 27% and 30% (Figure 6.3).
Figure 6.2: Capital costs for biogas upgrading systems by type and size
Source: Bauer, 2013.
Figure 6.3: Capital costs for biogas systems including upgrader by type and size
Sources: Bauer, 2013 and Urban, 2009.