Design Biogas Plant Pdf Viewer ((BETTER))
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Biogas is produced after organic materials (plant and animal products) are broken down by bacteria in an oxygen-free environment, a process called anaerobic digestion. Biogas systems use anaerobic digestion to recycle these organic materials, turning them into biogas, which contains both energy (gas), and valuable soil products (liquids and solids).
Digestate is the nutrient-rich solid or liquid material remaining after the digestion process; it contains all the recycled nutrients that were present in the original organic material but in a form more readily available for plants and soil building. The composition and nutrient content of the digestate will depend on the feedstock added to the digester. Liquid digestate can be easily spray-applied to farms as fertilizer, reducing the need to purchase synthetic fertilizers. Solid digestate can be used as livestock bedding or composted with minimal processing. Recently, the biogas industry has taken steps to create a digestate certification program, to assure safety and quality control of digestate.
With biogas systems, dairies, farms, and industry can reduce operational costs using their own organic wastes to power their equipment and buildings. Fair Oaks Dairy in Indiana produces 1.2 million cubic feet of biogas each day with manure from 9,000 dairy cows. Some of the biogas is upgraded to CNG and used to power trailers delivering milk to Fair Oaks processing plants, reducing their use of diesel fuel by 1.5 million gallons per year.
The biogas produced from biomass is a carbon-neutral renewable fuel. This fuel is usually flared or released into the environment as a waste gas. An energy generation technology that can efficiently produce electricity and heat with low emissions when operated in a distributed generation mode is most desired for the biogas applications. The DFC was developed to provide green electricity and heat from methane in a distributed generation mode and uniquely qualifies for this application. FCE has pursued biogas applications for the DFC since the start of commercialization of DFC power plants in 2003 and has used the operational experience with these early plants to improve the design (gas supply reliability, understanding of the contaminants, and control). FCE practice on biogas, knowledge of the contaminants, and discussion of the system design based on the initial project experience was discussed in an earlier paper [1] and an update is provided in this article.
The direct carbonate fuel cells were initially developed for natural gas fuels as a robust natural gas distribution network exists in the markets targeted by FuelCell Energy. The biogas produced by anaerobic digestion in wastewater treatment, food processing industry, and decomposition of wastes in landfills contains predominantly methane and CO2. It can be used in a DFC designed for the natural gas to produce ultra-green electricity and usable heat. Also, the byproduct heat of the DFC using the biogas is a good match for heat required by the anaerobic digestion process that produces the gas. The high CO2 content in the biogas negatively impact the performance of the anodic reaction of all fuel cell technologies including the carbonate fuel cell. However, a unique feature of the DFC is that its performance loss at the anode due to fuel dilution is compensated by a performance gain at the cathode due to higher reactant (CO2) concentration at the cathode. In fact, the DFC open circuit potential in biogas systems is slightly higher (approximately 4 mV) than the natural gas system. The stack performances of several DFC plants operating on the biogas and the pipeline natural gas at different customer sites are compared in Figure 3. Although natural gas and biogas compositions are different at all sites, a slight biogas performance advantage over natural gas is clearly evident at each site. On average, the biogas plants operate at approximately 0.5% higher fuel cell conversion efficiency.
The digester gas output volume as well as the chemical fuel value of the gas can vary depending on the digestion system variables. Additionally, the digester gas is not available for power production during maintenance operation. To ensure constant power production with digester gas, FCE has developed a fuel-flexible DFC design, where the power plant automatically blends in natural gas to adjust for the digester gas shortfall. An example of this fuel-flexible operation is shown in Figure 4. Fuel flow switched automatically from digester gas to natural gas as the digester switched off-line and later the plant switched to digester gas from natural gas, as the digester was brought on stream.
The municipal and non-municipal anaerobic wastewater treatment plants (WWTPs) represent a significant source of biogas in the USA. The output gas from the WWTPs employing a sulfide control process contains
The impurity levels in ADG, even with sulfur control technology, are significantly higher than those in natural gas. The type and level of contaminants are dependent on the gas source. An auxiliary fuel cleanup system is used for cleaning the biogas before the introduction to the fuel cell. The design of the contaminant removal system requires a detailed knowledge of the contaminant species, their levels, and potential variation with time. Usually, a dedicated auxiliary biogas treatment system as illustrated in Figure 5 (the biogas-specific cleanup is shown with the natural gas power plant block flow diagram in dotted lines) is used to control the contaminant levels in biogas for use in a fuel cell. The contaminant treatment process is carried out in several steps. In the first step, most of the sulfides are removed by treating with iron oxide under a controlled environment. The controlling parameters for this process are space velocity of the gas, residual oxygen content, relative humidity, condensate pH, etc. The iron oxide bed is not effective in removing organic sulfides. After moisture conditioning, a cleanup bed, usually an activated carbon bed, is employed for siloxanes escaping from the iron oxide treatment [6]. Finally, trace organic sulfur compounds present in the biogas leaving the dehumidifier is removed using a sorbent for high-temperature fuel cell use. Applications where organic sulfur content is low, and only one single bed is available, are employed to combine siloxanes and organic sulfur removal functions. A De-Ox catalyst bed incorporated in front of the pre-reforming catalyst bed is used to remove residual oxygen in the biogas.
Special attention is also required for performance monitoring of the cleanup system to ensure reliability of the gas cleanup system. The operating cost of the sulfur polishing system can be high due to frequent monitoring requirements and low sulfur intake capacity of the commercial sulfur polishing agents. FCE has developed two separate equipment solutions for inexpensive online sulfur monitoring and breakthrough detection. Both of these equipment solutions are currently under evaluation with DFC power plants operating on biogas.
FCE has placed over 25 biogas units ranging from 250 kW to 2.8 MW around the world, achieving an electricity conversion efficiency of 45% to 49% (LHV) without accounting for power consumption by the biogas auxiliary cleanup process. A vast majority of the plants are operating on biogas produced by the wastewater treatment plants; a few plants have operated on biogas produced during beer production process. Two sub-MW plants at Oxnard, CA, are operating on biogas produced by anaerobic digestion of onion juice. The Gills Onions Oxnard plant has won several environmental and economic leadership awards (go to www.gillsonions.com/validation; it provides details of awards and recognitions received). Although biogas from onion juice does not contain siloxanes, it does have very high level of sulfur compounds with total sulfur at about 10,000 ppm or approximately 1% by volume in the biogas. It is challenging to completely digest such high levels of sulfur compounds to H2S with a limited residence time in the digester. As a result, there is a considerable amount of organic sulfur, mainly propanyl mercaptan, in the raw biogas from the digester. As iron oxide media has almost no capacity for adsorption of these two organic sulfurs, multistages of organic sulfur removal beds are used with lead/lag option to get the maximum efficacy of the media. FCE and customers have been working together diligently and very effective, and an efficient sulfur removal has been obtained for the last several years. Two plants in California, USA, are operating on directed biogas which has similar gas composition as the natural gas.
Potential issues encountered for biogas applications primarily relate to the steadiness of fuel gas supply (gas supply and composition variations). The volume of flow would occasionally drop below the level needed for full-load operation. When this occurred, the fuel pressure would become too low and the unit would trip off-line. Also, the fuel content in the gas can vary diurnally as well as seasonally. FCE experience with early power plants has identified another important point relating to the digester gas availability. In real-world applications, digester plant operators do not consider maintaining a steady supply of ADG to be of high priority. Furthermore, it is a waste stream, which has little impact on their day-to-day operations. When the ADG supply is interrupted, which sometimes can be caused by maintenance activities or changes in sewage waste composition entering the plant, the fuel cell power plant needs to be able to respond.
Smooth operation of biogas pretreatment for fuel cells (for removal of sulfur compounds and siloxanes discussed previously) is an important component of the system for reliable operation. The operation of the early units was affected by the reliability of the pretreatment skids, which are supplied by the end user or a distributor. This has improved over time, as lessons learned from early units were incorporated into the design and maintenance of the newer systems. 2b1af7f3a8