The technical issues related to biogas purification aren’t really problems; it’s the business aspects and overall solutions that constitute the real challenges
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This post was last edited by lianglikun on 2016-5-20 at 11:21. First of all, we would like to state that our company is also capable of using pressure swing adsorption for the purification of biogas. We have built a biogas purification unit with a capacity of 5,000 Nm3 per hour, as well as smaller portable units. We mention this only to show that the data provided has a basis and isn’t arbitrary. The main technologies used for biogas purification are pressure swing adsorption and membrane separation. Pressure swing adsorption is an established gas separation technique in China, and it is more than sufficient for separating methane from carbon dioxide. Although the yield with this method may be slightly lower compared to membrane separation in conventional systems, this is because membrane separation typically uses multiple stages, while pressure swing adsorption usually operates in a single stage; at most, recycled gas can be reused in the process. If a two-stage system is used, a high yield can still be achieved. Membrane separation is also a mature technology. For current systems, when it comes to reducing the carbon dioxide content from 40% to 15 kg per unit volume, membrane separation is more energy-efficient compared to pressure swing adsorption, which reduces the carbon dioxide level to below 5 kg per unit volume. When the gas is sent to the pipeline network, the pressure is lower, making pressure swing adsorption even more energy-efficient. Below is a table showing the power consumption for different processes: Pressure washing, Pressure swing adsorption, Membrane separation. Notes: Operating pressure in MPaG: 1.5, 0.6, 1.3. Axial power of the biogas compressor in kW: 144, 102, 136. Power loss during CO2 compression in kW: 57.6, 40.8, 54.4 (based on 40% carbon dioxide content). Comparison of compression efficiency: 141%, 100%, 133% (based on pressure swing adsorption as a reference). Other electrical equipment and their power consumption: Washing water pump + cooling tower: 109 kW; Vacuum pump: 30 kW; Refrigeration dryer: 12 kW. Total electrical power consumption in kW: 253, 132, 148. Comparison of energy consumption: 192%, 100%, 112% (based on pressure swing adsorption as a reference). However, these are just minor technical issues. There are some real problems here that are difficult to discuss in detail. The key points summarized by us are as follows: calorific value, regulations regarding gas delivery to pipelines, permits required for pipeline connection, licenses for gas filling stations, radius within which straw can be collected, and comprehensive utilization of biogas sludge and liquid.
Let’s not discuss technology for now; operational issues are indeed major problems! ! !
The original poster is absolutely right; there are indeed many mature applications in the field of natural gas chemistry that can be used as references for biogas separation technologies. It has been proven that this is also the approach taken when selecting technologies for practical engineering applications, and the results confirm that such technologies are viable. Of course, there are still many areas that can be improved and optimized, but technical issues are not the factor limiting the development of this industry. Thank you for sharing the data; the design value for power consumption is quite high here – 30 KW for vacuum
Regarding energy consumption, the original poster only calculated the electricity used for the first stage of compression; the energy consumption for further compression to natural gas level was not taken into account. Since the compression pressure is lower in the early stage and higher in the later stage with PSA, the overall difference isn’t significant, right?
Indeed... operation is truly the issue that hinders the development of this industry
I didn’t even take the time to examine these values closely; they were taken from previous proposals. Separation technology is essentially carbon removal technology, and it is already a well-established technique in the chemical industry. Pressure swing adsorption originated from the process of carbon removal in ammonia synthesis, and its purpose is to address the issue of carbon removal.
The final pressure of the overall gas volume is the same in all cases. Therefore, the total energy consumption for processing those 60% of methane is identical. However, with PSA at low pressure, it is possible to remove those 40% of carbon dioxide, while with other methods, that 40% of carbon dioxide needs to be raised to over 10 kilograms before it can be removed; this difference in energy consumption represents the gap between the energy requirements of these different technologies
What are the purity and recovery rates of the product gas obtained through pressure swing adsorption?
The purity should definitely meet the lower limit of the calorific value standard for CNG, with a recovery rate of around 95% generally; it depends on the owner’s requirements.
In fact, when comparing aspects such as energy consumption, the differences among them are not very significant; after all, compressing biogas at normal pressure to 6 kilograms versus compressing it to 13 or 15 kilograms results in only a slight increase in the compression ratio of 2.5 points. After the washing process, dehydration is required to meet the requirements for pipeline transportation; furthermore, energy consumption is also involved. Compressing to a relatively high pressure is an issue, as the pressure in the main urban pipelines does not need to be that high; moreover, the pressure in the natural gas branch pipes is low. As a result, water washing and membrane separation become problematic in such situations. The pressure of the gas obtained through pressure swing adsorption is roughly equal to the pressure in the urban pipeline network. So from this perspective, PSA has certain advantages. The treatment of water after washing is also a considerable challenge; the pressure reduction used is not sufficient to remove all the CO2 from the water, and the volume of water that needs to be recycled as a result of washing is quite large. The overall operating costs will also be higher. The equipment investment for PSA is obviously high; especially since programmable valves operate frequently, reliability is a critical issue. Multiple pressure equalization attempts can help recover as much methane as possible, but this will reduce the equipment’s production capacity. As for membrane separation, there are likely to be certain difficulties in achieving long-term stable operation. Each of the three technologies has its own advantages; one shouldn’t favor only their own technology
Give it a like. The cost of purified biogas should be around 2-2.5 yuan at present; I’m not sure if this figure is still accurate. The commercialization of biogas in practical applications is indeed a major challenge. Using it as fuel for vehicles should present fewer obstacles compared to integrating it into the gas distribution network, though the investment required is high