Energy recovery during incineration

Energy recovery system is necessarily integrated with incinerator to utilize the generated heat energy. The overall efficiency of the incinerator is depended on the utilization of the recovered energy. The heat energy generated in the furnace can used for district heating as well as electricity generation. District heating has less revenue but higher efficiency, while lower efficiency shows on electricity generation.

The flue gases generated in the furnace have a high temperature of 1,000^oC-1200^oC. This temperature should be lower down before the gases passed on to the cleaning system. The cooling process shall be implement with a boiler, and the heat energy inside the gases can be transferred into water and become steam.  Then, the energy can be recovered to provide district heating and high-pressure steam can be pumped into turbine to generated electricity. It should be noticed that the feasibility of steam store is low, which means consumers should be contracted to the manager before the incineration project. As to the production of electricity, it is possible to generate it directly by incinerate prepared waste in pressurized fluidized bed.

Meanwhile, Water that may be used to cool down the furnace during the incineration should be recycled. Energy recovered from the water can be utilized for in-plant used straightforward. Moreover, the steam generated is possible for backup electric-driven equipment as well.

waste incineration

To convert the waste components into benign or useful materials, one of suitable solutions is using the thermal processing methods. These kinds of methods are implemented by elevating the temperature in designed equipment. Both pyrolysis and incineration are included in the thermal processing measures. However, the process occurred when the temperature increased in these two processes is totally different. Incineration requires the presence of substantial quantities of oxygen in the process, while pyrolysis requires the absence of oxygen.

Table 1. Calculation of volume reduction by various solid waste disposal systems 

	Original Volume as Fractions	Reduction Factor	Final Landfill Volume as Fraction as Original Volume
Sanitary Landfill
Combustible Waste          Bulky and Noncombustible Waste          Total Waste	0.8 0.2 1.0	0.166 0.5	0.133 0.100 0.233
Sanitary Landfill with Shredding and Resource Recovery
Total Waste	1.0	(0.1252)	(0.125)
Conventional Incineration
Combustible Waste          Bulky and Noncombustible Waste          Total Waste	0.8 0.2 1.0	0.0145 0.5	0.012 0.100 0.112
Incineration with Shredding and Resource Recovery
Total Waste	1.0	(0.0082)	(0008)
Pyrolysis Processes with/without Resource Recovery
Total Waste	1.0	(0.004-0.03)	(0.004-0.03)

In the past, incineration was popular in where the landfill capacity is insufficient. In practice, incineration converts the waste into flue gas, and heat with adequate quantities of air. A well-designed incinerator can significantly reduces both the weight and volume of the waste and produce residue that can be used as a fill material in construction business. As shown in the table 1, the pyrolysis process is the most efficient method to reduce the volume of the waste. Incineration shows less performance than pyrolysis, but still have a high efficiency on size reduction of waste.

In theory, the combustion gases should be comprised by carbon dioxide, water, nitrogen and oxygen entirely. All of these gases are normal constituents in the air. However, other constituents usually exist in the flue gas as well, including carbon monoxide and some other hazard gases. These gases are mostly generated due to the inefficient combustion. Thus, air pollution control equipment is necessary in the incineration plant. Then, gases discharged to the atmosphere can be treated to reach governmental standards for emission of chemical constituents as well as particulates. Moreover, the residue after the process should low heating values.

Before initiate the incineration process, it is important to consider several factors firstly. Air supply is always the most crucial factor in incineration design. Only fully circulating surplus airflow can introduce complete combustion and minimize the generation of pollution and hazard like carbon monoxide. The second factor is that different incinerator has different requirements to the refuse. Therefore, refuse preparation is sometimes necessary for some kinds of incinerators. After the incineration process is finished, the gas cleaning process is important to deal with the flue gases. Removing the pollutants like dioxin, furans and other emissions in the gases is compulsory for industries according to relevant legislation codes. Otherwise, these pollutants can cause harmful impacts on local and regional environment. Finally, the residue from the grate usually contains metal and it should be separated from the residue and be stored. Remaining ash may be used as an aggregate replacement or be disposed to landfill.

landfill gas collection

To utilize the landfill gas, extraction wells will gather the landfill gas from landfills at first like shown in below. Besides, landfill gas can also be extracted through horizontal trenches instead of vertical wells and both systems are effective at collecting. The figure of the horizontal trenches is attached as Appendix 5. The main collection header may connect to the leachate collection system for condensation collection purpose. And usually, a blower is needed to pull the gas from the collection wells to the collection header and further downstream. Since the landfill gas can increase the global warming greatly, the collection of the landfill gas must be processed carefully. However, 4 to 10 percent of landfill gas will escape the collection system inevitably. Hence, the capture technology still needs improvement to achieve higher capture efficiency.

For further utilization, a treatment system is necessary to remove the impurities, condensate and particulates inside the gas. When the gas is collected for direct use, it can be fuel of boiler, dryer, and process heater as an alternative to natural gas with cheaper price but lower heating value. Or it can go to power infrared heaters to provide power to local greenhouses. As for electricity generation, the landfill gas is usually introduced to a gas turbine or internal combustion engine.

s Flaring. Bristol: Environment Agency. 2002

Landfill gas

Gases including methane are usually generated with the accumulation of waste in the landfill, which is a result of the biological decomposition of wastes placed in. Among the composition of the landfill gas, the methane and carbon dioxide are the main components. As mentioned earlier, the methane is important as it has a great calorific value which gives rise to energy generation benefits. MSW landfill gas typically has roughly 45 to 60% methane and 40 to 60% carbon dioxide. With proper separating, the methane can be extracted from the landfill gas and become an alternative energy for fossil fuel. Moreover, it should be noticed that other minor components such as H₂S, NO_x, SO₂, CO, NMVOCs, PAHs, PCDDs, PCDFs are all harmful to human health at high doses.

In fact, the awareness of utilize the energy in the landfill gas actually has a long length of history. It is important to emphasize that the interests in landfill gas collection in the past are resulted by concerns for protecting adjacent properties, and also due to the escalating prices of energy. Besides, the reasons for extracting energy from waste are the same as the landfill gas collection. However, with the new concerns on climate nowadays, the interests in landfill gas utilization have become much more intensive now.

Anaerobic Digestion 2 - Stages

The main process of the anaerobic digestion is consisted of two phases, and there are three stages in second phase. The schematic chart is provided below.

The first phase is referring to hydrolysis, which is producing the necessary acid for bacteria. Usually, only large organic polymers like carbohydrates, fats, cellulose, and proteins make up the organic fraction of the waste. The microorganisms cannot utilize these polymers directly until they are broken down. To initiate the first phase, extracellular enzymes are introduced to break down the high-molecular-weight polymeric components into soluble components for next stage. In details, the enzyme amylase, cellulase and cellobiase break down the carbohydrates and cellulose to glucose; protease breaks the proteins into amino acids; lipase breaks down the fats to fatty acids. In addition, bacteria expend more energy than is salvageable during the first phase. By the end of the first stage, all the large polymer should be break down for bacteria metabolic reactions.

When the acids are available after the first stage, the second phase should be initiated. Three steps are concluded in this phase, which are acidogenesis, acetogenesis and methanogenesis. In the process of acidogensis, the acids are caused into further breakdown by biological process. Acidogenic bacteria are used and digest the sugars, fatty acids and amino acids into carbon acids, alcohol and hydrogen carbon dioxide. And then at the stage of actogenesis, acetogens breakdown the products from the preceding process into simple soluble molecules such as acetic acid and carbon dioxide.

The production of the methane is occurred in the final step, which is called as methanogenesis. In this step, soluble materials are converted into methane, carbon dioxide and water.  The equations of main reactions are listed below.
CH3COOH ------> CH4 + CO2                                      (acetic acids)
4CH3CH2COOH + 2H2O ------> 7CH4 + 5 CO2                                    (propionic acid)
2CH­­3CH2CH2COOH + 2H2O ------> 5CH4+ 3 CO2                   (butyric acid)
2CH3CH2OH ------> 3CH4 + CO2                                            (ethanol)
CH3COCH3+H2O------>2CH4+CO2                                         (acetone)


It can be seen that all the fermentations of different acids, ethanol and acetone has the same products including methane and carbon dioxide. Acetic acid is the most prevalent organic acid intermediate found in the methane fermentation of fats, carbohydrates, and proteins and about 70 percent of the methane produced in the subsequent anaerobic fermentation results from its degradation.

According to Van Niel, the organic compounds fermented by methane bacteria are completely oxidized to carbon dioxide, and this oxidation process is followed by a reduction process in which some or all of the carbon dioxide is reduced to methane (Van Niel – carbon reduction theory – 1938) Relevant equations is shown below.

Oxidation                     CH2COOH + 2H2O----à2CO2+8H
Reduction                    8H+CO2----àCH4+2H2O

Net                       CH3COOH----àCH4+CO2)

Anaerobic Digestion 1

As we all know, there is certain part of energy exists in the waste. To extract this part of energy, anaerobic digestion can be a choice. Anaerobic digestion is a phenomenon that some types of bacteria break down the biodegradable materials under specific requirement on the absence of oxygen. While the organic maters are decomposed, biogas like methane can be provided alternatively. In fact, anaerobic digestion has several hundreds years of history. It is started since Robert Boyle and Stephen Hale discovered the flammability of the gas generated while disturbing the sediment in 17th century. Due to its flammability as well as high heating value, the biogas generated during the digestion is considered as an alternative energy for fossil fuel. Also, the price for conventional fuels like oil has continued to rise, so biogas has the economically feasibility in the energy market. Since the certain types of microorganisms oxidize the organic wastes during anaerobic digestion, the size of the waste will be decreased as well. However, these species of microorganism cannot digest nonorganic components actually. So refuse preparation is important in this technology as well as in the incineration. Most of industries waste is not recommended to be dealt by anaerobic digestion, while sewage slurry is much more suitable in this case since it has a large fraction of organic constituents.

 Except the absence of oxygen, the proper operation of anaerobic digestion also requires other conditions including suitable moisture content, pH and alkalinity. The details of these factors are given below in the second section. The bacteria metabolize the glucose, amino acids and fatty acids to organic acids, and by products of carbon dioxide, hydrogen, ammonia and H2O. the amounts and types of organic acids produced vary and are not readily ascertained.

Get know about 'Green Energy'

With the development of the society, all the countries notice that using only fossile fuel can causes big problem like the glabal warming and also else. So how to replace fossile fuel with minimum side effect become ugent to governments. Some countries use natural gas and nuclear power to produce electricity or other energy that they want, but it seems the side effects are also obviously. Therefore, green energy, also know as renwable energy, starts to be noticed. (In addition, there is a debate about wether nuclear power is renwable energy or not.)

So, as you can see, there are many kinds of renewable energy show in the figure above. But there are also different limitations for each kind of renewable energy. For instance, solar power only can be used efficiently at the place where receives plenty of solar energy. In the same way, wind power also has a minimun requirement for the wind speed. Moreover, the relevant equiments fot these renewable energy also needs proper maintenance. In the UK, wind power is the most popolar one, it supplied almost 10% of eletricity demand for the country. At the coastal cities, wave power is also used commonly to produce electricity. And solar power is very mature technology now which is actually ustilized all over the coutry now.

Therefore, it can be seen that renewable energy becomes more and more important to out world now. It will not only reduces the effects of global warming, but also can propduces certain part of energy demand for each country.