Pyrolysis is the most recommended alternative for the thermochemical treatment of waste tires and extensively used for conversion of carbonaceous materials in Europe and the Asia-Pacific. Pyrolysis is a two-phase treatment which uses thermal decomposition to heat the rubber in the absence of oxygen to break it into its constituent parts, e.g., pyrolysis oil (or bio oil), synthetic gas and char. Cracking and post-cracking take place progressively as the material is heated to 450-500°C and above.

Process Description

The pyrolysis method for scrap tires recycling involves heating whole or halved or shredded tires in a reactor containing an oxygen free atmosphere and a heat source. In the reactor, the rubber is softened after which the rubber polymers disintegrate into smaller molecules which eventually vaporize and exit from the reactor. These vapors can be burned directly to produce power or condensed into an oily type liquid, called pyrolysis oil or bio oil.

Some molecules are too small to condense and remain as a gas which can be burned as fuel. The minerals that were part of the tire, about 40% by weight, are removed as a solid. When performed well a tire pyrolysis process is a very clean operation and has nearly no emissions or waste.

The heating rate of tire is an important parameter affecting the reaction time, product yield, product quality and energy requirement of the waste tire pyrolysis process. If the temperature is maintained at around 450^oC the main product is liquid which could be a mixture of hydrocarbon depending on the initial composition of waste material. At temperature above 700^oC, synthetic gas (also known as syngas), a mixture of hydrogen and carbon monoxide, becomes the primary product due to further cracking of the liquids.

The nature of the feedstock and process conditions defines the properties of the gas, liquid and solid products. For example, whole tires contain fibers and steel while shredded tires have most of the steel and sometimes most of the fiber removed.

Processes can be either batch or continuous. The energy required for thermal decomposition of the scrap tires can be in the form of directly-fired fuel, electrical induction and or by microwaves (like a microwave oven). A catalyst may also be required to accelerate the pyrolysis process.

Useful Products

The high acceptance of pyrolysis for the treatment of scrap tires is due to the fact that the derived oils and syngas can be used as biofuels or as feedstock for refining crude oil or chemical products. The pyrolysis oil (or bio oil) has higher calorific value, low ash, low residual carbon and low sulphur content.

The use of pyrolysis oil in cement kilns, paper mills, power plants, industrial furnaces, foundries and other industries is one of the best uses of scrap tires.  Pyrolysis of scrap tyres produces oil that can be used as liquid fuels for industrial furnaces, foundries and boilers in power plants due to their higher calorific value, low ash, residual carbon and sulphur content.

The solid residue, called char, contains carbon black, and inorganic matter. It contains carbon black and the mineral matter initially present in the tire. This solid char may be used as reinforcement in the rubber industry, as activated carbon or as smokeless fuel.

Recommended Reading: Environmental Impact of Tire Recycling

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Pyrolysis and the Promise of Biochar

The end application of biomass mostly depends on the feedstock type and the char conversion process. When processed under controlled conditions, biomass converts to char (or biochar). With the presence of high carbon content in biochar, they are highly dependent on the processing conditions of biomass (or fuel), e.g. wood char produced from pyrolysis at low or no air flow can expect to have high carbon and hydrogen with minimal minerals/inorganic presence.

Gas produced under same condition will have a high presence of heavy aromatic carbon and nitrogen gas. However, under the same conditions, if physical structure of biomass varies, the output results can fluctuate to a significant level.

The temperature, pressure, elemental composition, particle size, physical structure (e.g. density, moisture presence, molecular structure, pore size), heating rate, the maximum temperature of process, retention time during the conversion process can change the composition of biochar produced.

Biomass when converted to char has multiple applications with minimal effect on the environment. It has applications in toxic metal remediation and can remove harmful contaminants from soil which can damage plant growth and soil nutrients.

Char has potential to stabilise cadmium, lead, chromium, zinc, but they are found to be most effective in stabilisation of lead and copper.  Researchers have found the potential application of biochar in a range of applications, viz. carbon sequestration, solid waste management, green electricity production, wastewater treatment, iron making process and building construction.

Why Fossil Fuel is Preferred Over Biomass Fuel?

Despite the significant contrast of applications and proven to have minimal effect on the environment, why is biomass not preferred or unsuccessful to attract the commercial sector? The answer relies on biomass processing technologies that still need to develop economically feasible. Besides fuel cost, the initial setup of biomass-based technologies need high capital cost, operation and maintenance cost, which eventually lead to a significantly higher cost of end application when compared with fossil fuels.

In most FMCG, sugarcane and fruit-based industries, biomass is produced as their waste, and legal compliances expect them to dispose of their waste sustainably. Industries spend substantial money to dispose of their waste in agreement with legal and environmental regulations. Researchers termed it a negative cost, which means that industries intend to pay to take this biomass off from their facility.

This could bring a possible opportunity to biomass processing plants to get paid or acquire fuel at no or negative cost. But most processing facilities are far from fuel (or biomass waste) sources, and cost of transportation are significant enough to compare the economics of fuel acquirement with fossil fuel costs. Moreover, processing technologies need cleaning and maintenance which further add up to the cost.

The overall economics of biomass-based electricity and any other end-use process cost higher than fossil fuels, making it very difficult to attract industries to invest in biomass over fossil fuels. Research suggests that biomass processing facilities that are available within the periphery of 200km from the fuel source will cost biomass (or fuel) at zero to negative value, improving the overall economics to a significantly comparable level to fossil fuels.

The Way Forward

To address this issue, small-scale plants must be installed in nearby areas and critical focus is vital on economically small scale biomass processing plants. Considerable research work is going on with small scale gasification plants capable of producing electricity at a small scale, but that is still under pilot project and no large-scale implementation has been found so far. Pyrolysis plants are also under the research zone, producing biochar, but this method is still under research development.

To reach targets of global temperature and carbon emissions into the atmosphere set by the UN at Climate Summit 2015, this area of research is a potentially critical area that can play a significant role in overtaking biomass over fossil fuels.

The post Why Fossil Fuels are Preferred Over Biomass by Industries? first appeared on BioEnergy Consult.

Pyrolysis of Municipal Wastes

Pyrolysis process consists of both simultaneous and successive reactions when carbon-rich organic material is heated in a non-reactive atmosphere. Simply speaking, pyrolysis is the thermal degradation of organic materials in the absence of oxygen. Thermal decomposition of organic components in the waste stream starts at 350°C–550°C and goes up to 700°C–800°C in the absence of air/oxygen.

Pyrolysis of municipal wastes begins with mechanical preparation and separation of glass, metals and inert materials prior to processing the remaining waste in a pyrolysis reactor. The commonly used pyrolysis reactors are rotary kilns, rotary hearth furnaces, and fluidized bed furnaces. The process requires an external heat source to maintain the high temperature required.

Pyrolysis can be performed at relatively small-scale which may help in reducing transport and handling costs.  In pyrolysis of MSW, heat transfer is a critical area as the process is endothermic and sufficient heat transfer surface has to be provided to meet process heat requirements.

The main products obtained from pyrolysis of municipal wastes are a high calorific value gas (synthesis gas or syngas), a biofuel (bio oil or pyrolysis oil) and a solid residue (char). Depending on the final temperature, MSW pyrolysis will yield mainly solid residues at low temperatures, less than 450⁰C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800⁰C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is a liquid fuel popularly known as bio oil.

Wide Range of Products

Bio oil is a dark brown liquid and can be upgraded to either engine fuel or through gasification processes to a syngas and then biodiesel. Pyrolysis oil may also be used as liquid fuel for diesel engines and gas turbines to generate electricity.

Bio oil is particularly attractive for co-firing because it can be relatively easy to handle and burn than solid fuel and is cheaper to transport and store. In addition, bio oil is also a vital source for a wide range of organic compounds and specialty chemicals.

Syngas is a mixture of energy-rich gases (combustible constituents include carbon monoxide, hydrogen, methane and a broad range of other VOCs). The net calorific value (NCV) of syngas is between 10 and 20MJ/Nm³. Syngas is cleaned to remove particulates, hydrocarbons, and soluble matter, and then combusted to generate electricity.

Diesel engines, gas turbines, steam turbines and boilers can be used directly to generate electricity and heat in CHP systems using syngas and pyrolysis oil. Syngas may also be used as a basic chemical in petrochemical and refining industries.

The solid residue from MSW pyrolysis, called char, is a combination of non-combustible materials and carbon. Char is almost pure carbon and can be used in the manufacture of activated carbon filtration media (for water treatment applications) or as an agricultural soil amendment.

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Biomass gasification provides a means of deriving more diverse forms of energy from the thermochemical conversion of biomass than conventional combustion. The basic gasification process involves devolatization, combustion and reduction.

During devolatization, methane and other hydrocarbons are produced from the biomass by the action of heat which leaves a reactive char.

During combustion, the volatiles and char are partially burned in air or oxygen to generate heat and carbon dioxide. In the reduction phase, carbon dioxide absorbs heat and reacts with the remaining char to produce carbon monoxide (producer gas). The presence of water vapour in a gasifier results in the production of hydrogen as a secondary fuel component.

There are two main types of gasifier that can be used to carry out this conversion, fixed bed gasifiers and fluidized bed gasifiers. The conversion of biomass into a combustible gas involves a two-stage process. The first, which is called pyrolysis, takes place below 600°C, when volatile components contained within the biomass are released. These may include organic compounds, hydrogen, carbon monoxide, tars and water vapour.

Pyrolysis leaves a solid residue called char. In the second stage of the gasification process, this char is reacted with steam or burnt in a restricted quantity of air or oxygen to produce further combustible gas. Depending on the precise design of gasifier chosen, the product gas may have a heating value of 6 – 19 MJ/Nm³.

The products of gasification are a mixture of carbon monoxide, carbon dioxide, methane, hydrogen and various hydrocarbons, which can then be used directly in gas turbines, and boilers, or used as precursors for synthesising a wide range of other chemicals.

In addition there are a number of methods that can be used to produce higher quality product gases, including indirect heating, oxygen blowing, and pressurisation. After appropriate treatment, the resulting gases can be burned directly for cooking or heat supply, or used in secondary conversion devices, such as internal combustion engines or gas turbines, for producing electricity or shaft power (where it also has the potential for CHP applications).

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