**4. Explosion risk: formation of potentially explosive atmospheres**

Because of the presence of methane in its composition, biogas in combination with air can form potentially explosive atmospheres (**Table 1**). In Europe, safety measures against explosion risk are stipulated in Atex Directives 99/92/EC [24] and 2014/34/EU, which have inspired the preparation of checklist section, referred to the explosion risk. A crucial topic, reported in safety checklist, is the classification of plant areas [25], where explosive mixtures could be generated by biogas releases. This classification has to be carried out in terms of zones (Zone 0, Zone 1 and Zone 2), geometrical characterization (extent and volume) of hazardous areas [26] and persistence time:


Directive 99/92/EC states that, places where potentially explosive atmospheres can occur are marked with specific signs (**Figure 1**), which are characterized by the following distinctive features:

• triangular shape and

in the Directive 2000/54/EC [16]. Examples of control measures are exhaust ventilation to prevent exposure, adequate filters on the air intakes of vehicles (such as tractors used to move biomass) and personal protective equipment, such as suitably fitted respiratory devices, when

Design of workplaces and work processes, the choice of adequate equipment and working methods allow the control of occupational biohazard in the biogas plants. Any activities involving the movement of biomass and/or waste should be controlled, and site design and activities should be managed to avoid organic dust and/or bioaerosol release in the workplace. In particular, the biomass, such as silage, should be stored in closed silos or in platforms provided with containment walls and covered by a plastic material wrap. Livestock slurry storage tank should be equipped with immersion agitators to avoid air contamination, and moreover, the automatic transfer of slurry into the digester should be guaranteed by a pumping system. Working areas, where biomass is moved, should be considered as potential high exposure zones. An efficient system of forced ventilation is required if high-exposure activities are conducted within a confined space and, where practicable, employees should only work in these areas within a suitably controlled environment, such as a vehicle cab, or wear appropriate respiratory protective equipment (RPE). It is recommended that for exposure to bioaerosols, RPE is provided with the highest efficiency filters (P3). The replacement of the filters in the vehicle cabs' air handling system, cleaning of vehicle cabs and the instructions given to operators not to open cab doors and windows and remain in the vehicle have a significant effect on workers' exposure levels. These rules should be applied within a radius of 50 m from the operational areas, considering that bioaerosol levels typically return to background concentrations within this distance [23]. Such requirements clearly have an impact on site design and layout. In order to achieve these targets, the employers should amend working practices and operations and relocate office accommodation and welfare facilities to an area outside the potentially high-exposure zones. Dust control from the movement of vehicles is also recommended, and roadways should be properly constructed so that they can be cleaned and a vehicle wheels washing system should be planned. The workplace should be provided with adequate hand-washing and shower facilities and 'clean areas' in order to ensure that no contamination can affect external places. Employers should undertake an appropriate health surveillance of their workforce to ensure that early signs and symptoms of diseases, related to exposure to biological agents, are managed and reported. This may involve simple health screening or more detailed assessments, involving health questionnaires, lung function and blood serum test. All employees, who have undertaken health surveillance, should have a personal health record and the information must be kept for a period of 40 years and the findings of any health surveillance should be communicated to employees and any adverse findings should be deeply investigated and appropriate controls should be adopted. The training of site managers and personnel is a fundamental topic in order to verify the design and implementation of these prevention measures. It must be stressed that appropriate instructions, information and training, referred to the potential risks to their health and how they should be controlled, must be given to employees. Employers should also develop procedures for

working in areas close to where bioaerosols are generated.

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people who do not comply with the procedures and site rules.

**3.2. Prevention and protection measures for occupational biohazard**

• black letters on a yellow background with black edging

In **Figures 2** and **3**, the classification procedure of hazardous areas (outdoor and indoor place) is shown. It may be used as a basis to support the proper selection and installation of work equipments in hazardous zones. Classification of indoor places is particularly important because ventilation system design plays a fundamental role in order to dilute the potentially explosive atmosphere in the shortest times.

The first step of classification procedure consists of locating the potential sources of biogas release. On this subject, it has to be remembered that catastrophic elements failures are not considered as potential sources because they are beyond the concept of abnormality [27], reported in Technical Standards.

A plant component, such as valves, flanges, pumps, compressors, and so on, is considered as a potential source when its failures are expected during the operation. Zone classification depends on source release grade, ventilation degree and availability. Release grade


**Table 1.** Properties of gases.

**Figure 1.** Sign (zone where potentially explosive atmospheres can occur).

(continuous, primary, secondary) is determined by the analysis of element operating conditions [28]. On the contrary, ventilation degree depends on the volume of explosive atmosphere, which is strongly influenced by biogas mass flow. This last parameter depends on gas outflow typology (sonic or subsonic), which is determined by the comparison between critical pressure (pcr) and atmospheric pressure (pa ):

> hazardous places with regard to formation of explosive mixtures. In a biogas production plant, a potentially dangerous zone is the container (indoor place), which includes combined heat and power (CHP) unit (**Figure 4**). In indoor places, in order to assess the ventilation degree, sources emission contemporaneity must be considered. This is a necessary condition aimed at calculating average biogas concentration (Xm%) in indoor areas. Xm% depends on source release grade and can be calculated according to IEC 31-35 (Technical Standard). In case of continuous grade emis-

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*Qa* ⋅ *ρgas*

In case of primary and secondary grade releases (temporary period is considered), Xm% is

⋅ 100 (1)

sions (temporary period can be negligible), Xm% is calculated by the following equation:

*Xm* % <sup>=</sup> *Mgas* \_\_\_\_\_\_\_

/s) indicates ventilation air flow; and

) is the biogas density.

calculated by the following expression:

• Mgas (kg/s) is biogas mass flow;

**Figure 2.** Classification of an outdoor place.

where:

• Qa (m3

• ρgas (kg/m<sup>3</sup>


Ventilation degree can be high or medium for outdoor places, whereas it can be high or medium or low for indoor places. Three levels of ventilation availability are reported in Technical Standard (EN 60079-10-1):

Good: ventilation is continuously present;

Fair: ventilation is expected to be present during normal operation and its discontinuities are permitted, but they have to occur infrequently or for short periods and

Poor: ventilation, which does not meet the standard of fair or good.

Ventilation availability can be good or fair for outdoor zones, whereas it can be good or fair or poor for indoor areas. For outdoor places, this parameter depends on local minimum wind speed. If wind speed is bigger than 0.5 m/s, ventilation availability can be considered as good. For indoor areas, in order to assess ventilation availability, reliability of artificial ventilation system and presence of standby fans or an emergency ventilation plant has to be ensured. In case of fan failure, good availability usually requires automatic start-up of standby fan(s). Indoor areas are the most Biogas Production Plants: A Methodological Approach for Occupational Health and Safety... http://dx.doi.org/10.5772/intechopen.72819 191

**Figure 2.** Classification of an outdoor place.

hazardous places with regard to formation of explosive mixtures. In a biogas production plant, a potentially dangerous zone is the container (indoor place), which includes combined heat and power (CHP) unit (**Figure 4**). In indoor places, in order to assess the ventilation degree, sources emission contemporaneity must be considered. This is a necessary condition aimed at calculating average biogas concentration (Xm%) in indoor areas. Xm% depends on source release grade and can be calculated according to IEC 31-35 (Technical Standard). In case of continuous grade emissions (temporary period can be negligible), Xm% is calculated by the following equation:

$$X\_m \%= \frac{M\_{yu}}{Q \cdot \rho\_{yu}} \cdot 100\tag{1}$$

where:

(continuous, primary, secondary) is determined by the analysis of element operating conditions [28]. On the contrary, ventilation degree depends on the volume of explosive atmosphere, which is strongly influenced by biogas mass flow. This last parameter depends on gas outflow typology (sonic or subsonic), which is determined by the comparison between

Ventilation degree can be high or medium for outdoor places, whereas it can be high or medium or low for indoor places. Three levels of ventilation availability are reported in

Fair: ventilation is expected to be present during normal operation and its discontinuities are

Ventilation availability can be good or fair for outdoor zones, whereas it can be good or fair or poor for indoor areas. For outdoor places, this parameter depends on local minimum wind speed. If wind speed is bigger than 0.5 m/s, ventilation availability can be considered as good. For indoor areas, in order to assess ventilation availability, reliability of artificial ventilation system and presence of standby fans or an emergency ventilation plant has to be ensured. In case of fan failure, good availability usually requires automatic start-up of standby fan(s). Indoor areas are the most

permitted, but they have to occur infrequently or for short periods and

Poor: ventilation, which does not meet the standard of fair or good.

):

critical pressure (pcr) and atmospheric pressure (pa

**Figure 1.** Sign (zone where potentially explosive atmospheres can occur).

• pcr > pa → (sonic outflow)

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• pcr < pa → (subsonic outflow)

Technical Standard (EN 60079-10-1):

Good: ventilation is continuously present;


In case of primary and secondary grade releases (temporary period is considered), Xm% is calculated by the following expression:

$$X\_{\rm{m}}\,\%\,=\,\frac{M\_{\rm{go}}}{Q\_{\rm{s}}\cdot\rho\_{\rm{go}}}\cdot\left(1-e^{-c\,\mathrm{d}\_{\rm{i}}}\right)\cdot 100\tag{2}$$

**Figure 4.** Combined heat and power unit (indoor place)—Source: Maccarese S.p.a.

**High Medium Low**

Continuous (Zone 0 NE) (Zone 0 NE) (Zone 0 NE) Zone 0 Zone 0 Zone 0 Zone 0

Primary (Zone 1 NE) (Zone 1 NE) (Zone 1 NE) Zone 1 Zone 1 Zone 1 Zone 1 or

Secondary (Zone 2 NE) (Zone 2 NE) Zone 2 Zone 2 Zone 2 Zone 2 Zone 1 or

**Good Fair Poor Good Fair Poor Good, fair or** 

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Zone 2<sup>a</sup> Zone 1<sup>a</sup> + +

Zone 2<sup>a</sup> Zone 2<sup>a</sup> + +

**poor**

Zone 0<sup>b</sup>

Zone 0<sup>b</sup>

Zone 2 Zone 1

Zone 2 Zone 2

**Ventilation**

**Availability**

Non-hazardous

Non-hazardous

Non-hazardous

Zone 0 can be generated in poor states of ventilation.

Nonhazardous zone<sup>a</sup>

Zone 0 NE, 1 NE or 2 NE indicate areas, which have negligible extents.

zone<sup>a</sup>

zone<sup>a</sup>

zone<sup>a</sup>

Glossary: '+' means 'surrounded by'.

**Table 2.** Classification of hazardous zones.

a

b

**Release grade Degree**

where:


**Table 2**, which is reported [26] in EN 60079-10-1, is used to classify the hazardous zones.

**Figure 3.** Classification of an indoor place.

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**Figure 4.** Combined heat and power unit (indoor place)—Source: Maccarese S.p.a.


Glossary: '+' means 'surrounded by'.

*Xm* %  <sup>=</sup> \_\_\_\_\_\_\_ *Mgas*

<sup>E</sup> (s) is the release duration.

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**Figure 3.** Classification of an indoor place.

• C (s−1) represents the number of fresh air changes per time; and

where:

• t

*Q <sup>a</sup>* ⋅ *ρgas*

**Table 2**, which is reported [26] in EN 60079-10-1, is used to classify the hazardous zones.

⋅ (1 − *e* <sup>−</sup>*C*⋅*<sup>t</sup>*

*<sup>E</sup>*) ⋅ 100 (2)

a Zone 0 NE, 1 NE or 2 NE indicate areas, which have negligible extents.

b Zone 0 can be generated in poor states of ventilation.

**Table 2.** Classification of hazardous zones.
