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 will dictate whether a particular pyrolysis setup will be effective and profitable. The higher the oil yield from waste materials, the higher the efficiency and profitability of the process. The higher the oil yield from waste materials, the lower the production cost for each unit of oil.
On the other hand, the flue gas is the result of the combustion of non-condensable gases and exhaust gases after heating. Flue gas consists of many types of pollutants such as particulate matter, acid gases, and volatile organic compounds. Pollutants in flue gases pose a problem since they have an adverse effect on people and the environment.
Oil yield and flue gas are important aspects in a pyrolysis setup. As much as high oil yield is essential in the process, it will result in more problems where the flue gas poses difficulties. The following sections of this paper explore major factors affecting oil yield and purification of flue gases.
The oil yields differ based on factors such as feedstock used, the design of the reactor, the temperature of the process, the heating rate of the waste, the time the waste spends in the reactor, and the seal of the reactor. These variables affect the oil yield, and a slight change in one of them can make a significant difference, up to 50% or higher.
Feedstock used. The nature of the waste used determines the amount of oil produced. For example, plastic wastes such as polyethylene and polypropylene produce a higher quantity of oil while rubber waste, mixed waste, and biomass produce lesser amounts of oil. Also, moisture content in the waste decreases the amount of oil produced. Wet waste yields less oil than dry waste.
Reactor type. Rotating kiln, fixed bed, fluidized bed, and screw reactors all produce different yields. Some have higher consistency than others, and some produce a higher amount of char compared to others.
Temperature and Heating Rate. Temperature is the key parameter. Pyrolysis at low temperatures (350–450 ºС) normally results in heavy oils and char production. Pyrolysis at elevated temperatures (450–600 ºС) may lead to greater production of light oil and gases. Fast heating rate may facilitate liquid products generation due to the absence of secondary cracking. The slow heating rate may result in a greater quantity of char. It is up to your material which temperature would suit.
Residence Time. Residence time means the period of time the vapors and char stay in the hot zone. Short vapor residence time helps avoid secondary cracking, i.e., cracking of the vapor and resulting its conversion into gases. It helps increase the quantity of liquids produced.
Tightness of the System and Handling of Vapors. It will prevent air from getting into the pyrolysis chamber and ensure an inert atmosphere inside. Moreover, it will guarantee that all vapors go to the condenser.
Catalyst and additive. The effect of catalysts on reactions and production of oil is possible. The presence of catalysts will increase the share of gases and light oils if there is an acid reaction. Some types of catalysts can enhance oil qualities and decrease heavy fractions. They are quite expensive.
Feedstock preparation. It is necessary to shred, dry, eliminate metallic parts and stones from feedstocks in order to increase their quality and consequently improve oil yield and lower maintenance expenses.
There are variations in yields due to differences in feedstock types and processing methods. The values mentioned below are indicative in nature and the actual yields will be determined based on the parameters discussed in the previous section.
• Waste Plastics (PE, PP, PS) – 70-90% Oil by weight of dry material. Certain plastics such as PVC provide low oil yields and generate corrosive gases. Yield from mixed plastics is also lower compared to those derived from single plastics.
• Tyres & Rubbers – 35-45% oil, 30-40% carbon black/char and the remainder is gaseous. Tires contain sulfur and zinc derivatives which affect oil quality.
• Biomass (Wood, Agricultural Waste) – 20-40% bio-oil. High moisture and oxygen content makes bio-oil unstable and requires upgrading before using as fuel.
• Mixed Municipal Solid Waste – Highly variable yields. Range of oil yields would be 20-50%, depending on plastic content and moisture levels.
• Asphalt Roofs – 30-50% oil depending on content of bitumen and filler.
The data above suggests that waste plastics represent the best feedstock for oil generation. Mixing or contamination would require careful processing and lower yields.
Yield improvement is mostly a matter of input control process. Here are some measures that should be followed by the operators.
Use consistent input feed stock. In case of possibility, use one stream of materials. Get rid of all metals, stones, and inert fillers. Dry your feed stock to reduce water content.
Temperatures should be controlled. Use good quality thermocouples. Prevent temperature spikes. Choose proper temperatures depending on the analysis of your feed stock. In case of mixed plastics, temperatures from 400 to 500 °C will work fine. In case of tire, better results can be achieved using temperatures from 450 to 550 °C.
Change heating rate and vapor residence times. Higher heating rate and low residence time will result in higher yields of liquids. Make sure to optimize vapor paths for fast movement of vapor toward the condenser. There shouldn’t be any areas of high temperature because gases can break into lighter ones.
Improve condensation of gases. Use multistage condensation. Quick cooling of gas reduces second breaking.
Prevent air leakage. Conduct regular checks on seals and gaskets. Apply positive-pressure purging using nitrogen gas when necessary.
Use mild catalysts or absorbents only if you have evidence from the laboratory that supports their use. This should be conducted on a pilot scale. Note that the use of catalysts may cause poisoning of other units and make the handling process expensive.
Keep records. Document changes in feed properties, temperature distribution, processing speeds, and product yield. Changes could be indicative of fouling.
Conduct maintenance. Clean condensers, cyclones, and pipes regularly since fouling limits condensation and oil formation processes.
Flue gas is the gaseous effluent generated from the burning process and non-condensable gases formed by the combustion process. Flue gas composition will depend on the feedstock used and the burning conditions. The composition of the flue gas includes the following gases: CO₂, CO, N₂, O₂, H₂S, SO₂, NOₓ, VOC, PM, and trace metals or acid vapors.
Health and environment impacts. Some of the flue gases are harmful to health and the environment. SO₂ and NOₓ cause acid rain and respiratory effects. H₂S is very toxic. PM affects lung function. VOC causes smog formation and other health impacts.
Equipment impacts. The acidic gases and particulate matter could corrode equipment, damage filters, and cause deterioration of heat exchanger systems. Flue gas untreated will cause corrosion in equipment and clogging of the chimney.
Compliance with standards. Emission standards exist in almost all areas of the world. One must comply with the emission standards to operate within the legal requirements.
Safety. There are some components in flue gas that are either flammable or explosive at certain concentrations.
A comprehensive purification system will always consist of various types of purification technologies. In choosing which purification technology to implement, factors such as the nature of the fuel feed gas, emission standards, costs, and space availability will come into play. Here are some of the purification techniques that are widely used for gasification processes:
Cyclones/Demisters. These are among the initial defenses against contaminants in the gas. Cyclones employ the principle of centrifugation in order to separate particulates. Demisters separate oily droplets that can clog other pieces of downstream equipment. Cyclone/Demisters are easy to implement and use.
Quench/Condensation. This purification technique quickly lowers the temperature of the gas to minimize concentration. Quench Towers/Heat Exchangers remove oils and tars via condensation. Condensers are installed right after the reactor to remove oils and tars and prevent downstream wet scrubbers from being clogged.
Wet scrubbers. Acidic gases such as HCl, ammonia, H2S, and soluble VOCs are removed through wet scrubbers. This method works by bringing the gas in contact with a liquid phase. A packed bed and spray towers are used in this process to enhance gas-liquid interactions. Wet scrubbers can also be employed to eliminate fine particulate matter.
Injection of dry sorbent and bag filters. Dry sorbent injection involves the injection of alkaline substances like sodium bicarbonate and lime in the gas flow path to neutralize acids in it. The particulates are collected in a bag filter which acts as the baghouse downstream of the injection. The bag filters can be used for elimination of very fine particulates as well as solid products.
Activated carbon adsorption. Adsorption is performed using activated carbon to remove VOCs, dioxins, and odor compounds from exhaust gas. Activated carbon adsorbs the pollutants after the particulate control step is complete. Over time, the carbon gets saturated and needs to be either replaced or reactivated at high temperatures.
Thermal oxidation. In case of high concentrations of volatile and flammable VOCs or tar in exhaust gases, a thermal oxidizer (afterburner) is applied for oxidation of organics in high temperatures into carbon dioxide and water, thus reducing odor and VOC content. At the same time, a fuel source is needed for thermal oxidizer, and proper control of temperature and dwell time is required for NOₓ reduction.
Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). Both processes may be applied for NOₓ control. SNCR process consists of injection of ammonia or urea for NOₓ reduction. SCR process employs catalytic reaction for NOₓ reduction. On the other hand, it needs more pure gas and higher costs for construction.
Condensate and Wastewater Treatment. The wet scrubbing and quenching processes produce liquid effluent. Liquid effluent contains dissolved organics, acid, and particulates. Separation of oil and water, neutralization, and biological or chemical treatment is required for effluent treatment.
Stack and Final Polishing. After the cleaning processes, the gas may be run through a final polishing filter or bed, which may serve to polish the gas stream of any left-over impurities or odors. This is now prepared for discharge through the stack. The size of the stack will normally be defined according to local regulations.
Equipment design principles and sequence. Equipment train for pyrolysis flue gas typically includes the following units, in this particular order:
Cyclone/demister –> condenser/quench –> scrubbing or dry injection process –> baghouse –> activated carbon –> thermal oxidizer (if necessary) –> stack
The specific train depends on what is being done about certain pollutants. Design equipment so as not to damage sensitive units. A cyclone or similar unit should come before an activated carbon unit in order to avoid plugging up the bed too quickly.
Materials. Acid gases are frequently found in flue gases, requiring corrosion resistant materials. Some materials that are often used in scrubbers include stainless steel, lined steel and FRP (fiber reinforced plastics).
Bypass/Isolation and Redundancies. Bypass and isolation valves are essential in order to facilitate maintenance. Having redundant fans and pumps can save on downtime during maintenance.
Process: Bring process to steady state. Any changes to feedstock or temperatures may require time to establish stability. These changes need to be performed gradually and their effects should be observed.
Monitoring: Very important. Temperatures, pressures, and flows need to be monitored where necessary. Gas analysis instruments should be employed for monitoring concentrations of CO, oxygen, nitrogen oxides, sulfur oxides, hydrogen sulfide, and VOCs as required. Monitors for opacity or particulates should be used for monitoring concentrations of soot and dust.
Maintenance: Regular cleaning of condensers and heat exchangers. Any tar build-up should be cleaned before it becomes too solid to do so easily. Filters and bags should be changed based on schedule and pressure drops. Blockages on scrubber pumps and nozzles should be observed. Leaks and corrosion in ducting and seals should be monitored. Sorbent and activated carbon used as consumables should be replaced when below required level.
Disposal: Discard used sorbents, filter cake, and scrubber sludge in line with the law. Ensure proper storage and labeling for easy transportation.
Health and safety: Your employees should be trained on how to deal with emissions control chemicals. They need training in what to do when an alarm is triggered. Protect any individual who handles condensate, sludge, or dust from harm by providing protective equipment (PPE). You should have an MSDS for each of the chemical materials used.
Calibration: Regularly calibrate the gas analyzers and flow meters. Carry out periodic stack testing in coordination with an accredited organization. The information you gather will help you to optimize the purification train.
Documentation: Document everything from different batches of feedstock used to the process parameters, emission measurements, and waste disposal. You’ll have an easy time renewing the permit or during inspection. It could also enable you to recognize trends that may point to issues.
Contingency planning: Have measures in place for managing events such as increased levels of VOCs, water shortage for wet scrubbers, and loss of power.
Match Purification Needs with Feedstock
When you have a lot of plastic material, it means you have a lot of VOCs and tars. Hence, you should be more inclined towards condensers and carbon beds. With tire material, you have sulfur compounds and would hence like materials that will cope with the acidic gases.
Manage Between Capital Costs and Operating Costs
The highly efficient systems will require a high initial capital cost but will ensure you are saved from penalties, downtime, and maintenance. The less complicated systems will reduce capital costs but may result in increased operating costs.
Take into Consideration Feedstock Variability
In case your feedstocks are mixed, their composition will vary from time to time. It makes sense therefore to go for a system capable of handling different compositions of the feedstock. Controls that are flexible are a very useful consideration here. You also need sensors for rapid detection of emission changes.
Run Pilot Tests
Before scaling any system or material, it pays to conduct a pilot run.
Talk to Your Local Regulators
It always helps to talk to the regulators when making your decisions. Their ideas of an acceptable solution will vary depending on your location.
The relationship between the oil yield and purification of the flue gas needs mentioning. The preparation of the feed, the heat treatment, and management of vapor contribute to obtaining larger quantities of oil. The purification process takes place in several stages, eliminating the presence of particulates, acids, VOCs, and any odorous components. Monitoring, maintenance, and documentation guarantee that everything meets the requirements.
Consistent process control and maintenance result in the best outcomes. Measuring the effects and taking measures in relation to these effects will help maximize oil yields and the gas purity. Any changes in the properties of the feedstock or the conditions should require testing and adjusting the purification process.
In addition, we can prepare a checklist of the activities to carry out daily, weekly, and monthly or the gas train diagram depending on your feedstock separately.
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