Comparative technical-economical evaluation of gasification-based treatment of municipal solid waste

doi:10.21203/rs.3.rs-4770408/v1

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Comparative technical-economical evaluation of gasification-based treatment of municipal solid waste

https://doi.org/10.21203/rs.3.rs-4770408/v1

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A technical-economic analysis of municipal solid waste (MSW) gasification was carried out in the commune of Chillán, Chile. The MSW production was quantified and characterized in the 2015–2018 period. The percentage characterization of MSW corresponded to organic matter (61%), other waste (17%), plastic (10%), paper and cardboard (8%) and glass (4%). In the analysis, the countercurrent fixed-bed gasification technology was selected, due to the simplicity of operation and less difficulty in controlling the operating parameters. Flow diagrams and gasification mass and energy balances were developed incorporating three preliminary processing options for the wet fraction of MSW: biodigestion, drying and pressing, prior to gasification. Total energy efficiencies were 54.2%, 54.6%, and 61.4% respectively. Finally, a preliminary economic analysis was carried out considering income and costs for the three process alternatives. The approximate annual gross profits were estimated at 6,462,000 US $ ∙ year^− 1 for press-gasification, 6,139,000 US$ year^− 1 for drying-gasification and 4,600,000 US$ year^− 1 for biodigestion-gasification.

Gasification

electrical energy

energy balance

income

The global generation of urban or municipal solid waste (MSW) has increased considerably in recent decades. Currently this figure has grown to an equivalent of 1.2 kg of MSW inhab^− 1 day^− 1. For the year 2025 an increase in MSW production is expected, reaching 1.42 kg MSW inhab^− 1 day^− 1 [1]. Some of the causes of this increase correspond to high population growth, consumer habits in industrialized countries, and also changes in the habits of consumers in developing countries. In 2018 Chile already generated 7.7 Mt MSW year^− 1 [2]. Various MSW treatment techniques have been developed [3], such as landfill disposal, incineration, recycling, composting and biodegradation for energy purposes [4]. Additionally, gasification technologies are an alternative that has advantages over the previously mentioned technologies [5], since there is no formation of furans or carcinogenic dioxins compared to the incineration of MSW, there is also no free discharge of polluting gases into the atmosphere and it does not discriminate carbonaceous material such as biodigestion, which only works with organic waste. Gasification corresponds to a thermochemical process in which a carbon-rich substrate is transformed into a gas through the application of heat, directly or indirectly and in the presence of gasifying agents to generate a mixture of gases, called gas of synthesis or syngas, composed mainly of CO, CH₄, CO₂, N₂, H₂ [6]. Syngas is generated through a set of reactions that includes moisture vaporization, pyrolysis, combustion and reduction [7]. The gasifying agent is a gas, or a mixture of them, that injects oxygen to thermochemical reactions. Different gasification agents can be used, the most common being air, oxygen, carbon dioxide and water vapor [8], the latest having a direct effect on the economy of the process, since steam can generate an increase in cost with respect to of the air [9]. Air is the most common gasification agent; however its use tends to dilute the syngas, reducing its calorific value [10]. The syngas produced from solid waste can be classified into three categories according to its calorific value (CV), these are: low CV (3.5 to 10.0 MJ m^− 3), medium CV, (10.0 to 20.0 MJ m ^− 3) and high CP (20.0 to 35.0 MJ m^− 3). Low calorie syngas includes a significant amount of N₂ or CO₂ and can be used in combustion processes, where the fuel gas does not need to be of very high purity. The medium CV syngas is suitable for use in gas turbines. Finally, the high CV syngas is useful besides gas turbines, in production of synthetic natural gas and different fuels production [11].

The production of CO and H₂ was studied, with respect to five variants of gasifying agents, demonstrating the benefit of using the proportion 21% O₂/79% H₂O to obtain greater amounts of combustible gases. The total volume fraction of H₂ and CO decreased from 51.7–49.7% with a particle size that increased from 20 < d < 30 mm to 80 < d < 100 mm in a vapor atmosphere. With a increase in temperature from 600°C to 1000°C, the total volume fraction of H₂ and CO increased from 56.1–65.8%. In this case, the maximum production ratio of H₂ and CO occurred between 750 and 950 ° C. The optimal operating temperature would be between 800–850°C [12]. The cogasification of MSW with biomass, specifically Panicum virgatum [13], was also evaluated. Three proportions 0, 20 and 40% of MSW were applied, obtaining a syngas CV of 6.2, 6.5 and 6.7 MJ m^− 3 respectively. In a joint gasification study of wood, coal and plastic waste in a fluidized bed gasifier, it was found that the CV of the synthesis gas produced increased from 5.1 MJ m^− 3 to 7.9 MJ m^− 3 when the content of plastic waste in the mixture increased from 0–100% [14]. Another investigation used a gasifier with a moving grid and the RSM was introduced into the reactor without any separation or prior grinding. The basic composition of the synthesis gas corresponded to H₂, CH₄ and CO, with CV between 1.9 and 10.2 MJ kg^− 1 [15]. The moisture content has a significant effect on the effectiveness of the thermochemical conversion, because when increasing the moisture content from 0 to 40%, the CV of the MSW, decreases by 66%. Generally, the desirable moisture content for the gasification application is less than 20%. When it is greater than 60%, a secondary treatment is necessary to reduce humidity, such as an extruder [10]. In the particular case of Chile, the moisture content of MSW is between 40–60% [16].

The general objective of this study was to develop a technical-economic evaluation of a possible gasification and electricity generation plant, using Municipal Solid Waste (MSW) in the commune of Chillán, Chile.

2.1 Quantification and characterization of waste

In this stage, historical statistical antecedents of Municipal Solid Waste generation in the Chillán commune were compiled from especially annual declarations of waste in the single window system of the National Pollutant Emissions and Transfers Registry, during the period 2015–2018. The composition of these wastes was analyzed in terms of percentage of organic matter, paper, glass, among others, and their quantitative trends. For this purpose, consultations were made to the corresponding Department of Environment, Cleaning and Ornament of the Municipality of Chillán, together with the review of the documentation available via web in this regard.

2.2 Selection of the technology

According to the quantity and composition of the MSW, a technology approach was selected among the different gasification methodologies currently used commercially in the world, which have been successfully implemented, including upstream, downstream and circulating fluidized bed gasifiers [17]. Among the technological options, the one that presented greater compatibility or affinity with the characterized waste was selected and justified, considering the complexity factors or parameters influencing the operation of the reactor, among which MSW particle size, temperature of operation, type of gasifying agent, MSW moisture content, calorific value of syngas and quality of syngas. To achieve the selection of gasification technology, a comparative table was prepared, in order to facilitate the choice and justification based on the aforementioned technical terms.

2.3 Mass and energy balances

Flow diagrams were constructed for three possible process alternatives based on the moisture content of the MSW, since the water content in the sample affects the energy efficiency of the gasification process, reducing the calorific value of the fuel. The three alternatives correspond to: 1) Biodigestion of the wet material and gasification of the dry material (BG), 2) Drying of the wet material until the optimum moisture content is achieved for the gasification technology and then gasify all the material (DG), 3) Pressing of wet matter until reaching the optimum moisture content for gasification technology and then gasification of the whole material (PG). The mass and energy balance indices were evaluated for each of the flow diagrams considered, quantifying the efficiency of the processes, estimated as the energy produced over the input energy to the energy generation system, thus selecting the most efficient and suitable alternative. In the energy balance, the index 0.6 m³ of methane per kg of wet MSW [18] and 3.49 kWh of syngas per kg of dry MSW [19] was used.

2.4 Preliminary economic evaluation

For each of the process options energetically evaluated, a preliminary economic evaluation of a possible municipal project for the treatment of MSW in the commune of Chillán was carried out, considering operating costs for each kWh of energy produced. For the calculation of costs, a unit cost of 0.0576 US$ for each kWh of electricity produced by biodigestion and 0.036 US$ for each kWh of electricity generated by gasification [20]. In drying, a cost of US$ 17.0 per ton of water removed was also considered [21]. In the case of pressing, a value of 2.3 US $ per ton of dry solid was considered [22]. In relation to revenues, these were estimated based on the sale price of each electrical kWh produced [23]. To determine the revenue of the project, the monthly costs associated with the project were compared with the costs of the current treatment system for the Chillán MSW, which corresponds to the sanitary landfill [24]. Once the comparison was made, the profits of the alternatives were calculated to estimate the monthly or annual profits of each of the proposed alternatives.

3.1 Quantification and characterization of RSM

The generation of MSW in the commune of Chillán in t year^− 1 shows annual increases according to municipal declarations covering the 2014–2018 period, presenting an average of 70784.42 per year. The detail and percentage composition of the MSW provides information to estimate the characterized generation of MSW using the average of the global generation for the period 2015–2018, they are presented in Table 1.

Table 1

Characterization of MSW of Chillán commune.
Type of waste	t year^− 1	t h^− 1	% %
Organic matter	43178.5	5.0	61
Paper and plywood	5662.8	0.7	8
Plastic	7078.4	0.8	10
Glass	2831.4	0.3	4
Other wastes	12033.4	1.4	17

SUBDERE, 2018.

Metal content waste belongs to the category of "other waste" in which wood, textiles, rubber, cardboard for drinks and hygienic stand out. But as metal waste is within that category without an specification of its percentage, it was decided to estimate that percentage based on the population size of a similar Latin American country, that is, to identify a country that has a number of inhabitants similar to that of Chile, which is equivalent to 17.57 M, according to the census carried out in 2017 [25]. Ecuador presents a similar number, equivalent to 16.78 M inhabitants for the year 2017 [26], and in the characterization of its waste, reports a percentage of metals of 1.1% [27]. Following this idea, an estimate was made based on the population quantity, obtaining as a result 1.5% of metallic waste that must be segregated. It is important to differentiate wet and dry waste from combustible matter, since these figures were used in a later item related to mass and energy balances. A segregation according to the water content of MSW is presented in Table 2.

Table 2

Characterization of humid and dry MSW
Residue	t year^− 1	kg h^− 1
Humid	43178.50	4929.05
Dry	23959.26	2735.08
Total	67137.76	7664.13

3.2 Selection of gasification technology

To select the gasification technology, it was necessary to compare various complexity factors or operational parameters of each of the technologies, in order to determine the most applicable to this research [28]. The summarized operating data of each technology is presented in Table 3. Based on the information presented in that Table, the most appropriate technology for these wastes corresponds to the countercurrent fixed-bed gasifier or updraft technology, due to which is a simple and proven technology, with a low investment cost, compared to the fluidized bed gasifier, which has higher costs due to the operational configuration of the reactor atmosphere and the particle bed.

Table 3

Operational factors of most utilized gasification technologies.
Operation factors	Updraft	Downdraft	Fluidized
Operation temperature (°C)	1000	1000	850
Gas outlet temperature(°C)	200	800	850
Maximum moisture of RSM (%)	60	25	55
Calorific power of syngas (MJ m-3)	4.5-5.0	5.0–6.0	4.5–13.0
Operation pressure (bar)	1.0	1.0	1.0–19.0
Particle maximum size (mm)	51	51	6
Tar production (g m-3)	30–150	0.02-3.0	4.0–20.0
Gas efficiency (%)	90–95	85–90	89

In addition, the low outlet temperature of the syngas (250°C) is an advantage over other technologies, since it is not necessary to reduce the gas temperature. It is the technology with the most permissible range of moisture content of raw material (60%) which allows treating MSW, especially the organic fraction. It does not present a great difference in the calorific power of the gas (1.0-1.5 MJ m^− 3) compared to the most similar technology down-current. In relation to pressure, they can operate in atmospheric or pressurized conditions, increasing complexity and supporting a particle size similar to the down-current (< 51 mm). In addition, it has the highest level of hot gas efficiency (90–95%). The only disadvantage of this technology is the large amount of tar in the syngas, requiring a purification system before being used in the electrical generation unit. But this amount of tar can be reduced by controlling the moisture content of MSW to near 20%, to obtain tar concentrations below 60–70 g m^− 3 [30].

3.3 Mass and energy balances

To prepare the mass and energy balances of the three alternatives, it was necessary to know the operating conditions of the different equipment involved in the processes. The operating conditions of a biodigester were obtained from [31] and [32]. The operating characteristics of the gasifier, the heat exchanger and the electricity generator were obtained from [19]. The operating characteristics of the extruder were obtained from [22]. Figure 1 shows the energy flow diagrams with respect to gas production. It should be noted that in the BG alternative, the gas produced corresponds to a combination of biogas generated by the wet portion and syngas gas generated by the dry portion of the solid municipal waste, while in the DG and PG alternatives it is generated only syngas.

It is relevant to mention that for the process that includes anaerobic digestion, all the heat produced in the process was considered used for heating the reactor and that the consumption of thermal energy for drying and electrical energy in pressing was considered covered by the energy production of the process itself. The energy components corresponding to the power of thermal energy and electrical energy of each alternative are shown in Fig. 2, where it is clearly observed that the process alternatives DG and PG generate a greater amount of both thermal and electrical energy compared to the BG option.

In Fig. 3 the results of monthly net energy production in kWh both electrical and thermal are presented, along with the respective energy conversion efficiencies. The alternatives with the highest net electrical energy production correspond to the DG and PG treatments, with a net energy production of 4,441 MWhe month^− 1 and 4,410 MWhe month^− 1 respectively. The alternative with the highest net thermal energy production corresponds to the PG treatment with a total of 5081 MWh month^− 1. This alternative produces more energy because thermal energy is not used in the pressing process. The most efficient alternative also corresponds to the PG option, with 61.4% total energy efficiency, since it consumes a total power equivalent to 108.4 kW compared to the closest DG alternative in efficiency, which consumes 2002.9 kW.

3.4 Preliminary economic analysis

The operating cost in US$ month^− 1 of each of the proposed treatment option for MSW was calculated, and this cost then was compared with the current cost of MSW treatment, that is final disposal in a sanitary landfill, which reveals whether there is a saving for this item as shown in Table 3. For this purpose, the cost of landfill disposal of MSW was considered as 20 US$ t^− 1 [24]. It is observed that the cost of the gasification process of the DG and PG alternatives are identical since both processes produce the same amount of energy. The positive differential cost represents a higher cost compared to the landfill disposal while a negative value indicates a saving. It is observed that the DG alternative has the highest positive differential cost, while the BG alternative has a negative differential cost which means a saving relative to current MSW disposal. To estimate the income, a sale price of 0.158 US$ per kWhe produced, was considered [23]. The income of each alternative shown in Table 4 indicates that the BG and DG alternatives generate similar income, since they generate the same amount of electrical energy and considerably higher than the BG alternative.

Table 3

Monthly processing cost of proposed alternatives and current MSW disposal
Alternative	Process	Process cost (US$)	Differential cost (US$)
BG	Gasification	84097.7	-9223.0
BG	Biodigestion	56419.3	-9223.0
DG	Drying	30166.5	40301.1
DG	Gasification	159874.6	40301.1
PG	Pressing	3265.1	13399.6
PG	Gasification	159874.6	13399.6
Landfill	None	149740	0

Table 4

Monthly gross income for a sale price of 0.158 US$ per kWhe
Alternative	Type of Process	Energy (kWhe)	Income (US$)
BG	Biodigestion	979502.4	154,761
BG	Gasification	2336047.2	369,095
DG and PG	Gasification	4440960.0	701,671

When comparing the respective costs and monthly income of each alternative, the gross profit is obtained, as shown in Fig. 4. The most profitable alternative turns out to be the combination of pressing with gasification that achieves a preliminary profit greater than six million dollars per year, value over three hundred thousand dollars higher than the drying-gasification treatment. Strictly speaking, the practical profitability is even greater since it must be added the savings that the cost of the landfill means, which is close to one hundred and fifty thousand dollars per month.

A sustained rise in MSW was observed from year 2015 to 2018, in the commune studied, where the fuel fraction is about nineteen times the non-fuel fraction and the wet MSW fraction doubles the dry fraction. The gasification alternative selected in the analysis was updraft, due to a set of characteristics related to its greater operating flexibility. Among the three alternatives considered, pressing with gasification turned out to be the one that allows the highest energy efficiency, instead of combining biodigestion or drying with gasification. According to the preliminary economic analysis, the highest profits were obtained through the alternative that considers pressing and gasification, reaching a figure of more than six million dollars per year.

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Comparative technical-economical evaluation of gasification-based treatment of municipal solid waste

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Quantification and characterization of waste

2.2 Selection of the technology

2.3 Mass and energy balances

2.4 Preliminary economic evaluation

3. Results and discussion

3.1 Quantification and characterization of RSM

3.2 Selection of gasification technology

3.3 Mass and energy balances

3.4 Preliminary economic analysis

4. Conclusions

References

Status:

Version 1