The high rate of population growth, urbanization and economic expansion in the Middle East is not only accelerating consumption rates but also increasing the generation rate of all sorts of waste. The gross urban waste generation quantity from Middle East countries is estimated at more than 150 million tons annually. Bahrain, Saudi Arabia, UAE, Qatar and Kuwait rank in the top-ten worldwide in terms of per capita solid waste generation.
Saudi Arabia produces around 15 million tons of garbage each year. With an approximate population of about 28 million, the kingdom produces approximately 1.3 kilograms of waste per person every day. According to a recent study conducted by Abu Dhabi Center for Waste Management, the amount of waste in UAE totaled 4.892 million tons, with a daily average of 6935 tons in the city of Abu Dhabi, 4118 tons in Al Ain and 2349 tons in the western region. Countries like Kuwait, Bahrain and Qatar have astonishingly high per capita waste generation rate, primarily because of high standard of living and lack of awareness about sustainable waste management practices.
In Middle East countries, huge quantity of sewage sludge is produced on daily basis which presents a serious problem due to its high treatment costs and risk to environment and human health. On an average, the rate of wastewater generation is 80-200 litres per person each day and sewage output is rising by 25 percent every year. According to estimates from the Drainage and Irrigation Department of Dubai Municipality, sewage generation in the Dubai increased from 50,000 m3 per day in 1981 to 400,000 m3 per day in 2006.
Waste-to-Energy Prospects
Municipal solid waste in the Middle East is mainly comprised of organics, paper, glass, plastics, metals, wood etc. Municipal solid waste can be converted into energy by conventional technologies (such as incineration, mass-burn and landfill gas capture) or by modern conversion systems (such as anaerobic digestion, gasification and pyrolysis).
At the landfill sites, the gas produced by the natural decomposition of MSW is collected from the stored material and scrubbed and cleaned before feeding into internal combustion engines or gas turbines to generate heat and power. In addition, the organic fraction of MSW can be anaerobically stabilized in a high-rate digester to obtain biogas for electricity or steam generation.
Anaerobic digestion is the most preferred option to extract energy from sewage, which leads to production of biogas and organic fertilizer. The sewage sludge that remains can be incinerated or gasified/pyrolyzed to produce more energy. In addition, sewage-to-energy processes also facilitate water recycling.
Thus, municipal solid waste can also be efficiently converted into energy and fuels by advanced thermal technologies. Infact, energy recovery from MSW is rapidly gaining worldwide recognition as the 4th R in sustainable waste management system – Reuse, Reduce, Recycle and Recover.
The Akshaya Patra Foundation, a not-for-profit organization, is focused on addressing two of the most important challenges in India – hunger and education. Established in year 2000, the Foundation began its work by providing quality mid-day meals to 1500 children in 5 schools in Bangalore with the understanding that the meal would attract children to schools, after which it would be easier to retain them and focus on their holistic development. 14 years later, the Foundation has expanded its footprint to cover over 1.4 million children in 10 states and 24 locations across India.
The Foundation has centralised, automated kitchens that can cook close to 6,000 kilos of rice, 4.5 to 5 tonnes of vegetables and 6,000 litres of sambar, in only 4 hours. In order to make sustainable use of organic waste generated in their kitchens, Akshaya Patra Foundation has set up anaerobic digestion plants to produce biogas which is then used as a cooking fuel. The primary equipment used in the biogas plant includes size reduction equipment, feed preparation tank for hydrolysis of waste stream, anaerobic digester, H2S scrubber and biogas holder.
Working Principle
Vegetable peels, rejects and cooked food waste are shredded and soaked with cooked rice water (also known as ganji) in a feed preparation tank for preparation of homogeneous slurry and fermentative intermediates. The hydrolyzed products are then utilized by the microbial culture, anaerobically in the next stage. This pre-digestion step enables faster and better digestion of organics, making our process highly efficient.
The hydrolyzed organic slurry is fed to the anaerobic digester, exclusively for the high rate biomethanation of organic substrates like food waste. The digester is equipped with slurry distribution mechanism for uniform distribution of slurry over the bacterial culture.
Optimum solids are retained in the digester to maintain the required food-to-microorganism ratio in the digester with the help of a unique baffle arrangement. Mechanical slurry mixing and gas mixing provisions are also included in the AD design to felicitate maximum degradation of organic material for efficient biogas production.
After trapping moisture and scrubbing off hydrogen sulphide from the biogas, it is collected in a gas-holder and a pressurized gas tank. This biogas is piped to the kitchen to be used as a cooking fuel, replacing LPG.
Basic Design Data and Performance Projections
Waste handling capacity 1 ton per day cooked and uncooked food waste with 1 ton per day ganji water
Input Parameters
Amount of solid organic waste
1000 Kg/day
Amount of organic wastewater
~ 1000 liters/day ganji (cooked rice water)
Biogas Production
Biogas production
~ 120 – 135 m3/day
Output Parameters
Equivalent LPG to replace
50 – 55 Kg/day (> 2.5 commercial LPG cylinders)
Fertilizer (digested leachate)
~ 1500 – 2000 liters/day
Major Benefits
Modern biogas installations are providing Akshaya Patra, an ideal platform for managing organic waste on a daily basis. The major benefits are:
Solid waste disposal at kitchen site avoiding waste management costs
Immediate waste processing overcomes problems of flies, mosquitos etc.
Avoiding instances when the municipality does not pick up waste, creating nuisance, smell, spillage etc.
Anaerobic digestion of Ganji water instead of directly treating it in ETP, therefore reducing organic load on the ETPs and also contributing to additional biogas production.
The decentralized model of biogas based waste-to-energy plants at Akshaya Patra kitchens ensure waste destruction at source and also reduce the cost incurred by municipalities on waste collection and disposal.
An on-site system, converting food and vegetable waste into green energy is improving our operations and profits by delivering the heat needed to replace cooking LPG while supplying a rich liquid fertilizer as a by-product. Replacement of fossil fuel with LPG highlights our organization’s commitment towards sustainable development and environment protection.
The typical ROI of a plug and play system (without considering waste disposal costs, subsidies and tax benifts) is around three years.
Utilization of surplus biogas – After consumption of biogas for cooking purposes, Akshaya Patra will consider utilizing surplus biogas for other thermal applications. Additional biogas may be used to heat water before boiler operations, thereby reducing our briquette consumption.
Digested slurry to be used as a fertilizer – the digested slurry from biogas plant is a good soil amendment for landscaping purposes and we plan to use it in order to reduce the consumption of water for irrigation as well as consumption of chemical fertilizers.
The world is running out of fossil fuels, and we need to find new ways to generate energy. Converting waste into energy is a clean and efficient way to generate power. It doesn’t produce the same level of pollution as traditional fuel sources, and it helps reduce our dependence on fossil fuels.
We need to find ways to convert waste into energy today in order to address the issue of climate change. By converting our waste into energy, we can reduce our reliance on polluting fuels and help preserve our environment for future generations.
What is waste to energy?
Waste to energy is a process of turning waste into electricity. This is a clean and efficient way to generate power, and it doesn’t produce the same level of pollution as traditional fuel sources. Converting our waste into energy can help reduce our reliance on polluting fuels and preserve our environment for future generations.
Can all types of waste be used?
The different types of waste that can be used in waste to energy are municipal solid waste, agricultural waste, and industrial waste. Municipal solid waste is the most common type of waste that is used in this process. It includes everyday items like paper, plastic, and metal. Agricultural waste includes things like manure, straw, and wood chips. Industrial waste includes things like slag, ash, and boiler dust. Municipal solid waste is the most common type of waste that is used in waste to energy.
Is Waste to Energy the same as Biomass?
The similarities between waste to energy and biomass are that they are both renewable resources, and they can both be used to create energy. The main difference between them is that waste to energy uses organic material that would otherwise be thrown away (like food waste), while biomass uses plants specifically grown for the purpose of creating fuel.
Waste to Energy – Frequently Asked Questions
Is waste to energy effective?
Yes, waste to energy is an effective way to generate power. It doesn’t produce the same level of pollution as traditional fuel sources, and it helps reduce our dependence on fossil fuels. Converting our waste into energy can help preserve our environment for future generations.
There are many reasons to believe that waste to energy is a more efficient renewable energy source than other types of renewables. First, waste to energy facilities can be located near population centers, which reduces the amount of energy lost in transmission. Second, waste to energy plants tend to have higher capacity factors than other types of renewable energy sources, meaning that they produce more electricity per unit of capacity.
Finally, waste to energy plants can use a variety of feedstocks, including municipal solid waste, construction and demolition debris, and sewage sludge. This flexibility gives waste-to-energy plants a significant advantage over other renewable energy sources that are limited to a single feedstock.
Is waste to energy sustainable?
The short answer is yes – waste to energy (WtE) is a sustainable solution for managing municipal solid waste (MSW). But it’s important to consider the whole picture when making decisions about sustainability. That means taking into account factors like greenhouse gas emissions, financial costs, and other renewable energy options like solar and wind.
When it comes to conserving energy, there are many things that people can do to help out, both big and small. Saving energy at home can help reduce the amount of waste going to energy plants, and it can also save homeowners money on their monthly energy bills.
WtE plants use MSW to generate electricity, and they can actually help reduce greenhouse gas emissions. That’s because when MSW is incinerated, it doesn’t release methane, a powerful greenhouse gas that’s produced when MSW breaks down in landfills. In fact, WtE plants are so efficient at reducing methane emissions that they’re actually considered carbon-neutral.
WtE plants are also cost-effective, and the technology is constantly improving. In the past, WtE plants were criticized for being too expensive to build and operate. But new plants are much more efficient, and the costs have come down significantly.
What are the advantages and disadvantages of waste to energy?
The advantages of waste to energy are that it is a sustainable solution for managing MSW, it reduces greenhouse gas emissions, and it is cost-effective. The disadvantages of waste to energy are that it requires high initial investment, and it produces some air pollution. Overall, waste to energy is a good option for communities looking for a sustainable and cost-effective solution for managing MSW.
What are the alternatives to waste to energy?
The main alternative to waste to energy is landfill gas-to-energy, which captures methane gas produced by decomposing MSW in landfills and uses it to generate electricity. Landfill gas-to-energy is less expensive than waste to energy, but it has a higher greenhouse gas emissions footprint.
Other renewable energy options include solar and wind power. Solar and wind power are both carbon-neutral, but they are more expensive than waste to energy. Waste to energy is a good option for communities looking for a sustainable and cost-effective solution for managing MSW. It has some disadvantages, but overall it is a good option for communities looking to reduce their environmental impact.
Why are we converting Waste to Energy?
We need to convert waste to energy today because the world is running out of fossil fuels. The use of coal, oil, and natural gas has created an unprecedented level of pollution, which is damaging our environment and contributing to climate change. In order to reduce our dependence on these polluting fuels and address the issue of climate change, we need to find ways to convert waste into energy.
Sweden is one of the best proponents of waste-to-energy in the world
In recent years, waste to energy (WtE) has become increasingly popular as a means of generating electricity. However, not everyone is convinced that WtE is the best option for the environment. Some critics argue that WtE actually damages the environment and is not worth the investment.
One of the major criticisms of WtE is that it emits pollutants into the air. When waste is burned, it releases harmful chemicals and particulates into the atmosphere. These pollutants can have a negative impact on human health, as well as the environment. In addition, WtE plants are often located in close proximity to populated areas, which means that the pollution they emit can affect a large number of people.
Food waste and waste to energy are two important topics that we should be thinking about more. With the right infrastructure in place, food waste can be used to create energy, which can help to power our homes and businesses. In addition, by reducing food waste, we can also help to reduce greenhouse gas emissions.
Another criticism of WtE is that it is actually less efficient than other means of generating electricity. WtE plants typically have lower efficiency rates than coal-fired power plants, for example. This means that more waste needs to be burned in order to generate the same amount of electricity. This can lead to more pollution and more damage to the environment.
Critics also argue that WtE plants are expensive to build and operate. The initial investment can be significant, and the operating costs can be high. This means that WtE may not be the most cost-effective option for generating electricity.
Despite these criticisms, some experts believe that WtE can be a valuable tool for generating electricity. WtE plants can help to reduce the amount of waste that is sent to landfill, and they can provide a source of renewable energy. In addition, WtE plants can create jobs and boost the economy.
Ultimately, the decision of whether or not to use WtE should be based on a careful consideration of all the pros and cons. WtE may not be right for everyone, but it could be the best option for some.
Thailand’s annual energy consumption has risen sharply during the past decade and will continue its upward trend in the years to come. While energy demand has risen sharply, domestic sources of supply are limited, thus forcing a significant reliance on imports.
To face this increasing demand, Thailand needs to produce more energy from its own renewable resources, particularly biomass wastes derived from agro-industry, such as bagasse, rice husk, wood chips, livestock and municipal wastes.
In 2005, total installed power capacity in Thailand was 26,430 MW. Renewable energy accounted for about 2 percent of the total installed capacity. In 2007, Thailand had about 777 MW of electricity from renewable energy that was sold to the grid.
Biomass Potential in Thailand
Several studies have projected that biomass wastes can cover up to 15 % of the energy demand in Thailand. These estimations are primarily made from biomass waste from the extraction part of agricultural activities, and for large scale agricultural processing of crops etc. – as for instance saw and palm oil mills – and do not include biomass wastes from SMEs in Thailand. Thus, the energy potential of biomass waste can be much larger if these resources are included. The major biomass resources in Thailand include the following:
Wood residues from wood and furniture industries (bark, sawdust, etc.)
Biomass for ethanol production (cassava, sugar cane, etc.)
Biomass for biodiesel production (palm oil, jatropha oil, etc.)
Industrial wastewater from agro-industry
Livestock manure
Municipal solid wastes and sewage
Thailand’s vast biomass potential has been partially exploited through the use of traditional as well as more advanced conversion technologies for biogas, power generation, and biofuels. Rice, sugar, palm oil, and wood-related industries are the major potential biomass energy sources in Thailand. The country has a fairly large biomass resource base of about 60 million tons generated each year that could be utilized for energy purposes, such as rice, sugarcane, rubber sheets, palm oil and cassava.
Biomass has been a primary source of energy for many years, used for domestic heating and industrial cogeneration. For example, paddy husks are burned to produce steam for turbine operation in rice mills; bagasse and palm residues are used to produce steam and electricity for on-site manufacturing process; and rubber wood chips are burned to produce hot air for rubber wood seasoning.
In addition to biomass residues, wastewater containing organic matters from livestock farms and industries has increasingly been used as a potential source of biomass energy. Thailand’s primary biogas sources are pig farms and residues from food processing. The production potential of biogas from industrial wastewater from palm oil industries, tapioca starch industries, food processing industries, and slaughter industries is also significant. The energy-recovery and environmental benefits that the KWTE waste to energy project has already delivered is attracting keen interest from a wide range of food processing industries around the world.
Food residuals are an untapped renewable energy source that mostly ends up rotting in landfills, thereby releasing greenhouse gases into the atmosphere. Food residuals are difficult to treat or recycle since it contains high levels of sodium salt and moisture, and is mixed with other waste during collection. Major generators of food wastes include hotels, restaurants, supermarkets, residential blocks, cafeterias, airline caterers, food processing industries, etc.
According to EPA, about 63.1 million tons of food waste was thrown away into landfills or incinerators the United States in 2018. As far as United Kingdom is concerned, households threw away 6.6 million tons of food each year. These statistics are an indication of tremendous amount of food waste generated all over the world.
The proportion of food residuals in municipal waste stream is gradually increasing and hence a proper food waste management strategy needs to be devised to ensure its eco-friendly and sustainable disposal. Currently, only about 3 percent of food waste is recycled throughout U.S., mainly through composting. Composting provides an alternative to landfill disposal of food waste, however it requires large areas of land, produces volatile organic compounds and consumes energy. Consequently, there is an urgent need to explore better recycling alternatives.
Anaerobic digestion has been successfully used in several European and Asian countries to stabilize food wastes, and to provide beneficial end-products. Sweden, Austria, Denmark, Germany and England have led the way in developing new advanced biogas technologies and setting up new projects for conversion of food waste into energy.
Anaerobic Digestion of Food Waste
Anaerobic digestion is the most important method for the treatment of organic waste, such as food residuals, because of its techno-economic viability and environmental sustainability. Anaerobic digestion generates renewable energy from food waste in the form of biogas and preserves the nutrients which are recycled back to the agricultural land in the form of slurry or solid fertilizer.
The relevance of biogas technology lies in the fact that it makes the best possible use of various organic wastes as a renewable source of clean energy. A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. Thus, anaerobic digestion of food waste can lead to climate change mitigation, economic benefits and landfill diversion opportunities.
Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer. Food waste can either be used as a single substrate in a biogas plant, or can be co-digested with organic wastes like cow manure, poultry litter, sewage, crop residues, slaughterhouse wastes, etc.
Renewable Energy from Food Residuals
The feedstock for the food waste-to-energy plant includes leftover food, vegetable refuse, stale cooked and uncooked food, meat, teabags, napkins, extracted tea powder, milk products, etc. Raw waste is shredded to reduce to its particle size to less than 12 mm. The primary aim of shredding is to produce a uniform feed and reduce plant “down-time” due to pipe blockages by large food particles. It also improves mechanical action and digestibility and enables easy removal of any plastic bags or cling-film from waste.
Fresh waste and re-circulated digestate (or digested food waste) are mixed in a mixing tank. The digestate is added to adjust the solids content of the incoming waste stream from 20 to 25 percent (in the incoming waste) to the desired solids content of the waste stream entering the digestion system (10 to 12 percent total solids). The homogenized waste stream is pumped into the feeding tank, from which the anaerobic digestion system is continuously fed. Feeding tank also acts as a pre-digester and subjected to heat at 55º to 60º C to eliminate pathogens and to facilitate the growth of thermophilic microbes for faster degradation of waste.
From the predigestor tank, the slurry enters the main digester where it undergoes anaerobic degradation by a consortium of Archaebacteria belonging to Methanococcus group. The anaerobic digester is a CSTR reactor having average retention time of 15 to 20 days. The digester is operated in the mesophilic temperature range (33º to 38°C), with heating carried out within the digester. Food waste is highly biodegradable and has much higher volatile solids destruction rate (86 to 90 percent) than biosolids or livestock manure. As per conservative estimates, each ton of food waste produces 150 to 200 m3 of biogas, depending on reactor design, process conditions, waste composition, etc.
Biogas contains significant amount of hydrogen sulfide (H2S) gas that needs to be stripped off due to its corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria that oxidizes H2S into elemental sulfur. The biogas produced as a result of anaerobic digestion of waste is sent to a gas holder for temporary storage. Biogas is eventually used in a combined heat and power (CHP) unit for its conversion into thermal and electrical energy in a cogeneration power station of suitable capacity. The exhaust gases from the CHP unit are used for meeting process heat requirements.
The digested substrate leaving the reactor is rich in nutrients like nitrogen, potassium and phosphorus which are beneficial for plants as well as soil. The digested slurry is dewatered in a series of screw presses to remove the moisture from slurry. Solar drying and additives are used to enhance the market value and handling characteristics of the fertilizer.
Diverting Food from Landfills
Food residuals are one of the single largest constituents of municipal solid waste stream. Diversion of food waste from landfills can provide significant contribution towards climate change mitigation, apart from generating revenues and creating employment opportunities. Rising energy prices and increasing environmental pollution makes it more important to harness renewable energy from food scraps and create a sustainable food supply chain.
Anaerobic digestion technology is widely available worldwide and successful projects are already in place in several European as well as Asian countries that makes it imperative on waste generators and environmental agencies to root for a sustainable food waste management system.
MSW is a poor-quality fuel and its pre-processing is necessary to improve its consistency, storage and handling characteristics, combustion behaviour and calorific value. Technological improvements are taking place in the realms of advanced source separation, resource recovery and production/utilisation of recovered fuel in both existing and new plants for this purpose. In recent years, there has been an increase in the pelletization of municipal solid wastes.
What is Pelletization of MSW?
Pelletization of municipal solid waste involves the processes of segregating, crushing, mixing high and low heat value combustible waste material and solidifying it to produce fuel pellets or briquettes, also referred to as Refuse Derived Fuel (RDF).
The process is essentially a method that condenses the waste or changes its physical form and enriches its organic content through removal of inorganic materials and moisture. The calorific value of RDF pellets can be around 4000 kcal/ kg depending upon the percentage of organic matter in the waste, additives and binder materials used in the process.
The calorific value of raw MSW is around 1000 kcal/kg while that of fuel pellets is 4000 kcal/kg. On an average, about 15–20 tons of fuel pellets can be produced after treatment of 100 tons of raw garbage. Since pelletization enriches the organic content of the waste through removal of inorganic materials and moisture, it can be very effective method for preparing an enriched fuel feed for other thermochemical processes like pyrolysis/ gasification, apart from incineration.
Production of MSW Pellets
RDF production line consists of several unit operations in series in order to separate unwanted components and condition the combustible matter to obtain the required characteristics.
The main unit operations are screening, shredding, size reduction, classification, separation either metal, glass or wet organic materials, drying and densification. These unit operations can be arranged in different sequences depending on raw MSW composition and the required RDF quality.
Various qualities of fuel pellets can be produced, depending on the needs of the user or market. A high quality of RDF would possess a higher value for the heating value, and lower values for moisture and ash contents. The quality of RDF is sufficient to warrant its consideration as a preferred type of fuel when solid waste is being considered for co-firing with coal or for firing alone in a boiler designed originally for firing coal.
Uses of MSW Pellets
MSW pellets can be used for heating plant boilers and for the generation of electricity. They can also act as a good substitute for coal and wood for domestic and industrial purposes. The important uses of RDF are found in the following spheres:
Cement kilns
RDF power plants
Coal-fired power plants
Industrial steam/heat boilers
Pellet stoves
The conversion of solid waste into fuel pellets provides an alternative means for environmentally safe disposal of garbage which is currently disposed off in non-sanitary landfills. In addition, the pelletization technology provides yet another source of renewable energy, similar to that of biomass, wind, solar and geothermal energy. The emission characteristics of RDF are superior compared to that of coal with fewer emissions of pollutants like NOx, SOx, CO and CO2.
Biomass is a versatile energy source that can be used for production of heat, power, transport fuels and biomaterials, apart from making a significant contribution to climate change mitigation. Currently, biomass-driven combined heat and power, co-firing, and combustion plants provide reliable, efficient, and clean power and heat.
Feedstock for biomass energy plants can include residues from agriculture, forestry, wood processing, and food processing industries, municipal solid wastes, industrial wastes and biomass produced from degraded and marginal lands.
The terms biomass energy, bioenergy and biofuels cover any energy products derived from plant or animal or organic material. The increasing interest in biomass energy and biofuels has been the result of the following associated benefits:
Potential to reduce GHG emissions.
Energy security benefits.
Substitution for diminishing global oil supplies.
Potential impacts on waste management strategy.
Capacity to convert a wide variety of wastes into clean energy.
Technological advancement in thermal and biochemical processes for waste-to-energy transformation.
Biomass can play the pivotal role in production of carbon-neutral fuels of high quality as well as providing feedstock for various industries. This is a unique property of biomass compared to other renewable energies and which makes biomass a prime alternative to the use of fossil fuels. Performance of biomass-based systems for heat and power generation has been already proved in many situations on commercial as well as domestic scales.
Biomass energy systems have the potential to address many environmental issues, especially global warming and greenhouse gases emissions, and foster sustainable development among poor communities. Biomass fuel sources are readily available in rural and urban areas of all countries. Biomass-based industries can provide appreciable employment opportunities and promote biomass re-growth through sustainable land management practices.
The negative aspects of traditional biomass utilization in developing countries can be mitigated by promotion of modern biomass-to-energy technologies which provide solid, liquid and gaseous fuels as well as electricity as shown. Biomass wastes can be transformed into clean and efficient energy by biochemical as well as thermochemical technologies.
The most common technique for producing both heat and electrical energy from biomass wastes is direct combustion. Thermal efficiencies as high as 80 – 90% can be achieved by advanced gasification technology with greatly reduced atmospheric emissions. Combined heat and power (CHP) systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.
Biochemical processes, like anaerobic digestion and sanitary landfills, can also produce clean energy in the form of biogas and producer gas which can be converted to power and heat using a gas engine.
In addition, biomass wastes can also yield liquid fuels, such as cellulosic ethanol, which can be used to replace petroleum-based fuels. Cellulosic ethanol can be produced from grasses, wood chips and agricultural residues by biochemical route using heat, pressure, chemicals and enzymes to unlock the sugars in lignocellulosic biomass. Algal biomass is also emerging as a good source of energy because it can serve as natural source of oil, which conventional refineries can transform into jet fuel or diesel fuel.
Gasificationof municipal wastes involves the reaction of carbonaceous feedstock with an oxygen-containing reagent, usually oxygen, air, steam or carbon dioxide, generally at temperatures above 800°C. The process is largely exothermic but some heat may be required to initialise and sustain the gasification process.
The main product of the gasification process is syngas, which contains carbon monoxide, hydrogen and methane. Typically, the gas generated from gasification has a low heating value (LHV) of 3 – 6 MJ/Nm3.The other main product produced by gasification is a solid residue of non-combustible materials (ash) which contains a relatively low level of carbon.
Syngas can be used in a number of ways, including:
Syngas can be burned in a boiler to generate steam for power generation or industrial heating.
Syngas can be used as a fuel in a dedicated gas engine.
Syngas, after reforming, can be used in a gas turbine
Syngas can also be used as a chemical feedstock.
Gasification has been used worldwide on a commercial scale for several decades by the chemical, refining, fertilizer and electric power industries. MSW gasification plants are relatively small-scale, flexible to different inputs and modular development. The quantity of power produced per tonne of waste by gasification process is larger than when applying the incineration method.
The most important reason for the growing popularity of gasification of municipal solid wastes has been the increasing technical, environmental and public dissatisfaction with the performance of conventional incinerators.
Plasma Gasification
Plasma gasification uses extremely high temperatures in an oxygen-starved environment to completely decompose input waste material into very simple molecules in a process similar to pyrolysis. The heat source is a plasma discharge torch, a device that produces a very high temperature plasma gas. It is carried out under oxygen-starved conditions and the main products are vitrified slag, syngas and molten metal.
Vitrified slag may be used as an aggregate in construction; the syngas may be used in energy recovery systems or as a chemical feedstock; and the molten metal may have a commercial value depending on quality and market availability. The technology has been in use for steel-making and is used to melt ash to meet limits on dioxin/furan content. There are several commercial-scale plants already in operation in Japan for treating MSW and auto shredder residue.
Advantages of MSW Gasification
There are numerous MSW gasification facilities operating or under construction around the world. Gasification of solid wastes has several advantages over traditional combustion processes for MSW treatment. It takes place in a low oxygen environment that limits the formation of dioxins and of large quantities of SOx and NOx. Furthermore, it requires just a fraction of the stoichiometric amount of oxygen necessary for combustion. As a result, the volume of process gas is low, requiring smaller and less expensive gas cleaning equipment.
The lower gas volume also means a higher partial pressure of contaminants in the off-gas, which favours more complete adsorption and particulate capture. Finally, gasification generates a fuel gas that can be integrated with combined cycle turbines, reciprocating engines and, potentially, with fuel cells that convert fuel energy to electricity more efficiently than conventional steam boilers.
Disadvantages of Gasification
The gas resulting from gasification of municipal wastes contains various tars, particulates, halogens, heavy metals and alkaline compounds depending on the fuel composition and the particular gasification process. This can result in agglomeration in the gasification vessel, which can lead to clogging of fluidised beds and increased tar formation. In general, no slagging occurs with fuels having ash content below 5%. MSW has a relatively high ash content of 10-12%.
The Kingdom of Saudi Arabia produces around 15 million tons of municipal solid waste (MSW) each year with average daily rate of 1.4 kg per person. With the current growing population (3.4% yearly rate), urbanization (1.5% yearly rate) and economic development (3.5% yearly GDP rate), the generation rate of MSW will become double (30 million tons per year) by 2033. The major ingredients of Saudi Arabian MSW are food waste (40-51 %), paper (12-28 %), cardboard (7 %), plastics (5-17 %), glass (3-5 %), wood (2-8 %), textile (2-6 %), metals (2-8 %) etc. depending on the population density and urban activities of that area.
In Saudi Arabia, MSW is collected and sent to landfills or dumpsites after partial segregation and recycling. The major portion of collected waste is ends up in landfills untreated. The landfill requirement is very high, about 28 million m3 per year. The problems of leachate, waste sludge, and methane and odor emissions are occurring in the landfills and its surrounding areas due to mostly non-sanitary or un-engineered landfills. However, in many cities the plans of new sanitary landfills are in place, or even they are being built by municipalities with capturing facilities of methane and leachate.
Recycling Prospects in Saudi Arabia
The recycling of metals and cardboard is the main waste recycling practice in Saudi Arabia, which covers 10-15% of the total waste. This recycling practice is mostly carried out by informal sector. The waste pickers or waste scavengers take the recyclables from the waste bins and containers throughout the cities. The waste recycling rate often becomes high (upto 30% of total waste) by waste scavengers in some areas of same cities. The recycling is further carried out at some landfill sites, which covers upto 40% of total waste by the involvement of formal and informal sectors.
The recycled products are glass bottles, aluminum cans, steel cans, plastic bottles, paper, cardboard, waste tire, etc. depending on the area, available facilities and involved stakeholders. It is estimated that 45 thousand TJ of energy can be saved by recycling only glass and metals from MSW stream. This estimation is based on the energy conservation concept, which means xyz amount of energy would be used to produce the same amount of recyclable material.
Waste-to-Energy Potential in Saudi Arabia
The possibilities of converting municipal wastes to renewable energy are plentiful. The choice of conversion technology depends on the type and quantity of waste (waste characterization), capital and operational cost, labor skill requirements, end-uses of products, geographical location and infrastructure. Several waste to energy technologies such as pyrolysis, anaerobic digestion (AD), trans-esterification, fermentation, gasification, incineration, etc. have been developed. Waste-to-energy provides the cost-effective and eco-friendly solutions to both energy demand and MSW disposal problems in Saudi Arabia.
As per conservative estimates, electricity potential of 3 TWh per year can be generated, if all of the KSA food waste is utilized in biogas plants. Similarly, 1 and 1.6 TWh per year electricity can be generated if all the plastics and other mixed waste (i.e. paper, cardboard, wood, textile, leather, etc.) of KSA are processed in the pyrolysis, and refuse derived fuel (RDF) technologies respectively.
Conclusion
Waste management issues in Saudi Arabia are not only related to water, but also to land, air and the marine resources. The sustainable integrated solid waste management is still at the infancy level. There have been many studies in identifying the waste related environmental issues in KSA. The current SWM activities of KSA require a sustainable and integrated approach with implementation of waste segregation at source, waste recycling, WTE and value-added product (VAP) recovery. By 2032, Saudi government is aiming to generate about half of its energy requirements (about 72 GW) from renewable sources such as solar, nuclear, wind, geothermal and waste-to-energy systems.
Cofiring of biomass involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels. Cofiring of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels.
Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Cofiring of biomass has the major advantage of avoiding the construction of new, dedicated, biomass power plant. An existing power station is modified to accept the biomass resource and utilize it to produce a minor proportion of its electricity.
Cofiring of biomass may be implemented using different types and percentages of biomass in a range of combustion and gasification technologies. Most forms of biomass are suitable for cofiring. These include dedicated energy crops, urban wood waste and agricultural residues such as rice straw and rice husk.
The fuel preparation requirements, issues associated with combustion such as corrosion and fouling of boiler tubes, and characteristics of residual ash dictate the cofiring configuration appropriate for a particular plant and biomass resource. These configurations may be categorized into direct, indirect and parallel firing.
1. Direct Cofiring
This is the most common form of biomass cofiring involving direct cofiring of the biomass fuel and the primary fuel (generally coal) in the combustion chamber of the boiler. The cheapest and simplest form of direct cofiring for a pulverized coal power plant is through mixing prepared biomass and coal in the coal yard or on the coal conveyor belt, before the combined fuel is fed into the power station boiler.
2. Indirect Cofiring
If the biomass fuel has different attributes to the normal fossil fuel, then it may be prudent to partially segregate the biomass fuel rather than risk damage to the complete station.
For indirect cofiring, the ash of the biomass resource and the main fuel are kept separate from one another as the thermal conversion is partially carried out in separate processing plants. As indirect co-firing requires a separate biomass energy conversion plant, it has a relatively high investment cost compared with direct cofiring.
Parallel Firing
For parallel firing, totally separate combustion plants and boilers are used for the biomass resource and the coal-fired power plants. The steam produced is fed into the main power plant where it is upgraded to higher temperatures and pressures, to give resulting higher energy conversion efficiencies. This allows the use of problematic fuels with high alkali and chlorine contents (such as wheat straw) and the separation of the ashes.
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