**3. Methodology**

To calculate the potential to generate biogas from waste created during the beef production chain and the reduction of GHG in different scenarios, various hypotheses and parameters were considered based on the ultimate inventory of Brazilian greenhouse gas emissions [12, 25] and the research carried out by D'Avignon [26], as described below.

The slaughter age of animals fed only on pasture is around 36 months4 , and is inversely proportional to the increased intensification of the productive system, being reduced to 18 months with confinement or 26 months with semi-confinement.

In Brazil, the average time of confinement after weaning is 4 months, though it is possible to expand this to six months, in the driest period, between May and October, while for semi-confinement, the average time can be increased by two months, rising to three months.

It is estimated that biogas with 60% methane is generated because of animal waste per day (4.05 m3 /animal/day). This value was calculated based on the methane information emitted per head of cattle, in accordance with the latest national inventory of greenhouse gas emissions [12].

For the generation of biogas from the effluents produced during animal slaughter, the value of 7,4872 m3 /head slaughtered5 was estimated by D'Avignon [26], for a large part of abattoirs existing in the country in 2010, responsible for approximately 90.8% of cattle slaughtered in the country. In this research the need for electric energy in the productive process was estimated at 39 kWh per head of cattle slaughtered.

Biogas can thus be generated by two distinct sources: the waste produced by the animal and the effluents generated during animal slaughter. However, with the use of bio-digesters, neither are altered in relation to enteric fermentation, irrespective of changes in the productive system. Intensive and extensive systems emit methane, but with a difference in the number of days. The lower the number of days, the lower the emission of methane due to enteric fermentation. Considering all parameters and hypothesis, biogas production potential is given by the following equation:

<sup>5</sup> This calculation was based on the sum of the potential of biogas with cattle fat and rumen of around

<sup>4</sup> There are cases in some parts of Brazil with a slaughter age above 36 months, and even reaching

<sup>44</sup> months. However, in most of Brazil, with an extensive system, the slaughter age is around 36 months.

*Biogas Generation from Bovine Confinement: An Energy Policy Option for Brazil DOI: http://dx.doi.org/10.5772/intechopen.99828*

$$\mathbf{g}\_{\text{biaya}} = \left[ \left( \mathbf{N}\_{\text{conf}} \right) \* \left( \mathbf{number of days} \right) \* \left( \mathbf{q}\_{\text{out\\_fr}} \right) + \left( \mathbf{N}\_{\text{ab}} \right) \* \left( \mathbf{q}\_{\text{bar}} \right) \right] / FC \left( \mathbf{m}^{\flat} \right) \tag{1}$$

where:


If the biogas is applied to the generation of electricity, there is an increase in the abatement of emissions through the reduction of the use of non-renewable sources and many GHG emitters, normally used by a large part of the agricultural sector. This potential reduction also should be calculated in relation to the grid. The use is proposed of the emission factors calculated by MCTI [27] and applied in the inventories existing for the 2006–2015 period. To analyze the years between 2000 and 2005, the factor observed in 2006 was applied, and for future scenarios the authors proposed the use of the MCTI study [25] which calculated the factors for 2020, 2025, and 2030, while for 2015–2020, 2020–2025, and 2025–2030, the calculation will be based on the linear tendency in each period, measuring the annual rates of emission factors in the initial and end years. From this the emission factor results are obtained for the national grid, as described in **Table 2**.

In the case of projections for electricity demands, it is proposed to use the average rate calculated by EPE [4] with a growth in energy sources projected until 2050 for the agricultural sector. According to EPE [28], per year, the electricity consumption of the agricultural sector will be almost 47,101,500 MWh6 . This is equivalent to an increase of around 42.95% in electricity consumption in the 2015–2030 period. Between 2015 and 2030, the annual rate of growth in the consumption of electricity is estimated at 3.81%. These estimations are important to measure how biogas can contribute to meet the growing demand for electricity in the agricultural sector.

<sup>6</sup> In the study made of projections for energy demand, EPE [10], the total consumption of energy (without detailing the type of source) is given in millions of TEP for the year 2030. In the distribution chart for each source, it is shown that the share of electricity will be 27% in 2030. Based on this information and knowing that the conversion of TEP to kWh is 11.63x103, the result is obtained in kWh and afterwards multiplied by 103 to determine the final value in MWh.

