The Oil Yield And Flue Gas Purification Of Pyrolysis Machines
1. Introduction — why oil yield and flue gas matter
2. Key factors that affect oil yield
3. Typical oil yields by common feedstocks
4. How to optimize oil yield in operation
5. What is in flue gas and why purification is necessary
6. Flue gas purification methods and system design
7. Operation, monitoring, maintenance, and record keeping methods and system design
8. Practical tips and trade-offs
9. Conclusion — steady process, steady product, cleaner air
The oil yield is a factor that determines if a pyrolysis system is efficient and profitable. The more oil that is obtained per unit of waste material means that a system is more efficient and profitable. The more oil that is obtained per unit of waste material, the lower the cost of production per unit of oil.
Flue gas is a product of the combustion of non-condensable gases and exhaust gases during the process of heating. The flue gas contains various pollutants such as particulate matter, acidic gases, and volatile organic compounds. The pollutants in flue gas are a problem if they are not handled properly, as they can have negative effects on human beings and the environment.
The oil yield and flue gas have to be considered in a pyrolysis system. The more oil that is obtained, the more problems are likely to occur if flue gas is a problem. The rest of this article will discuss some of the major factors that affect oil yield and how flue gas is purified.
Oil yield varies depending on the feedstock used, the reactor design, the temperature used, the rate at which the waste is heated, the residence time of the waste in the reactor, and the reactor seal. All these parameters affect the yield of the oil. A change in any of the parameters can affect the yield by as much as 50% or more.
Feedstock used. The type of waste used affects the yield of the oil. Plastics like polyethylene and polypropylene produce a lot of oil. Rubber waste, mixed waste, and biomass produce less oil. Moisture content in the waste reduces the yield of the oil. Dry waste produces more oil than wet waste.
Type of reactor used. Rotary kiln, fixed bed, fluidized bed, and screw reactor produce different amounts of oil. Some produce more uniform results than others. Some produce more char than others.
Temperature and heating rate. Temperature is the primary control. Low-temperature pyrolysis (350-450 °C) generally favors the production of heavier oil and char. Higher temperatures (450-600 °C) may favor gas production and the formation of lighter oil. Rapid heating rates may favor higher liquid production for some feedstocks, since secondary cracking is minimized. Slow heating rates may favor char formation. The best temperature depends on the material you want to pyrolyze.
Residence time. Residence time refers to the duration the vapors and char spend in the hot zone. Shorter vapor residence time minimizes secondary cracking, which converts the vapor into gas. This maximizes liquid production. Longer vapor residence time may crack the vapor into permanent gases, which minimizes liquid production.
System leak tightness and vapor handling. This will help to ensure that the air does not get into the pyrolysis system and to maintain the inert environment. Also, it will help to ensure that the vapors get to the condensers without being lost. This will improve the yield and safety of the plant.
Catalysts and additives. The catalyst may affect the chemical reactions and the yield of the oil. The addition of catalysts may increase the gas and light oil fractions if they are acidic. Some catalysts may improve the quality of the oil and reduce the heavy fractions. However, they are expensive and must be handled with care.
Feedstock preparation. Shredding and drying the material and removing metal and stone will improve the quality of the material. This will improve the yield of oil and reduce maintenance costs.
Yields differ based on the type of feedstock and the process used. The approximate values given below are to give a rough idea about what to expect. The actual yield will depend on the factors mentioned in the previous section.
Waste plastics (PE, PP, PS): 70-90% oil by weight of dry material. Some plastics like PVC yield less oil and more corrosive gases. Mixed plastics will give lower yields than single plastics.
Tires and rubber: 35-45% oil, 30-40% carbon black/char, and the rest is gas. Tires contain sulfur and zinc compounds, which impact the oil quality.
Biomass (wood, agricultural waste): 20-40% bio-oil, with high water content and oxygen. Bio-oil is unstable and must be upgraded to be used as fuel.
Mixed municipal solid waste: Varies greatly. Oil yield will be between 20-50%, depending on the plastic content and moisture levels.
Asphalt roofing shingles: 30-50% oil, depending on the bitumen content and filler material.
The above values indicate that plastics are the most suitable feedstock for oil production. Mixed or contaminated materials must be processed with care and may give lower yields.
Yield enhancement is primarily an input process control issue. Here are some steps that can be followed by the operators.
Begin with uniform feedstock. If possible, use a single material stream. Remove all metals, stones, and inert filler. Dry your feedstock to decrease water content.
Temperature control must be precise. Employ good-quality thermocouples and a control loop. Prevent temperature surges. Optimize temperature settings based on your feedstock analysis. For mixed plastics, temperatures of 400-500 °C are generally optimal. For tires, 450-550 °C is more effective.
Vary heating rate and vapor residence time. To maximize liquid yield, a higher heating rate and shorter vapor residence time are required. Optimize the vapor path to ensure rapid gas transfer to the condenser. Eliminate hot spots where vapors can crack to form lighter gases.
Enhance vapor condensation. Employ multi-stage condensers to separate various fractions. Cool the gas quickly to minimize secondary cracking. Keep the condenser surfaces clean and at proper temperatures.
Prevent air leaks. Check seals and gaskets periodically. Apply positive pressure nitrogen purging as required during maintenance.
Evaluate mild catalysts or sorbents only if you have laboratory results to support their use. Pilot-scale testing is required. Remember that catalysts can poison other processing units and increase handling costs.
Record results. Record changes in feed composition, temperature patterns, processing rates, and product output. Small changes may signal fouling or feedstock variations.
Schedule maintenance. Clean condensers, cyclones, and lines periodically. Fouling decreases the efficiency of condensation and oil production.
Flue gas is the gas stream created by the burning process and the non-condensable gases released from the system. Flue gas composition varies depending on the type of feedstock used and the burning conditions. Flue gas may be composed of the following: carbon dioxide (CO₂), carbon monoxide (CO), nitrogen (N₂), oxygen (O₂), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), nitrogen oxides (NOₓ), volatile organic compounds (VOC), particulate matter (PM), and trace metals or acid vapor.
Health and environmental risks. Flue gas components can be hazardous to health and the environment. SO₂ and NOₓ can cause acid rain and respiratory problems. H₂S is highly toxic. Particulate matter is hazardous to the lungs. VOC can cause the formation of smog and health risks.
Equipment risks. Acidic components and particulate matter can corrode equipment, damage filters, and degrade heat transfer equipment. Flue gas, if not treated, can cause equipment failure and block the chimney.
Compliance with regulations. Emission standards are in place in most areas of the world. To be within regulatory limits, one must be in compliance with local air quality standards to operate legally.
Safety. Certain components of the flue gas are flammable or explosive at given concentrations.
A complete purification system will often be a combination of different purification technologies. The selection of purification technology will depend on the type of feedstock gas, the desired emissions quality, cost constraints, and available space. Some of the purification technologies commonly used in gasification plants include the following:
Cyclone separators/demisters. Cyclone separators/demisters are the first line of defense against impurities in the gas. Cyclone separators utilize centrifugal separation to remove particulate matter. Demisters remove oily droplets that would otherwise plug downstream equipment. Cyclone separators/demisters are simple to install and operate.
Quench/condensation. Quench/condensation rapidly cools the gas stream to reduce concentration levels. Quench towers/heat exchangers remove oils and tars by condensing them. Condensers are used after the main reactor to remove oils and tars, thus protecting downstream wet scrubbers from plugging.
Wet scrubbers. Wet scrubbers remove acidic gases such as HCl, ammonia, H2S, and soluble VOCs. Wet scrubbers work by bringing the gas in contact with a liquid. A packed bed or spray tower is often used to increase contact between the gas and liquid phases. Wet scrubbers can also be used to remove fine particulate matter.
Dry sorbent injection and bag filters. Dry processes inject alkaline sorbents, such as sodium bicarbonate or lime, into the gas stream to neutralize acids. A baghouse downstream of the injection collects the treated particles. Bag filters remove very small particulates and solid reaction products. They are often used when water discharge is limited.
Activated carbon adsorption. Activated carbon removes VOCs, dioxins, and odors. It is used following particulate control. Eventually, the carbon becomes saturated and must be replaced or reactivated by high-temperature treatment. In dioxin or furan destruction, high-quality carbon and effective upstream particulate control are essential.
Thermal oxidation. For high concentrations of combustible VOCs or tars, a thermal oxidizer (afterburner) is used to high-temperature oxidize organics to CO₂ and H₂O. It reduces odor and VOC mass. However, fuel is required for a thermal oxidizer, and temperature and residence time must be carefully controlled to avoid NOₓ formation.
Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). Both methods can be used for controlling NOₓ emissions. SNCR uses the injection of urea or ammonia to reduce NOₓ emissions. SCR uses a catalyst for the efficient reduction of NOₓ emissions. However, it requires cleaner gas and higher capital costs.
Condensate and Wastewater Treatment. Wet scrubbing and quenching result in the production of liquid effluent. The liquid effluent contains dissolved organics, acid, and particulates. Oil and water separation, pH neutralization, and biological or chemical treatment must be used in the treatment of the effluent before it is discharged into the environment.
Stack and Final Polishing. After the main cleaning steps, the gas can be passed through a final polishing filter or bed, which can be used to polish the gas and remove any remaining impurities and odors. The gas is now ready for the stack and can be discharged. The height of the stack is usually determined by local regulations.
Design principles and order. The normal train of equipment for pyrolysis flue gas is as follows:
Cyclone/demister –> condenser/quench –> wet scrubber or dry sorbent –> baghouse –> activated carbon –> thermal oxidizer (optional) –> stack
The exact train will depend on the exact pollutants and control goals. Place equipment in a position that will prevent damage to sensitive equipment. For example, it is a good idea to have a cyclone or other particle control device ahead of activated carbon beds in order to prevent rapid bed plugging.
Materials selection. Acid gases are commonly present in flue gas and require corrosion-resistant materials. Commonly used materials in scrubbers include stainless steel, lined steel, and FRP (fiber reinforced plastic).
Redundancy and bypass. Bypass and isolation valves are required for maintenance. Redundant fans and pumps are also useful in order to minimize downtime.
Operation: Run to steady state. Changes to feedstock or temperature take time to stabilize. Changes should be made slowly and outcomes measured.
Monitoring: This is crucial. Temperature, pressure, and flow should be monitored at critical points. Gas analyzers should be used to monitor CO, O2, NOx, SO2, H2S, and VOCs if required by regulation. Opacity monitors or particulate monitors should be used to monitor soot and dust levels.
Maintenance: Condensers and heat exchangers should be cleaned regularly. Tar buildup should be removed before it hardens. Filters and bags should be changed according to schedule and pressure drop. Scrubber pumps and nozzles should be checked for blockages. Ducting and seals should be checked for leakage and corrosion. Consumable sorbent and activated carbon should be changed before they degrade to below specified levels.
Waste handling: Dispose of spent sorbents, filter cake, and scrubber sludge according to law. Store them properly and label them for safe transportation.
Health and safety: Train your staff on how to handle emissions control chemicals. Train them on how to react in case of alarm conditions. Provide PPE for those who come in contact with condensate, sludge, or dust. Ensure that there is a material safety data sheet (MSDS) for all the chemicals used.
Performance testing and calibration: Calibrate gas analyzers and flow meters frequently. Perform periodic stack tests with accredited organizations to ensure compliance with regulations. Use the results to optimize the purification train.
Record keeping: Keep records of different batches of feedstocks used, process conditions, emissions monitoring results, maintenance activities, and waste disposal. This will make it easier to renew permits and undergo inspections. It can also be used to find trends that may be indicative of potential problems.
Contingency planning: Develop procedures for dealing with unexpected situations such as a rise in VOCs, a failure in the water supply for a wet scrubber system, or a loss of electrical power.
Match Purification with Feedstock
If you are working with a lot of plastics, you will have more VOCs and tars. Therefore, you will want to focus more on condensers and carbon beds. If you are working with tires, you will have sulfur compounds and will want materials that will handle acidic gases well.
Balance Capital and Operating Costs
High-efficiency systems will cost more in capital investment but will save you in penalties, lost operating time, and maintenance. Less complex systems will save you in capital investment but could cost you more in operating expenses or penalties.
Plan for Variability
Mixed feedstocks will change over time. Therefore, you will want a system that will handle a wide range of conditions. It is a good idea to include controls that can adapt easily. Sensors will help you detect changes in emissions quickly.
Test on a Pilot Scale
Before you scale up a new feedstock or a new purification system, you will want to run a test on a small scale. This will help you make better decisions about a full-scale system.
Communicate with Regulators
It is a good idea to discuss your plans with local regulators. What is considered an acceptable solution will depend on where you are.
Oil yield and purification of the flue gas are related. Preparation of the feed, heating, and handling the vapor ensure higher oil yield. Purification is done in a multi-step process, which removes particulates, acids, VOCs, and odors. Monitoring, maintenance, and record-keeping ensure the system is always in compliance and in good working order.
Process control and maintenance, if done in a consistent manner, ensure the best results. Measured results and response to the results ensure more oil yield and better gas purity. If there is a change in the feedstock or the conditions, test and adjust the purification process accordingly.
A checklist for daily, weekly, and monthly activities, or a gas train diagram based on your feedstock, can be provided separately.
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