*4.3.5 Source of variation*

The variance in the Aveiro's Pilot-scale Bubbling Fluidized Bed Reactor for gasification can be visualized by monitoring the temperature and syngas composition:

1.Temperature: It is a crucial process variable that modifies the syngas composition, and its variance might result from the following improvement opportunities.

Cooling system: Gasification is partial oxidation when an excess char is produced. It is also oxidized and produces CO2, H2O, and heat, increasing the reactor's temperature. The current equipment has a complicated cooling system comprising 16 small pipes along and around the reactor, supplying and returning cooling water (**Figure 2**). When the temperature increases, the operator introduces a few centimeters of some 16 pipes. This kind of technology depends entirely on the operator's expertise. Thus, the temperature variance of the reactor will change from operator to operator. Furthermore, some 16 small pipes are inaccessible to the operator, clearly a poor engineering design.

Equivalence ratio: To produce an oxidation reaction, it is necessary to have oxygen, so the temperature will also depend on the equivalence ratio. A single flowmeter indicates oxygen and airflow, so the chosen equivalence ratio will be inaccurate. Furthermore, flowmeters are just indicators. Thus, the flow control is performed by partially opening or closing a valve. Therefore, the flowmeter measurement adds an error to the flow ratio because an operator performs this opening and closing of the valve. Hence, the precision depends on how well an operator's eyesight is.

Refuse-derived Fuel Composition: The composition of RDF is given by moisture, volatile matter, fixed carbon, elementary composition, impurities, and ashes, and those elements might impact the temperature by promoting specific exothermic reactions.

2. Syngas Composition

Equivalence Ratio: The equivalence ratio corresponds to the ratio between the oxygen content in the oxidant supply required for complete stoichiometric combustion. Usually, ER is between 0.2 and 0.4. ER < 0.2 results in incomplete gasification, excessive char formation, and low calorific value of the product gas. Whereas ER > 0.4 results in excessive formation of CO2 and H2O, rather than CO and H2, it also decreases the calorific value of the gas.

Refused Derived Fuel: The composition of RDF is given by moisture, volatile matter and fixed carbon, elementary composition, impurities, and ashes, and those elements might impact the gas composition by promoting the production of products with low calorific value.

The following fishbone diagram (**Figure 11**) helps to understand the source of the variance.

The Analyze phase offers statistical methodologies and tools for isolating vital data that will reveal the number of defective products. Business issues are transformed into statistical problems, exposing their causes and potential

**Figure 11.** *Fishbone diagram.*

solutions. The process settings, experimental findings, efficiency metrics, and gas composition standard deviation are detailed in **Table 3**. In contrast, **Table 6** presents an FMEA that hints at alternative solutions to the source of variance.
