Generating Electricity from Municipal Solid Waste

We live in a throwaway society that accumulates vast quantities of waste every day. While this comes with pressing challenges, there are also opportunities for professionals including electrical engineers to process at least some of the waste to produce much-needed renewable energy.

According to the U.S. Energy Information Administration (EIA), in 2018 a total of 68 U.S. power plants generated around 14 billion kilowatt-hours of electricity from 29.5 million tons of combustible municipal solid waste (MSW). Biomass, which comes from plants and animals and is a source of renewable energy, was responsible for more than half (about 51%) of the electricity generated from waste. It also accounted for about 64% of the weight of the MSW used. The rest of the waste used was from other combustible materials including synthetic materials made from petroleum and plastics. Glass and metal are generally not noncombustible.

WTE_Plant_Belgium

Waste-to-Energy is now widely accepted as a part of sustainable waste management strategy.

Municipal Solid Waste in the U.S.

Burning MSW is not only a sustainable way to produce electricity, it also reduces the volume of waste that would inevitably end up in landfills. Instead, the EIA estimates that burning MSW effectively reduces waste volumes by about 87%.

But, while more than 268 million tons of MSW are generated in the United States every year, in 2017, only 12.7% of it was burned to recover energy. More than half (52.1%) went to landfill, about a quarter (25.1%) was recycled, and the rest (10.1%) was used to generate compost.

According to a U.S. Environmental Protection Agency (EPA) fact sheet on sustainable materials management published in November 2019, the total MSW generated in 2017 by material, comprised:

  • Paper and paperboard, primarily containers and packaging 25%
  • Food 15.2% (see below)
  • Plastics 13.2% (19.2% of the total materials that ended up in landfill were plastics)
  • Yard trimmings 13.1% (most of this type of waste is composted)
  • Rubber, leather and textiles 9.7%
  • Metals 9.4%
  • Wood 6.7%
  • Glass 4.2%
  • Other 3.5%

Indicating tremendous human waste in its worst form, 22% of the material that ended up in landfill was classified as food. Trashed food was also the product category with the highest landfill rate, at an alarming 75.3%. Nearly a quarter (22%) of materials that were combusted with energy recovery were food, and overall, food was also the highest product category to recover energy, with a rate of 18.4%.

The total MSW combusted to generate energy was made up of the following materials:

  • Food 22%
  • Plastics 16.4%
  • Rubber, leather, and textiles 16.1%
  • Paper and paperboard 13.2%
  • Wood 8.4%
  • Metals 8.6%
  • Yard trimmings 6.2%
  • Glass 4.3%
  • Other 4.3%

Generating Electricity from MSW

There are a variety of technologies for generating electricity from municipal solid waste, but in the US the most common system involves mass burning of MSW in a large incinerator that has a boiler that produces steam, and a generator that produces electricity. Another entails processing MSW into fuel pellets for use in smaller power plants.

Waste materials destined to be processed to generate electricity

Generating electricity in mass-burn WTE plants is remarkably straightforward and follows seven basic steps:

  1. The MSW is dumped out of garbage trucks into a large pit.
  2. A crane with a giant claw attachment is used to grab the waste and dump it into a combustion chamber.
  3. The waste, which now becomes the fuel, starts to burn, releasing heat.
  4. The heat that is released turns water in the boiler into high-pressure steam.
  5. The steam turns the turbine generator’s blades and produces electricity.
  6. The mass-burn plant incorporates an control system to prevent air pollution by removing pollutants from the combustion gas before it is released through a smoke-stack.
  7. Ash is inevitably produced in the boiler and the air pollution control system, and this has to be removed before another load of waste can be burned.

While the volumes burned as fuel in different plants vary, for every 100 pounds of MSW produced in the US, potentially, more than 85 pounds could be burned to generate electricity.

Of course, the USA isn’t the only country that uses waste-to-energy plants to generate electricity from MSW. And in fact, when compared to a lot of other countries, the percentage of MSW burned with energy recovery in the U.S. is minimal. At least nine countries are named by the EIA as bigger producers of electricity from municipal waste. In Japan and some European countries, for instance, there are fewer energy resources and not much open space available for landfills. So generating electricity from MSW is an obvious opportunity.

The four leading nations identified by the EIA as burning the most MSW with energy recovery are:

  • Japan 68%
  • Norway 54%
  • Switzerland 48%
  • France 35%
  • The United Kingdom 34%

One thing’s for certain, the percentages are all set to continue increases globally as the move towards sustainability gains momentum. And U.S. percentages are going to increase too.

Waste-to-Energy Sector in China: Perspectives

China is the world’s largest waste generator, producing as much as 175 million tons of waste every year. With a current population surpassing 1.37 billion and exponential trends in waste output expected to continue, it is estimated that China’s cities will need to develop an additional hundreds of landfills and waste-to-energy plants to tackle the growing waste management crisis.

garbage-china

China’s three primary methods for municipal waste management are landfills, incineration, and composting. Nevertheless, the poor standards and conditions they operate in have made waste management facilities generally inefficient and unsustainable. For example, discharge of leachate into the soil and water bodies is a common feature of landfills in China. Although incineration is considered to be better than landfills and have grown in popularity over the years, high levels of toxic emissions have made MSW incineration plants a cause of concern for public health and environment protection.

Prevalent Issues

Salman Zafar, a renowned waste management, waste-to-energy and bioenergy expert was interviewed to discuss waste opportunities in China. As Mr. Zafar commented on the current problems with these three primary methods of waste management used by most developing countries, he said, “Landfills in developing countries, like China and India, are synonymous with huge waste dumps which are characterized by rotting waste, spontaneous fires, toxic emissions and presence of rag-pickers, birds, animals and insects etc.” Similarly, he commented that as cities are expanding rapidly worldwide, it is becoming increasingly difficult to find land for siting new landfills.

On incineration, Zafar asserted that this type of waste management method has also become a controversial issue due to emission concerns and high technology costs, especially in developing countries. Many developers try to cut down costs by going for less efficient air pollution control systems”. Mr. Zafar’s words are evident in the concerns reflected in much of the data ­that waste management practices in China are often poorly monitored and fraudulent, for which data on emission controls and environmental protection is often elusive.

Similarly, given that management of MSW involves the collection, transportation, treatment and disposal of waste, Zafar explains why composting has also such a small number relative to landfills for countries like China. He says, “Composting is a difficult proposition for developing countries due to absence of source-segregation. Organic fraction of MSW is usually mixed with all sorts of waste including plastics, metals, healthcare wastes and industrial waste which results in poor quality of compost and a real risk of introduction of heavy metals into agricultural soils.”

Given that China’s recycling sector has not yet developed to match market opportunities, even current treatment of MSW calls for the need of professionalization and institutionalization of the secondary materials industry.

While MSW availability is not an issue associated with the potential of the resource given its dispersion throughout the country and its exponential increase throughout, around 50 percent of the studies analyzed stated concerns for the high moisture content and low caloric value of waste in China, making it unattractive for WTE processes.

Talking about how this issue can be dealt with, Mr. Zafar commented that a plausible option to increase the calorific value of MSW is to mix it with agricultural residues or wood wastes. Thus, the biomass resources identified in most of the studies as having the greatest potential are not only valuable individually but can also be processed together for further benefits.

Top Challenges

Among the major challenges on the other hand, were insufficient or elusive data, poor infrastructure, informal waste collection systems and the lack of laws and regulations in China for the industry. Other challenges included market risk, the lack of economic incentives and the high costs associated with biomass technologies. Nevertheless, given that the most recurring challenges cited across the data were related to infrastructure and laws and regulations, it is evident that China’s biomass policy is in extreme need of reform.

China’s unsustainable management of waste and its underutilized potential of MSW feedstock for energy and fuel production need urgent policy reform for the industry to develop. Like Mr. Zafar says, “Sustainable waste management demands an integration of waste reduction, waste reuse, waste recycling, and energy recovery from waste and landfilling. It is essential that China implements an integrated solid waste management strategy to tackle the growing waste crisis”.

Future Perspectives

China’s government will play a key role in this integrated solid waste management strategy. Besides increased cooperation efforts between the national government and local governments to encourage investments in solid waste management from the private sector and foster domestic recycling practices, first, there is a clear need to establish specialized regulatory agencies (beyond the responsibilities of the State Environmental Protection Administration and the Ministry of Commerce) that can provide clearer operating standards for current WTE facilities (like sanitary landfills and incinerators) as well as improve the supervision of them.

It is essential that China implements an integrated solid waste management strategy to tackle the growing waste crisis

It is essential that China implements an integrated solid waste management strategy to tackle the growing waste crisis

Without clear legal responsibility assigned to specialized agencies, pollutant emissions and regulations related to waste volumes and operating conditions may continue to be disregarded. Similarly, better regulation in MSW management for efficient waste collection and separation is needed to incentivize recycling at the individual level by local residents in every city. Recycling after all is complementary to waste-to-energy, and like Salman Zafar explains, countries with the highest recycling rates also have the best MSW to energy systems (like Germany and Sweden).

Nevertheless, without a market for reused materials, recycling will take longer to become a common practice in China. As Chinese authorities will not be able to stop the waste stream from growing but can reduce the rate of growth, the government’s role in promoting waste management for energy production and recovery is of extreme importance.

Solid Wastes in the Middle East

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.

Biogas from Kitchen Waste at Akshaya Patra Foundation

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.

Akshaya-Patra-Kitchen-BioGas

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.

akshayapatra-kitchen

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.

Future Plans

Our future strategy for kitchen-based biogas plant revolves around two major points:

  • 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.

Biomass Energy in Thailand

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.

Thailand_paddy

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:

  • Woody biomass residues from forest plantations
  • Agricultural residues (rice husk, bagasse, corn cobs, etc.)
  • 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.

Renewable Energy from Food Residuals

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.

food-waste

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.

Pelletization of Municipal Solid Waste

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.

RDF

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.

Everything You Need to Know About Biomass Energy Systems

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.

biomass-energy-systems

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.

Gasification of Municipal Wastes

Gasification of 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.

utishinai-gasification-plant

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.

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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%.

Recycling and Waste-to-Energy Prospects in Saudi Arabia

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.

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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.

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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.