By Salman Zafar, Renewable Energy Advisor

Introduction

The enormous increase in the quantum and diversity of waste materials and their potentially harmful effects on the general environment and public health, have led to an increasing awareness about an urgent need to adopt scientific methods for safe disposal of wastes. While there is an obvious need to minimize the generation of wastes and to reuse and recycle them, the technologies for recovery of energy from wastes can play a vital role in mitigating the problems. These technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal while meeting the pollution control standards.

Waste-to-energy plants based on gasification are high-efficiency power plants that utilize municipal solid waste as their fuel rather than conventional sources of energy like coal, oil or natural gas. Such plants recover the thermal energy contained in the garbage in highly efficient boilers that generate steam that can then be sold directly to industrial customers, or used on-site to drive turbines for electricity production. The recovery of energy from solid wastes offers several benefits which include:

• Substantial reduction in the total quantity of waste depending upon the waste composition and the adopted technology.
• Significant reduction in environmental pollution.
• Improvement in the quality of residual waste.
• Reduction in the demand for land for waste disposal.
• Reduction in transport cost, as garbage is not required to be carried to a faraway place for dumping.
• Improved commercial viability of the waste disposal project from the sale of energy/products.

Gasification

Gasification processes involve the reaction of carbonaceous feedstock with an oxygen-containing reagent, usually oxygen, air, steam or carbon dioxide, generally at temperatures in excess of 800°C. It involves the partial oxidation of a substance which implies that oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur. The process is largely exothermic but some heat may be required to initialise and sustain the gasification process.

The main product is a syngas, which contains carbon monoxide, hydrogen and methane. Typically, the gas generated from gasification will have a net calorific value of 4 - 10 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 different ways, for example:

• Syngas can be burned in a boiler to generate steam which may be used for power generation or industrial heating.
• Syngas can be used as a fuel in a dedicated gas engine.
• Syngas, after reforming, may be suitable for use in a gas turbine
• Syngas can also be used as a chemical feedstock.

Gasification plants, based on syngas production, are relatively small scale, flexible to different inputs and modular development. Producing syngas to serve multiple end-uses could complicate delivery of the plants but it could provide a higher degree of financial security.  

Gasification of Municipal Solid Waste

The most important reason for the growing popularity of thermal processes for the treatment of solid wastes has been the increasing technical, environmental and public dissatisfaction with the performance of conventional incineration processes. MSW is difficult to handle, segregate and feed in a controlled manner to a waste-to-energy facility. MSW has a high tendency to form fused ash deposits on the internal surfaces of furnaces and high temperature reactors, and to form bonded fouling deposits on heat exchanger surfaces. The products of the combustion of MSW are also very aggressive, in that the flue gases are erosive and the relatively high levels of chloride containing species in the flue gases can lead to high rates of metal wastage of heat exchange tube surfaces due to high temperature corrosion.

While evaluating gasification or other thermal technologies, the degree of pre-processing required in conversion of MSW into a suitable feed material is a major criterion. Unsorted MSW is not suitable for most thermal technologies because of its varying composition and size of some of its constituent materials. It may also contain undesirable materials which can play havoc with the process or emission control systems.

The main steps involved in pre-processing of MSW include manual and mechanical separation or sorting, shredding, grinding, blending with other materials, drying and pelletization. The purpose of pre-processing is to produce a feed material with consistent physical characteristics and chemical properties. Pre-processing operations are also designed to produce a material that can be safely handled, transported and stored.

Figure 1 

Advantages of Gasification

There are numerous solid waste gasification facilities operating or under construction around the world. Gasification 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

During gasification, tars, heavy metals, halogens and alkaline compounds are released within the product gas and can cause environmental and operational problems. Tars are high molecular weight organic gases that ruin reforming catalysts, sulfur removal systems, ceramic filters and increase the occurrence of slagging in boilers and on other metal and refractory surfaces. Alkalis can increase agglomeration in fluidized beds that are used in some gasification systems and also can ruin gas turbines during combustion. Heavy metals are toxic and accumulate if released into the environment. Halogens are corrosive and are a cause of acid rain if emitted to the environment. The key to achieving cost efficient, clean energy recovery from municipal solid waste gasification will be overcoming problems associated with the release and formation of these contaminants. 

Types of Gasifiers for MSW Treatment

Gasification technology is selected on the basis of available fuel quality, capacity range, and gas quality conditions. The main reactors used for gasification of MSW are fixed beds and fluidized beds. Larger capacity gasifiers are preferable for treatment of MSW because they allow for variable fuel feed, uniform process temperatures due to highly turbulent flow through the bed, good interaction between gases and solids, and high levels of carbon conversion. Table 1 shows the thermal capacity ranges for the main gasifier designs.

• Fixed Beds

Fixed bed gasifiers typically have a grate to support the feed material and maintain a stationary reaction zone. They are relatively easy to design and operate, and are therefore useful for small and medium scale power and thermal energy uses. The two primary types of fixed bed gasifiers are updraft and downdraft.

In a downdraft gasifier, air is introduced into a downward flowing packed bed or solid fuel stream and gas is drawn off at the bottom. The air/oxygen and fuel enter the reaction zone from above decomposing the combustion gases and burning most of the tars. Downdraft gasifiers are not ideal for waste treatment because they typically require a low ash fuel such as wood, to avoid clogging.

In an updraft gasifier, the fuel is also fed at the top of the gasifier but the airflow is in the upward direction. As the fuel flows downward through the vessel it dries, pyrolyzes, gasifies and combusts. The main use of updraft gasifiers has been with direct use of the gas in a closely coupled boiler or furnace. Because the gas leaves this gasifier at relatively low temperatures, the process has a high thermal efficiency and, as a result, wet MSW containing

50% moisture can be gasified without any pre-drying of the waste.

Slagging fixed bed gasifier, which is high-pressure and oxygen-injected, has commercial potential for gasifying MSW. In theory, the high temperatures crack all tars and other volatiles into non-condensable, light gases. Also under these conditions, the ash becomes molten and is tapped out, as is done in iron blast furnaces.  

• Fluidized Beds

Fluidized beds are an attractive proposition for the gasification of MSW. In a fluidized bed boiler, a stream of gas (typically air or steam) is passed upward through a bed of solid fuel and material (such as coarse sand or limestone). The gas acts as the fluidizing medium and also provides the oxidant for combustion and tar cracking. Waste is introduced either on top of the bed through a feed chute or into the bed through an auger. Fluidized-beds have the advantage of extremely good mixing and high heat transfer, resulting in very uniform bed conditions and efficient reactions. Fluidized bed technology is more suitable for generators with capacities greater than 10 MW because it can be used with different fuels, requires relatively compact combustion chambers and allows for good operational control. The two main types of fluidized beds for power generation are bubbling and circulating fluidized beds.

In a Bubbling Fluidized Bed (BFB), the gas velocity must be high enough so that the solid particles, comprising thebed material, are lifted, thus expanding the bed and causing it to bubble like a liquid. A bubbling fluidized bed reactor typically has a cylindrical or rectangular chamber designed so that contact between the gas and solids facilitates drying and size reduction (attrition). As waste isintroduced into the bed, most of the organics vaporize pyrolytically and are partially combusted in the bed. Typical desired operating temperatures range from 900° to 1000 °C.

A circulating fluidized bed (CFB) is differentiated from a bubbling fluid bed in that there is no distinct separation between the dense solids zone and the dilute solids zone. The capacity to process different feedstock with varying compositions and moisture contents is a major advantage in such systems.  

Emerging Trends

Gasification with pure oxygen or hydrogen

Gasification with pure oxygen or pure hydrogen (or hydrogasification) may provide better alternatives to the air blown or indirectly heated gasification systems. This depends greatly on reducing the costs associated with oxygen and hydrogen production and improvements in refractory linings in order to handle higher temperatures. Pure oxygen could be used to generate higher temperatures, and thus promote thermal catalytic destruction of organics within the fuel gas. Hydrogasification is an attractive proposition because it effectively cracks tars within the primary gasifying vessel. It also promotes the formation of a methane rich gas that can be piped to utilities without any modifications to existing pipelines or gas turbines, and can be reformed into hydrogen or methanol for use with fuel cells.

Plasma gasification

Plasma gasification or plasma discharge 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. Plasma gasification has two variants, depending on whether the plasma torch is within the main waste conversion reactor or external to it. 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.

Thermal depolymerization

Such processes use high-energy microwaves in a nitrogen atmosphere to decompose waste material. The waste absorbs microwave energy increasing the internal energy of the organic material to a level where chemical decomposition occurs on a molecular level. The nitrogen blanket forms an inert, oxygen free environment to prevent combustion. Temperatures in the chamber range from 150 to 350⁰C. At these temperatures, metal, ceramics and glass are not chemically affected. 

Conclusion

A solution to the waste problems confronted by municipalities requires a strategy that integrates several technologies including, waste reduction, recycling, landfilling and waste-to-energy. Waste-to-energy, which converts the non-recyclable and combustible portion of the waste to electricity, reduces the amount of materials sent to landfills, prevents air/water contamination, improves recycling rates and lessens the dependence on fossil fuels for power generation. Another area that would increase the viability of waste gasification is the improvement of waste sorting and pre-treatment methods. Preparation of a homogenous RDF remains one of the most difficult tasks in thermochemical conversion of solid waste. It involves a large amount of mechanical processing and close supervision, which greatly impact operating costs and can account for as much as 40% of the total plant capital costs. If shredding and sorting of the waste can be made simpler and more effective, gasification would become even more advantageous. Similarly, waste gasification will be most successful in communities where there is good recycling practice. A better job of recycling glass and food wastes by city residents will improve the gasification reactions.

Salman Zafar is an independent renewable energy advisor with vast expertise in biomass energy, waste-to-energy conversion, anaerobic digestion, municipal solid waste management and renewable energy systems. Apart from managing the renewable energy advisory firm, BioEnergy Consult, he has alliances with several leading international companies and non-governmental agencies to foster sustainable energy solutions worldwide. Salman is a prolific writer with many publications to his credit. His articles have been appearing in reputed journals, magazines and web-portals on a wide array of topics related to renewable energy and waste management. Salman hold Masters degree in Chemical Engineering from Aligarh Muslim University, Aligarh (India). He is based in India and can be reached at salman.alg@gmail.com