The system was designed to be inherently safe; that is, to operate at atmospheric pressure with a mild updraft to a flue stack to draw flammable gases through the combustion zone where they are ignited and through a secondary flare mounted in the stack before they are discharged to atmosphere. However, there were also a number of safety issues.
The control objectives of the process were to:
1. Ensure safe operation of the process by combusting all products of incomplete combustion
2. Maintain T-004 at the heating-side set point temperature of 400, 500 or 600 °C 3. Monitor T-002 which will provide process information for implementation of the
control strategies (see Section 5.5)
Additional monitoring features include:
3. Ensuring no polycyclic aromatic hydrocarbons (PAH’s) form and to limit particulate matter and NOx emissions from the system
4. Ensuring the char meets quality standards (not discussed here)
Points 3 and 4 are not part of the control strategy as the temperature is being controlled to affect these outcomes; however, testing of these will be conducted during the
commissioning phase to ensure the standards are met.
The variables that can be manipulated in this process are:
x The tertiary air flow (either ON or OFF) which controls the secondary air and draft in the system, over and above the natural draft to the system
x The LPG flow rate to the base main burner – the burner has four operation modes: OFF, low (6 kW), medium (23 kW) and high (6 kW and 23 kW)
x The LPG flow rate to the pilot burner in the combustion chamber – the pilot burner has two modes of operation: ON or OFF
The flue (being sufficiently high) creates a natural system draft and a slightly negative system pressure. This is also augmented by the tertiary air which enters the flue stack and creates a draw to bring in additional secondary air. It is important that PSystem < Patm for safety reasons i.e. to prevent egress of flammable gases from the system; all gases are drawn through the base burner then up through the stack flare giving two opportunities for
products of incomplete combustion to be destroyed. This removes the possibility of an explosion occurring. The differential pressure sensor, PI-001, in the base section will monitor the pressure in the system. The design of the secondary air inlets as open holes in the base plate (with an area equal to 2 % of the basal area) provides an environment where the base of the system is essentially open to the atmosphere.
The temperature sensor T-004 will be used to monitor the flue temperature and will be used to control the base burner heat output. The amount of LPG will be adjusted accordingly. If the flue temperature gets too high, then the LPG will be decreased and then the tertiary air flow can be increased. This will increase the secondary air and cool the system. All these potential scenarios can be found in the hazard and control operability in Table 5.2 below.
To ensure pyrolysis gases were completely combusted, a variety of approaches were taken. The expected proportions of gases are given in Table 5.1 above with ranges of minimum and maximum expected proportions. Early on in the heating phase only water vapour will flow out of the reactor as the wood chips dry. Later, beyond 280 qC, pyrolysis gases will evolve. Their exact flow rate depends on the rate at which heat penetrates the particle bed inside the reactor. These gases will be drawn through the base burner and the flue stack flare as explained above. Both the burner and flare pilot flame are independently controlled and are always burning. The flammability limits of the gases are contained in Table 5.1; these refer to each flammable gas concentration in air required to maintain a naked flame. The draw from the stack and the tertiary air entering at the venturi will, at low flow stages of the process, dilute the flammable gas concentrations below the flammability limit; however, this is not a problem as the gases will pass through two independently controlled burners, the base burner and the flare. The flare ensures complete combustion.
The base burner combustion zone will operate at a T>850 °C, which will ensure no PAHs form but less than 1200 °C to prevent NOx formation. The flare system will not exceed this temperature either; the temperature sensor, T-006, will be installed above the flare to monitor this situation and, if exceeded, the tertiary air will be adjusted.
Another safety feature to consider when designing a pyrolyser, is the security of ignition in order to combust all flammable gases, where it is necessary to maintain a pilot light (pilot- 001 and pilot-002) and/or a constant spark system. The pyrolysis off gases will contain volatiles as aerosols and soot particles; there is a risk of a soot and aerosol cloud forming
and to the overflow vent which opens a flap that is gravity weighted. The gases will take the path of least resistance and so the force of the explosion will be vented upwards and away from bystanders.
A spark arrester in the form of wire mesh was placed around the flare at the top of the flue. This will catch for any burning solid char particles that manage to pass through the system and to ensure no sparks leave the system.
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Table 5.2: Control scenarios
Scenario TFlue (°C) T-004 TCore (°C) T-002 3° Air Supply LPG Supply
LPG flow rate is too high, no volatiles
production >700 <200 Increase Medium Decrease Low Decrea requ Incre
The chamber is too hot as pyrolysis gases are being produced. We will decrease the 3° air to get products of incomplete combustion
so the temperature doesn't get too high. They will instead be combusted at the flare.
>700 200<T<SP Increase Medium Decrease Low The L dri produc of t
Volatile production has finished but the soak time has not been meet so turn on the LPG
to finish the process
>700 200<T<SP Increase Medium Increase High Increa LPG is
The LPG is on full and pyrolysis gases are being produced increasing both the flue and
core temperatures above their set points
>700 >SP Increase Medium Decrease High Increas and re Dec tempe
The burner is on full as the flue temperature is less than 700 °C and the core
temperature is less than 100 °C
<700 ≤ 100 Decrease Med/High Increase High The bu has it
Volatiles are being produced and the LPG supply is on low <700 200<T<700 Increase Low/Med Increase or no change Syng needs reach the L
No volatiles are being produced and
the LPG supply in so low <700 <700 Decrease Low Increase High
Syng point is
sup
The inner core temperature is
around the set point temperature N/A ~SP No change No change
Synga is
Carbon monoxide levels after the flare are too high