Mass Balance for Biomass and Biogas Production From Organic Waste

Running header: Mass Balance for Biomass and Biogas production1

Mass Balance for Biomass and Biogas production from Organic Waste

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Mass Balance for Biomass and Biogas production 2

Introduction

The recent rise in global oil prices and excessive environmental degradation has brought

new ideas and concepts to the fore. Renewable energy in all areas of applications such as

electricity and fuel has emerged. The global direction in terms of fuel and energy needs has now

fully shifted to innovation and research in renewable energy. While these energy sources have to

be environmentally green, other ideals such as economic sustainability have to be considered. In

light of these issues, several governments and private organizations have delved deep into the

foray of renewable energy. Perhaps one of the most preferred energy renewable energy sources

is biomass and biogas. There is increasing advocating for the adoption of biogas as a renewable

energy sources due to its natural processes of production and easily available raw materials.

Animal and plant wastes are easily available and the biogas produced can be used for several

purposes right from domestic such as cooking and lighting to industrial uses such as electricity

production. Biogas production has been found to have two basic advantages besides its energy

uses. One of the advantages that accrues on biogas is that is provides a cheaper means of waste

disposal and secondly, the outcome of the process may be used to enrich soils for agriculture.

There are different kinds of plants that can be used in biogas production and they include

sugarcane waste, switch grass, corn silage and wet algae. These different organic materials

require different processes before the production process and have different yield potentials. This

paper reviews the different biogas production processes and potentials of organic products after

undergoing anaerobic digestion.

Theoretical framework of biogas productionThe concept of biogas production is pegged on the idea that organic compounds have

chemical bound energy and with controled release, the energy can be harnessed for important


uses. Hutňan, et al. (2010) explains that while most organic wastes may appear as solids, they

contain up to 90% moisture. Thermo-incineration may be used in the disposal of such wastes but

environmental concerns dictate that such a process cannot be used hence the idea of

biodegradation.

The process of biodegrading can be done either with or without air. The process of

treating organic waste with air is referred to as composting while the process of treating the

waste without air is referred to as digestions (Melegari, et al., 2012). Composting is a relatively

cheap process of degradation that takes much little time and is in fact a very cheap process

leading to the production of CO2 and compost (Salerno, Nurdogan, & Lundquist, 2009). On the

other hand, anaerobic digestion is a much slower and complex process that can only be used on

selected organic waste. However, the result of anaerobic digestion is methane rich gas that can

be used as an energy source. Johansson & Burnham (2005) define anaerobic digestion as:

“a process of controlled decomposition of biodegradable materials under

managed conditions where free oxygen is absent, at temperatures suitable for

naturally occurring mesophilic or thermophilic anaerobic and facultative

bacteria and archaea species, that convert the inputs to biogas and whole

digestate“.

There are four major steps in anaerobic digestion, that is, hydrolysis, acidogenesis,

acetogenesis and methanogenesis. The process of hydrolysis is the first step in the anaerobic

digestion process. The idea of anaerobic digestion relies on microorganisms (introduced bacteria)

breaking down the chemical components of the organic waste in order to cause a reaction that

will lead to the production of the required gasses (Jin, Bierma, & Walker, 2012). However, at the


beginning of the process, the organic wastes are usually large organic compounds that must be

chemically broken down into smaller micro molecular compounds that will allow intended

molecular reactions. The process of breaking the macromolecular structure of the organic

compounds is referred to as hydrolysis (Jin, Bierma, & Walker, 2012). Hydrolysis can be

achieved through either through chemical, mechanical or thermal pretreatment of the organic

waste. With the additional use of hydrolytic microorganisms and/or combined with chemical

catalytic reactors, the hydrolysis process results in acetates and hydrogen.

The acetates and hydrogen can be directly used as methanogens, however in some cases

molecules such as volatile fatty acids with longer chain lengths may have to broken down

further. The process of breaking down volatile fatty acids into compounds that can be used as

methanogens is referred to acidogenesis (Ahn, et al, 2010). This process is done by introducing

acidogenic bacteria (also referred to as fermentative bacteria).

After volatile fatty acids have been broken down into ammonia and hydrogen sulphide,

the next step is to break down these products further into acetic acid. This process is referred to

as acetogenesis (Ahn, et al, 2010).

The final stage of anaerobic digestion is methanogenesis where methanogenic archaea

converts the products of the other processes in order to obtain methane, CO2 and water (Ahn, et

al, 2010). These compounds are therefore the objective of the whole process and methanogenesis

has been found to the PH sensitive. The process is best carried out at PH level between 6.5 and 8

(Salerno, Nurdogan, & Lundquist, 2009). The remaining solid which cannot be broken down by

the microorganisms together with the dead bacteria make up the solid digestate. The diagram

below depicts the process of anaerobic digestion


B

Source: Johansson & Burnham ( 2005)

The entire processes and aspects regarding anaerobic digestion are extremely wide and

complex. This research will not delve further into this process rather the next section reviews the

different potentials in the organic wastes.

Sugarcane

The production of biogas from sugarcane waste is one of the most common methods of

biogas production that utilizes anaerobic digestion. Several research and scholarly works indicate

the biogas production process based on different nations and economies. In Brazil for instance,


commercial sugar cane is used for the production of ethanol and sugar (Melegari, et al., 2012).

The ethanol is used for both domestic purposes and industrial uses as gasoline for vehicles. The

country is one of the earliest adopters of ethanol powered cars since 1973 and the country is

considering further progress in this foray. Thus over the recent past, industrial production of

sugarcane has increased by 47% reaching about 570,000 million tonnes (Melegari, et al., 2012).

With regard to biogas, the residue of the juice extraction process is necessary. Normally,

in order to extract the juice from sugar cane, the harvest is subjected to high mechanical

pressures and temperatures. The remaining material, referred to as vinasse, is produced at high

temperatures and has a high chemical oxygen demand (OCD), high polluting power and organic

matter. Due to the extremely high potassium level in the vinasse, several of Brazil’s sugar plants

use the vinasse as fertilizer and directly apply to the fields (Melegari, et al., 2012). However, as

the vinasse may be good fertilizer, it may be pollutants and thus governments are keen in

restricting direct use of vinasse as fertilizer. The use of high amounts of the vinasse may be

drained into rivers causing pollution to the water used by several parts of the population.

Thus to treat vinasse before disposal, the process of anaerobic digestion is preferred and

the results are biogas and bio fertilizer. Melegari, et al.( 2012) indicates that organic digestion of

vinasse leads to the production of the organic power and a bio fertilizer of the same richness as

before anaerobic digestion process. The biogas produced has been found to contain 40-75%

methane, CO2 of between 25 and 40% and other compounds.

The production of biogas from sugar cane is quite commendable. Melegari, et al.( 2012)

assets that in industrial sugar cane plants, the production of 1 litre of ethanol, produces about 10

liters of vinasse. If the vinasse is subjected to anaerobic digestion, about 1m3 of vinasse produces


about 14.6m3 of biogas. According to the study by Melegari, et al. (2012), the biogas produced

with 55% methane has an energy value of 20.8 MJm-3. Further studies indicate very impressive

estimation when it comes to biogas potential from sugarcane waste. Either Ahn, et al. ( 2010)

notes that while the biogas is very high, it is further augmented by the fact that the material used

is a byproduct of an equally economically impressive process. The use of sugarcane has the

inherent advantage that the pretreatment process has been achieved at a negligible cost. In

addition to this impressive process is the fact that the end product is methane rich gas and an

equally nutrient-rich bio-fertilizer.

Switch grass

There is increasing interest in biogas production from animal feeds. Animal feeds such as

switch grass have a high organic energy and can be used either a co-digester or main digester in

the biogas production. Switch grass (Panicum virgatum) is a productive North American native,

perennial warm season grass. The perennial productive switch grass can be used as for ethanol

production through cellulosic processes. Other processes such as thermal combustion and

thermochemical conversion may also be used in the process of obtaining the organic energy.

Ahn, et al. (2010) argues that research has increased in the possibility of using switch grass in

anaerobic co-digestion process in biogas production. Co-digestion can be done with either

sewage or manure solids.

The idea of anaerobic digestion of switch grass is grounded on the fact the switch grass

has a high organic energy and thus a high potential for biogas production. In fact Ahn, et al,

(2010) postulate that switch grass cells’ are primarily composed of cellulose and hemicelluloses.

The estimates have it that the cellulose concentrations ranges from 273 to 322 g kg-1 dry matter


(DM). Similarly, the hemicelluloses concentration ranges from 235 to 279 g kg-1 dry matter

depending on the age of the plant material (Ahn, et al, 2010). When the cell wall components

undergo the hydrolysis process, sugars are produced which can easily be converted into methane

gas by anaerobic digestion. Hence, in the production of biogas using switch grass, the rate at

which the hydrolysis takes places determines the rates of the entire process. Zupančič & Grilc,

(2012) argue that most biogas plants that use switch grass tends to utilize dry anaerobic digestion

and this has been found to the most economically viable digestion process.

Ahn, et al, (2010) decries the pretreatment stage of working with switch grass. Since the

plant is not a raw material for another process, the pretreatment stage is mechanically

demanding. This is so because the process of anaerobic digestion requires that the switch grass

be ground into fine particles before subjecting it to the digestion process. The process of

obtaining finely ground switch grass particles is energy intensive and may be cost prohibitive to

undertake. Johansson & Burnham, (2005) purport that while some may argue that the process of

grinding must not be very fine, other experiments have arrived at the conclusion that coarsely

ground grass particles do not digest very well.

Corn silage

There is also increasing interest in the use of maize as a substrate for anaerobic digestion

for the production of methane gas. The idea of using corn is grounded on the fact that several

corn plantations are very huge and such farms could use the additional energy from biogas

plants. Maize is an energy crop just as other crops used in biogas plants and can be used both as

a grain and as silage. However, maize grains are not the best substrate for use in biogas plants

due to the cost implications. Most of maize plantations have the primary goal of ensuring that the


end product is pure dry grains. Though, on some occasion the market price for dry grains tends

to come so low such that there is economic justification for sales.

The use of silage has however been the preferred substrate for anaerobic digestion. Jin,

Bierma, & Walker (2012) argue that the use of silage has the advantage in the fact that fresh or

ensiled silage has no significant difference in terms of biogas production.

The production of methane gas in using silage has been experimented in the laboratories.

In the experiment done by Hutňan, et al., (2010) batch laboratory reactors achieved a specific

methane production of 0.270-0.289 Nm-3kg-1 of the TS. Flows reactors produced much lower

levels of methane gas.

Hutňan, et al.(2010) studied the production of methane gas by using corn silage based in

two different methods in the anaerobic digestion process. Ideally, corn silage can produce

methane gas whether the process undergoes acidification process or not. There were comparisons

between acidified and nonacidified silage. In the comparison, it was found that anaerobic

digestion of nonacidified maize was stable. However, acidified maize silage had several

advantages over the nonacidified maize. For instance, the methanogenic reactor start-up period

was much shorter as compared to nonacidified silage. It was also established that methane

production from acidified maize was higher by 7% as compared to the nonacidified corn silage.

Acidified silage also produced much lower amounts of sludge as compared with nonacidified

maize. Finally, the biogas produced by acidified silage had higher methane content as compared

to non acidified corn silage (Hutňan, et al., 2010). In general therefore, while maize grains and

silage can be used for the production of methane gas, maize grains may be seen to be quite an

expensive process. Still, the use of silage may pose the problem of reactor stability because the


low nitrogen content in the silage makes the anaerobic digestion process unstable. This problem

may be solved by introducing reactor agent with high nitrogen content to stabilize the anaerobic

reaction process.

Wet Algae

The production of biogas from wet algae has also been a topic of discussion and review

for quite a considerable period of time. Waste grown microalgae has been found to have a high

potential for bio-fuel and biogas such as methane. Processing of the algae could begin from

harvesting such algae from ponds and using such algae for fuel extraction or anaerobic digestion

for methane gas production. However, much of the technology that would allow lipid extraction

for bio fuels is still under development and therefore anaerobic digestion is perhaps the best

means of making use of organic energy in the wet algae.

Salerno, Nurdogan, & Lundquist (2009) indicate the use of wet algae is methane gas

productions have been found to have low yields. Estimates show that wet algae yield as low as

0.25-0.50 L CH4/g volatile solid introduced (Salerno, Nurdogan, & Lundquist, 2009). This was

considered a low yield as compared to waste water sludge and other types of energy crops. It was

established that much of the algae remained undigested to a tune of 45% (Johansson & Burnham,

2005). The reason for such low yields was attributed to the resistive cell walls that work against

the digestion by the bacteria introduced. The cell walls created a ring around the cells of the plant

ensuring that degradation by bacteria was reduced.

Studies have indicated that increasing temperatures during the pretreatment stage may

reduce the cell wall resistivity of wet algae. Thermochemical pretreatment of green microalgae at


about 100 degrees Celsius for a period of 8 hours increased the methane yield by one third

(Salerno, Nurdogan, & Lundquist, 2009).

There was a problem of inhibitory ammonia concentration that may also lead to the low

methane yield in the anaerobic digestion of wet algae. The high protein content in the wet algae

of about 40-50% or a C: N ratio of 6:1 has been found to considerably contribute to the high

ammonia concentration levels during the digestion process. To counter the high ammonia

concentration, the wet algae should be co-digested with high carbon, low nitrogen substrates

(Salerno, Nurdogan, & Lundquist, 2009). This will reduce the ammonia toxicity levels in the

tanks as well as increase the output of ammonia per unit reactor tank.

There is a cost implication when it comes to using wet algae as main substrate for

anaerobic digestion. The first cost implication is the harvesting process, in that harvesting the

algae is both time consuming and the harvested quantities are much lower as compared to the

other energy crops. Either, the long pretreatment hours required to increase the methane yield is

cost prohibitive. With 8 hours of continuous thermo chemical treatment that does not result in

any other product is rather repugnant.

Conclusion

Recent changes in the global social, political and economic aspects of the world have

created the need to adjust to renewable source of energy. The use of renewable energy sources

will spur sustainable growth as well as maintain the energy needs of ever growing urban centers.

Several renewable sources of energy such as solar, wind, geothermal, hydro power and now

biomass and biogas have been bought to the fore. Biogas, in particular has been considered as

possible preferred technology in renewable energy as it makes use of easily available materials


and the outcome can used for several purposes. Biogas plants produce methane rich gas by

digesting energy crops using a process called anaerobic digestion. Different energy crops have

different yields as well as different processes required for the pretreatment process. Sugar cane

and corn silage have the least pretreatment procedures and tend to depict higher biogas yields.

On the other hand, switch grass and algae may have similar yields however high pretreatment

costs have proved to be quite prohibitive. In this regard, corn silage or sugar cane has been found

to be most effective energy crops for biogas productions.

References


Ahn, et al. (2010). Evaluation of Biogas Production Potential by Dry Anaerobic Digestion of

Switchgrass–Animal Manure Mixtures. Appl Biochem Biotechnol 160 , 965–975.

Hutňan, et al. (2010). Biogas Production from Maize Grains and Maize Silage. Polish J. of

Environ. Stud. Vol. 19, No. 2 , 323-329.

IEA Bioenergy. (2010). Algae – The Future for Bioenergy? Summary and conclusions from the

IEA Bioenergy ExCo64 Workshop. IEA Bioenergy ExCo64 Workshop.

Jin, G., Bierma, T., & Walker, P. (2012). Biogas production from switchgrass under

experimental conditions simulating U.S. digester operations. J Environ Sci Health , 470-

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Johansson, T. B., & Burnham, L. (2005). Renewable energy: sources for fuels and electricity.

Boston, MA: Thomson.

Melegari, et al. ( 2012). Production potential of biogas in sugar and ethanol plants for use in

urban buses in Brazil. Journal of Food, Agriculture & Environment Vol.10 (1) , 908-910.

Salerno, M., Nurdogan, Y., & Lundquist, T. J. (2009). Biogas Production from Algae Biomass

Harvested at Wastewater Treatment Ponds. Written for presentation at the 2009

Bioenergy Engineering Conference. Washington: ASABE.

Zupančič, G. D., & Grilc, V. (2012). Anaerobic Treatment and Biogas Production from Organic

Waste. In S. Kumar, Management of Organic Waste. ISBN: 978-953-307-925-7,.

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Mass Balance for Biomass and Biogas Production From Organic Waste