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The rapidly increasing population and urbanization growth has resulted in higher demands on finite resources such as land space, water, food and energy. It has also intensified environmental challenges, which include pollution and waste management issues. These issues are quite detrimental to the global goal of sustainable development and hence, have ignited global interest in sustainable strategies for energy utilization, production and waste management.

  • Oil sludge:

Objectives, needs and necessities of industrial waste design is one of the hazardous resources of the environment and at the same time can be turned into valuable resources in industrial development. In addition to its effective role in the economic cycle, waste management in developing countries is considered as one of the important indicators of development at the macro level. In the oil, gas and petrochemical industries, due to the consumables, products and technology used, significant amounts of hydrocarbon waste are produced, the reduction or control of which is based on environmental standards is of great importance. Therefore, the attitude of macro management to how to establish a hydrocarbon waste management system can prevent serious problems due to improper disposal of these materials. The proposed plan in the field of management and recycling of industrial hydrocarbon waste is the use of distillation and pyrolysis methods. With these characteristics, the most important needs and objectives of the plan can be summarized as follows:

- Necessity and importance of the subject:

creating jobs

Cleaning contaminated areas

Conservation of biological resources

Enforce waste laws and regulations

Health promotion

- Objectives of the Center for Management and Disposal of Special Industrial Waste:

Organizing refinery and petrochemical wastes

Reducing environmental and health risks

Reduce the environmental costs of industrial waste

Concentration and optimal waste management

Comprehensive and reliable monitoring

Creating an appropriate statistical system and focusing on information and statistics

Reduce accidents and risk

Creating added value (energy, materials and financial resources)

- Spatial and technical option of the plan:

Due to the nature of the industrial waste management plan and the management of the private sector on it, currently only one spatial option has been proposed for the plan and these studies are based on the environmental and socio-economic conditions of the spatial option. The desired location option for the implementation of the study plan is located in Khuzestan province. In general, barometers such as acceptable distance from population centers, access to transportation hubs, distance from the sources of materials are used to select this location.

- Strategies for energy use, production and waste management:

A direct and overlooked consequence of the increase in waste worldwide is the increase in the volume of municipal wastewater, especially sewage sludge.

In facility water treatment, the initial treatment of raw wastewater such as the initial strain of the received sludge is done to remove large particles such as sand, sand and rock. Subsequently, the wastewater is deposited in sedimentation tanks using gravity, which allows the formation and removal of slurry sludge in the bottom of these reservoirs.

This marks the point of production of the primary sludge. The solid phase in sludge consists of a homogeneous combination of proteins, carbohydrates, oils, minerals and microorganisms. This mixture of organic, inorganic and living organisms leads to the formation of an unstable, volatile and rotten substance with toxic elements. Subsequent treatments are mainly biological (composting or digestion), physical (eg pressure, heat, vibration, microwave) or chemical (oxidation, alkaline regulation) methods with the aim of stabilizing organic matter (destroying pathogens, removing odors and Pollution reduction).

  • Sludge-to-Energy Recovery Methods:

Anaerobic digestion is a biological conversion method which is widely used due to its low cost and ability to utilize organic waste with high moisture content without reducing the high calorific value of the produced biogas (combination of methane and carbon dioxide). The biogas obtained from the digester can be cleaned and further upgraded to produce bio-methane which can be a direct substitute for natural gas or the biogas can be converted to heat and electricity via cogeneration using thermal reactors.

Incineration is another prominent process currently used to manage sewage sludge, but the traditional method was not to recover energy, but to reduce the volume of waste and destroy harmful elements.

In contrast to combustion, pyrolysis takes place in completely inert atmosphere (devoid of oxygen) at moderate to high temperature (300–900C) to produce pyrolytic oil, biochar and non-condensable gases (CO, H2, CO2, CH4 and light hydrocarbons)

Biochar, incompressible gases and bio-oils can be used from solid fuels, gas and liquid for electricity and energy. Alternatively, biochar can be used in adsorption or catalyst applications. Finally, gasification involves the thermochemical conversion of organic compounds through partial oxidation (less oxidative than stoichiometric requirements) at high temperatures (650 to 1000 ° C) to maximize gaseous products (CO, H2, CO2, and light hydrocarbons); In particular, the gas is synthesized by CO.

  • Pre-Processing of Sludge:

The constituents of sludge are made of blends of organic matters such as carbohydrates, proteins, fats and oils, a range of microorganisms (both living and dead), and inorganic elements which are characterized by high energy content. Nevertheless, the properties of sewage sludge are highly variable and dependent on its origin, wastewater treatment system, environmental requirements, seasonal variations and production processes such that simple processing such as drying can easily improve its organic contents and calorific value significantly. This makes the variability in sludge’s chemical composition more extreme in comparison with traditional biomass and coal samples.

The proximate analyses of sludge are such that the volatile matter of biomass is higher while coal has lower volatile content in comparison to sludge. Also, the fixed carbon of coal and biomass is higher than that of sludge as established by past works. Nonetheless, the ash content (mostly aluminium, calcium, iron, magnesium, sodium, phosphorus, silicon and titanium) of sludge is higher than that of biomass and coal due to its high inorganic content [7]. Similarly, the ultimate analyses of sludge reveal higher nitrogen (from protein and peptides), higher hydrogen and comparable carbon contents to lignite and biomass. The sulphur and oxygen content remain higher than biomass but comparable to that of lignite. Wet sludge has approximately 98 wt% moisture content and after mechanical dewatering processes, free water and some of the interstitial water can be removed, leaving about 73–84% of the water content. Irrespective of this dewatering process, the remnant moisture (mostly vicinal water) might require the application of thermal energy for rapid drying. The use of heat can reduce the moisture to very small content ~5.6% which is mostly chemically bonded water from inorganics such as calcium or aluminium hydroxides.

Anaerobic digestion:

Anaerobic digestion is a biological process that occurs in an inert environment for the conversion of organic compounds into biogas by the use of microorganisms. The use of naturally occurring bacteria for biodegradation involves a series of biochemical stages including hydrolysis, acidogenesis (fermentation), acetogenesis and methanogenesis. These metabolic stages are used for mass and volume reduction of the sludge while the organic contents are converted to biogas by the pathogens. The hydrolysis stage involves the conversion of the non-toxic organics into simple sugars, fatty acid and amino acids. Afterward, the acidogenesis and acetogenesis stage aids the fermentation of the hydrolysis products into acetate, carbon dioxide and hydrogen gas which are further converted to methane through methanogenesis.

High-methane biogas can be recovered for heat and power generation using boilers, turbines and generators, or alternately upgraded for use as biomethane.

The remnant after this digestion process has high nutritional contents (phosphorus, potassium and nitrogen) which can be used as compost and/or fertilizers for agricultural and soil reclamation purposes if it conforms with environmental standards. In practice, a unit can consist of multiple digesters forming multi-stage systems to enhance gas recovery with various treatment methods used at each stage.

The combination of different energy technologies such as anaerobic digestion followed with pyrolysis has been presented by Cao and Pawlowski for maximization of energy efficiency in comparison to the use of the technologies independently. The use of anaerobic digestion is particularly attractive as it fulfils most of the requirements in European Waste reuse and recovery hierarchy and its use in combination with food waste has been proposed by Morales-Polo et al. due to the added benefit of nutrient enrichment, increase in alkalinity, reduced ammonia and enhanced stability of the process. Furthermore, ~20–40% increase in production of bio-methane was obtained from the co-digesting of mixed sludge and organic food waste by boosting the decomposition of acetate during methanogenesis

Combustion:

Combustion of all solid fuels is similar to the combustion of sewage sludge. This includes the oxidation of fuel at high temperatures to obtain heat, carbon dioxide, water vapor, and other trace gases. However, the use of combustion technology for waste such as sludge can be used mainly to generate heat (normal combustion) or to reduce the volume of waste (combustion). Conventional use of heat generated by combustion technology is for heating or generating electricity through heat engines, while incineration systems may or may not use the heat generated by combustion because their main purpose is to burn harmful elements from waste before final disposal. Or again.

Pyrolysis:

Pyrolysis is the thermal decomposition or degradation of fuel without any oxidizing agent in an inert (non-reactive) medium. It is used to produce bio-oil, solid coal and gaseous fuels and is referred to as incomplete gasification. This includes converting airless sewage sludge to medium operating temperatures (350-600 ° C), although there are some pyrolysis reactors operating at temperatures as high as 900 ° C. The output of this process depends on the process temperature, in which the coal efficiency decreases with increasing temperature.

Pyrolysis of sludge takes place in an inert environment at high temperatures, so an external heat source (electric or thermal) is needed to supply heat to initiate the reaction. The use of heat from partial combustion of biogas or bio-oil resulting from the process itself has been seriously investigated to ensure the self-stability of thermal decomposition, especially in energy dissipation applications.

Due to the heterogeneous nature of waste, decomposition of sewage sludge occurs in different stages, so that after drying at 200 ° C, partial decomposition of organic matter decomposes. Dead organisms and lipids are followed at a temperature of 200 to 300 ° C. The proteins, organic polymers and cellulosic compounds are then decomposed at 700 ° C. These reaction steps occur simultaneously in reactors (mostly fluidized bed) due to the high heating rate and primarily produce heavy fibers, light gases and particles at temperatures below 600 ° C.

Condensation of gaseous vapors from the reactor to bio-oil with a calorific value above ~ 33 MJ / kg occurs after cooling. Pyrolytic products of bio-oil, biochar and incompressible gases can all be used downstream, which is why pyrolysis attracts attention as a waste-free energy recovery process. Biofuels can be used as combustion fuel to generate heat or electricity, to be used as a liquid fuel, or to produce synthetic gas for the production of chemicals. Similarly, biogas can be burned as fuel or converted to synthetic gas, which can be processed for liquid fuel or chemical synthesis. Biochar, on the other hand, has a variety of applications of direct combustion as a solid fuel, absorption in catalytic applications and agricultural applications.

Gasification:

Thermochemical conversion of the organic content of wastewater sludge to high value gases such as H2 and CO known as synthetic gas, as well as CO2, CH4, H2O and other hydrocarbons is the main basis for gasification. This reaction occurs in a semi-oxidized reaction atmosphere at high temperature (800-1000 ° C). Gasification can be done using air, carbon dioxide, oxygen, steam or a mixture of these gases.

The gasification reaction can be divided into four sub-stages, which are: drying of the sample (70-200 ° C), evaporation (350-600 ° C), oxidation of volatiles and conversion to coal gas. Hence, it can be described as an incomplete combustion or prolonged pyrolysis reaction in which gas-solid, gas-gas and liquid cracking reactions are required in order to maximize the efficiency of the gaseous product.

The main challenges of sludge gasification are ash issues due to the high content of mineral compounds, tar minimization and sludge composition (moisture, heavy metals, nitrogen and sulfur).