**1. Introduction**

Climate change is an urgent problem of our time since it threatens the equilibrium of our planet and, with it, the livelihood of billions of people and species [1]. Our reliance on fossil fuels, urbanization, population growth, and the increase in municipal solid waste (MSW) have influenced climate change, prompting us to reconsider how we produce and consume energy [2]. Global gross final energy consumption was 370EJ in 2017, with oil accounting for 38.7%, coal 20.3%, natural gas 21.1%, nuclear 2.1%, and renewables 17.8% (13% biomass, 3% hydro, 0.9% wind, 0.7% solar and 0.23% geothermal). The gross final energy consumption in continents in 2017 in renewable energy was: Africa 54.5%, Americas 16.0%, Asia 15.9%, Europe 12.7%, Oceania 11.9%, and the world 17.8% [3]. These figures demonstrate the dominance of fossil fuels over renewables. Therefore, substantial work remains to be done to shift the balance toward renewable energy and prevent climate change's effects.

Intergovernmental organizations and policymakers are the cornerstones of combating climate change, looking for cost-effective change by shifting toward nonconventional energy sources, for example, the EU emissions trading system (EU ETS) [4]. In this regard, some countries have formally stated a deadline to stop all coal

burning. For example, the UK has set a deadline of 2025 to phase out coal use and, to expedite the process, has imposed a carbon tax of £18 per ton of carbon dioxide equivalent [5, 6]. It is worth noting that the UK remains in the EU ETS until December 31, 2020, aligning with the withdrawal agreement [7]. Likewise, Netherlands and Italy plan to phase out coal burning by 2030 and 2025 [6]. Other countries, such as Portugal, have already phased out coal use. Indeed, it completed the project 2 years ahead of schedule (from 2023 to 2021), and it now intends to use the coal-burning facilities to generate green hydrogen [8, 9].

All efforts to accelerate the phase-out of coal are also favorable from an economic point of view since the cost of emitting greenhouse gases continues to rise as policymakers increase their efforts to curb pollution-induced climate change. Carbon Pulse predicts that EU carbon prices will triple by 2030, reaching €90 [10]. Other forecasts place the price of CO2 equivalent at between €32 and €65 per ton by 2030 [11]. **Figure 1** illustrates the upward trend in carbon emission futures prices over time.

In light of rising carbon prices, renewable energy sources appear viable for meeting global energy demand while reducing the reliance on fossil fuels in the energy sector [14]. As companies are phasing out coal burning, they turn to biomass combined with other feedstocks like MSW, reducing carbon footprint. The technology to pass from coal to biomass is already mature [6]. Indeed, Valmet upgrades old units based on bubbling fluidized bed (BFB) or circulating fluidized bed (CFB) technology or converting existing grate, oil, or pulverized coal boilers to BFB. The latest Valmet solution is biomass gasification, which partially replaces fossil fuel with biomass and RDF on a large scale, providing fuel flexibility and decreasing CO2 emissions economically [15]. Gasification is a thermochemical conversion of carbonaceous materials into a combustible gas through partial oxidation and oxidizing agents, namely air, vapor, oxygen, or carbon dioxide [1, 16]. Gasification has numerous advantages over combustion, such as larger molecules being completely broken down into syngas. Gasification has an oxygen-deficient atmosphere. Thus, it prevents the formation of furans and dioxins since their formation demands enough oxygen. Another advantage is that the resulting syngas can produce energy or chemicals like ammonia [17].

Despite the advantages of gasification over combustion, some elements remain to improve, particularly analyzing biomass blends with other feedstocks (cogasification) [18]. Co-gasification offers additional advantages. For example, RDF can

**Figure 1.** *Carbon emissions futures price (euros/ton) [12, 13].*

#### *Application of the Six Sigma DMAIC Methodology to the Gasification Process DOI: http://dx.doi.org/10.5772/intechopen.111850*

blend materials with little added value. It also reduces CO2 emissions by avoiding the extraction of new fossil fuels. Furthermore, blends with RDF improve the overall feedstock's LHV since RDF has a higher LHV. It also increases the CH4 and C2H4 concentrations, decreasing CO concentration, which may be related to the interaction between the thermal cracking of the plastic and the catalytic ashes contained in RDF [19]. Finally, blends of RDF with biomass dilute some negative features of the RDF char, like high ash and chlorine contents, allowing its energetic valorization in existing gasification facilities [20].

Another improvement element is business process optimization, an integrated activity to make business processes manageable, reaching the best asset utilization and performance through measurable factors like efficiency and quality [21]. Fortunately, it is unnecessary to reinvent the wheel to improve efficiency and quality since many methodologies and tools for business process optimization have been designed and proved successful, for example, Six Sigma DMAIC [22] and the design of experiments. DMAIC is an essential part of the Six Sigma methodology that can be executed independently as a quality improvement method. Define, Measure, Analyze, Improve, and Control [23].

Six Sigma DMAIC methodology improves the bottom line of a product, service, or process by reducing waste and resources and increasing customer satisfaction. Although many believe Six Sigma aims to reach Six Sigma levels of quality, the truth is that Six Sigma and DMAIC aim to improve profitability. Therefore, efficiency and quality are excellent value by-products of its correct implementation [23]. According to Mikel Harry (the creator of Six Sigma), six areas drive its implementation [23]: (1) basic organizational capabilities, (2) industrial process variations, (3) business process variation, (4) engineering/design process, and documentation, (5) quality of specifications, and (6) supplier capabilities.

Even though the Six Sigma DMAIC methodology has proven successful in improving a process, it remains a considerable gap between gasification and Six Sigma DMAIC in the literature because little information or null is available. In this regard, the objective of this chapter is to explore the synergies between gasification and Six Sigma DMAIC by (1) Analyzing the Six Sigma DMAIC, history, and achievements, (2) proposing an integrated Six Sigma DMAIC framework for continuous incremental improvement and optimization of co-gasification, enhancing efficiencies of the overall process, and (3) analyzing the effects of blending biomass with refuse-derived fuels (RFD).

To achieve that, a set of experimental co-gasification runs was performed, changing blending percentages and equivalence ratio (ER) to maximize energetic efficiency based on Low Heating Value (LHV) and gas composition.

## **2. Six Sigma DMAIC methodology**

Six Sigma has been used, tasted, and adapted to different industries and businesses from 1985 until now, optimizing processes and improving profitability. **Figure 2** [24–33] shows a historical background of the evolution and the use of Six Sigma in different fields, among them mechanical design, electrical design, manufacturing, value creation, environmental sustainability, education, etc.

A quantum leap in Six Sigma occurred in 2000 when Mikel Harry published the book *The Breakthrough Management Strategy*. That book provides a strategy called The Breakthrough Management Strategy that gathers the experiences of 15 years to reach Six Sigma Quality through a highly efficient method. In other words, Six Sigma is the

**Figure 2.** *Gasification history.*

Land of Oz, and the Breakthrough Strategy is the Yellow Brick Road. This strategy is based on eight phases: Recognize, Define, Measure, Analyze, Improve, Control, Standardize, and Integrate (RDMAICSI) [23]. The five core phases are called DMAIC, which may implement as a standalone method [34].

The Define phase aims to understand the why of the project and what it is intended to reach. In this phase, the objectives and scope must be defined [35–37]. In the Measure phase, the applicable measurement systems and tools focusing on data

collection and reporting are reviewed to identify the opportunity for improvement and the baseline performance. Critical variables are measured and collected in this phase [35, 38, 39].

The Analyze phase provides statistical methods and tools to isolate critical information that will expose the number of defective products. Here, practical business problems are shifted into statistical problems, and it is glimpsed the cause of the problems and possible solutions [35, 38]. Next, the Improve digs into the key variables that cause the problem. It may also encompass the tool Design for Six Sigma (DFSS) to guarantee a complete understanding of the problem or customer's requirements and expectations before design completion or selection of the optimum solution [3, 6, 7]. Finally, the Control phase sustains the Six Sigma initiative through continuous monitoring to avoid falling into the same problem [3, 6, 7].

The application of Six Sigma DMAIC has been quite extensive since it has proven successful in guiding companies to reduce mistakes in day-to-day operations, focusing on eliminating or reducing lapses in quality at the earliest possible time of occurrence by implementing quality control programs to detect and correct commercial, industrial, and design faults [23]. In addition, the correct implementation of Six Sigma DMAIC results in an economic benefit. For example, Motorola's savings was \$15 billion over 11 years, General Electric's savings of 2 billion, Honeywell's Savings of \$1.2 billion, Texas Instruments'savings of \$600 million, Johnson & Johnson's savings of \$ 500 million, among many more [40].

Companies have adopted the Six Sigma DMAIC methodology to improve their processes and margins. Before Six Sigma, improvements in quality programs or process improvements usually had no evident impact on a company's net income. Organizations that cannot track the effect of quality improvements on profitability do not identify what must be changed to increase their profit margins. Thus, implementing this methodology for gasification might bridge the gap between Six Sigma and gasification to the continuous incremental improvement and optimization of gasification, enhancing efficiencies of the overall process. **Table 1** shows examples of applying the Six Sigma DMAIC methodology in some processes and equipment, namely boilers, heat exchangers, ovens, compressors, cooling towers, etc.



#### **Table 1.**

*Application of the Six Sigma DMAIC methodology in processes and equipment.*
