Gasification as a Method of Disposing Horticultural Waste (Environmental Technology Essay)

Essay question 1

Gasification as a Method of Disposing Horticultural Waste.

Gasification is a thermochemical conversion technique involving turning carbonaceous materials into gaseous products by use of a gasification agent and at very high temperatures. This is done under conditions of limited oxygen. Gasification yields gases like carbon monoxide, carbon dioxide, nitrogen, and hydrogen gases as the largest fractions.


How Gasification Works

Gasification is a four-step process. The first step involves dehydration of the feedstock. Water content in the feedstock is evaporated and can be used in later processes. The next step involves pyrolysis, which involves exposure of the feedstock to very high temperatures. this results in the breaking of chemical bonds and devolatisation. This leads to the release of gases such as methane, tar vapors, and hydrogen. Combustion takes place next which provides the energy required for gasification. The last process involves the reaction of the remaining char with carbon (ii) oxide and steam to yield hydrogen and Carbon monoxide.

The Five Processes of Gasification - ALL Power Labs

By-products of Gasification Process

The by-products of gasification are either gaseous or liquid. The gaseous products are methane and hydrogen. Water in the form of steam is also an important by-product.

Factors That Affect the Gasification Process

The moisture content of the biomass influences the effectiveness of gasification. A moisture content of ~5% is suitable for gasification. The ratio of the carbon, hydrogen, and oxygen elements is an important factor to consider. Feedstock with low lignin and cellulose are recommended for gasification.

Advantages of the Gasification Process

It is easier to control emission in gasification because the produced syngas is at a higher temperature and pressure. And hence it is easier to control emissions. Gasification can capture carbon iv oxide more efficiently and at a lower cost compared to conventional technologies.

Also, gasification reduces the amount of solid residues. Much of the feedstock is turned into gaseous byproducts hence the remaining amount of residues is small. It is an energy-efficient technique for reducing the volume of solid waste and the recovery of energy.

Gasification is a process that produces a wide range of products that can be utilized to perform other functions. Examples of the by-products of gasification include gasoline, ethanol, formaldehyde, acetic acid, DME. Some of the products, upon separation, can be converted to heavy and light tars, solvents and fertilizer.

Essay Question 2.

How Different Factors Affect the Process of Pyrolysis

Moisture content in the feed will affect the heat transfer process in pyrolysis which will give undesirable effects on gas production. High moisture content, for example, leads to a decrease in gaseous products.

High amounts of volatile matter in biomass lead to increased production of biofuel through pyrolysis. volatile matter determines the efficiency of conversion of biomass to fuel.

The amount of fixed carbon and char affect the amount of char formed after devolatilisation. An increase of fixed carbon and ash component would increase the amount of char formed.

Deriving Moisture and Ash Content for Wood Samples

Moisture is lost and evaporates at 1000C, so to obtain the moisture content we will use the weight change at 1500C.

Sample A; 14.60 is the value for the amount of moisture contained within the wood sample.

Sample B; 4.80 is the value for the amount of moisture contained within the wood sample.

Deriving Ash Content

Ash is obtained when the wood sample is burnt down completely. So to obtain the ash content, we add up all the weight changes and subtract it from 100%.

Sample A; 14.60+25.15+23.75+10.20= 73.7

100-73.7 = 26.3

Value for Ash content for sample A is 26.3

Sample B; 4.80+33.15+34.25+15.70= 87.9

100-87.9= 12.1

Value for Ash content for sample B is 12.1

Determination of Volatile Matters and Fixed Carbon for both samples

Determination of volatile matters involves subtracting the moisture when the samples were heated from the moisture when the biomass fuel is heated.

%VM=m1- m2     x100


Fixed carbon is determined by

%FC= 100- %VM- %Ash

Where the %VM is the percentage volatile matters

                     %ash is the percentage in ash contents.

So, to obtain the percentage of fixed carbon and volatile matters, we add up the values of moisture and ash content, then subtract it from 100%

Sample A; 26.3+14.60= 40.9

100-40.9= 59.1

The percentage of fixed carbon and volatile matters is 59.1 for sample A

Sample B; 12.1+4.80= 16.9

100-16.9= 83.1

The percentage of fixed carbon and volatile matters for sample B is 83.1

Which Sample Has a Higher Energy Content of Biomass?

Sample B has a higher energy content due to a higher amount of fixed carbon and volatile matters than sample B.

Essay question 3

Determining the Type of Gasification Reactor.

The downdraft is gasifier type A while the updraft is gasifier type B.

Working Principles of Gasifier Type A and Type B

The downdraft version of the fixed bed gasifier works on the fact that the product gas outlet is located on the opposite side to the air inlet. Air is introduced in the reactors at a high velocity creating a jot zone with temperatures rising above 15000C due to the combustion of part of the char. The remainder of the char is gasified to carbon iv oxide in the next zone. The heat from the combustion is conducted to the pyrolysis zone where fresh biomass is pyrolysed when passing.

The updraft version of the fixed bed works in a counter-current mechanism, where the biomass flows in opposite directions to the gasification agent, for example, the air in this case. They are way simpler than downdraft gasifiers. The biomass is added at the top of the gasifier whereas the gasifying agent is added at the bottom of the gasifier on the opposite side of the gasifier. The fuel then goes through different levels in the reactor with different levels of temperature. The product gas flows out without interacting with the biomass.

Advantages and Disadvantages of Gasifier Type A and Type B.

The advantage of the updraft gasifier is that it has a small pressure drop in comparison to B. it is also highly efficient in the gasification process. The main disadvantage is that it produces a lot of tar. The advantage of the downdraft gasifier is that it has a lower amount of tar in comparison to the updraft gasifier. This is because tar is broken down in the oxidation zone of the gasifier. Its main disadvantage, however, is its lower efficiency due to the counter-current mechanism. It results in the expenditure of higher temperatures with lesser than optimum results.

Recommendation for Use of the Updraft Gasifier

I highly recommend the use of the updraft gasifier due to its good qualities and easy working mechanism in comparison to the downdraft gasifier.

I would recommend gasifier type A which is the updraft gasifier. They have fixed bed biomass. They can cover a wide range of fuels and fuel sizes. They can take up biomass with moisture contents as high as 60%.  The relatively low temperature of the raw syngas is suited to the gas cleanup units. It also has a small pressure drop in comparison to the downdraft gasifier.

Essay Question 4

Anaerobic digestion is the natural process that breaks down degradable organics in the absence of molecular oxygen to release biogas. It results from a series of biological pathways that lead to the breaking down of food waste.

Anaerobic digesters operate under mesophilic, thermophilic, or even psychrophilic conditions. Hydrolysis is the process by which complex molecules are converted to simple sugars, amino acids, and short-chain fatty acids. These processes a catalyzed by specific enzymes.  The process of anaerobic breakdown of material also involves acetogenesis,  acidogenesis and methanogenesis. Acidogenesis involves the conversion of the products of hydrolysis into volatile fatty acids. These then are converted to acetic acid by the process of acetogenesis. Later on, in methanogenesis, methane, water, and carbon iv oxide are produced.

Factors that affect the working of this process include PH, temperature, loading rate, retention time, and mixing, just to name a few. The breakdown of these biochemical compounds requires bacteria that are PH-specific. Large-scale digesters have optimum digestion with temperatures ranging from 20- 450C. the loading rate is the measure of the biological conversion capacity of the system. Overloading leads to system failure through inadequate mixing.

Calculating the energy of Biogas produced

Energy content= biogas ( Nm3) x methane content (%) x energy value of methane (9.97 kwh/Nm3)

Plant A;   35×58  x9.97 = 20239.1 kwh

Plant B; 16X55X9.97= 8773.6 kwh

Calculating Electricity Yielded

Energy =power x time

Plant A; 20239.1 = f x 8400 hours

Power= 2.409 joules

Plant B; 8773.6= f x 8400 hours

Power = 1.0445 joules

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