UNIVERSITÀ DEGLI STUDI DI PISA
SCUOLA DI INGEGNERIA
CORSO DI LAUREA MAGISTRALE IN INGEGNERIA ENERGETICA
Thermoeconomic evaluation of cogeneration plants
based on Municipal Solid Waste gasification in the
Brazilian scenario
Relatori Candidato
Prof. Umberto Desideri Sofia Russo
Prof. Silvio de Oliveira Júnior
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A mia sorella,
grande amore della mia vita
(To my sister,
big love of my life)
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1. Abstract...4
2. Objectives...5
3. Municipal Solid Waste: the Brazilian scenario………...6
3.1 Definition of Municipal Solid Waste………...6
3.2 Composition and characteristics of Municipal Solid Waste………..……8
3.3 Generation of Municipal Solid Waste……….………10
3.4 Collection and final disposal of MSW………12
3.5 Public policy and sources of funding for MSW management………..17
3.6 Final considerations……….21
4. Energy recovery from Municipal Solid Waste………....24
4.1 Alternatives for the energy recovery from MSW………24
4.2 Advantages of energy recovery from MSW in the Brazilian scenario………..………29
4.2.1 The Brazilian energy overview………..……. 29
4.2.2 The problem of energy access………. 31
5. Gasification of Municipal Solid Waste……… 34
5.1 Process description……….. 34
5.2 Operating process parameters………. 36
5.3 Performance process parameters……….39
5.4 Available technologies………... 41
5.5 Syngas treatment and emissions………. 46
5.6 Syngas utilization……… 48
5.7 Advantages and comparison with other MSW treatment alternatives………. 50
5.8 Gasification of MSW in Brazil………... 53
6. Simulation of syngas production through a BFB gasifier using Aspen Plus 8 ®……... 56
6.1 Methodology……….……….. 56
6.2 Waste characterization……… 61
6.3 Type of gasifier……….. 66
6.4 Model assumptions and description………... 67
6.5 Model operating conditions……….. 72
6.6 Model validation……… 74
6.7 Results and analysis……….. 76
6.7.1 Syngas composition……….. 76
6.7.2 Cold gas efficiency (CGE) and Low Heating Value (LHV)……… 77
6.7.3 Exergy efficiency………78
7. Simulation of steam turbine CHP cogeneration plant using Aspen Plus 8 ®…………. 80
7.1 Methodology……….. 80
7.1.1 Cogeneration requirements………. 86
7.1.2 Absorption chillers……… 88
7.2 Scenarios creation criteria………. 89
7.3 Model assumptions and description………. 92
7.4 Model operating parameters……….. 96
7.5 Results and discussion……….. 99
7.5.1 Power production only………... 99
7.5.2 Cogeneration……….. 103
7.5.3 Refrigeration system configuration……… 107
8. Thermoeconomic analysis of the cogeneration system……….. 110
8.1 Exergy cost balance……… 110
8.2 Exergy cost creating criteria……… 114
8.3 Results and discussion……… 116
9. Conclusions and future developments……… 122
Acknowledgments………. 124
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1.
Abstract
The present work gives an energetic and thermoeconomic evaluation of electric and cooling power cogeneration plants based on energy recovery from Municipal Solid Waste in Brazil. For MSW gasification, a BFB gasifier is utilized; in the cogeneration section, an extraction-condensation steam turbine is coupled with an absorption chiller. The modeling and simulation of the gasification-cogeneration plants are carried out using the software Aspen Plus 8.
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2.
Objectives
The general objective of the present work is to evaluate the alternative of gasification for energy recovery of Municipal Solid Waste (MSW) in Brazil. First, an overview of the Brazilian MSW generation and management as well as of the gasification technology is presented. In order to figure out the possible composition of the syngas, a BFB gasifier is modelled and simulated using Aspen Plus 8 ®. A sensitivity analysis is performed on the gasifier according to the steam to solid waste ratio; energy and exergy performance parameters are utilized for estimating the efficiency of the gasifier. The produced syngas is supposed to be burnt and used for heat and power production through a Rankine steam cycle in a cogeneration plant. The plant is simulated with a range of representative thermal input found according to the MSW generation. The aim is to estimate the maximum potential power production in the “Power production only” scenario, and the maximum steam available for cooling power production in the “Cogeneration” scenario. Low-pressure steam is supposed to be used in absorption chillers for chilled water production. First and Second Law efficiency parameters are employed for the evaluation of the cogeneration plant. A thermoeconomic analysis is finally developed in order to allocate the cost of the products using an exergy basis.
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3.
Municipal solid waste: the Brazilian scenario
Waste: the Brazilian scenario
3.1 Definition of Municipal Solid Waste
The term “solid waste” is generally associated to every material item that does not have any more value associated to the final product. Considering the importance of the solid waste issue in the world, in the field of its production, collection, treatment and disposal, it was necessary to give some standard and legal definitions.
Referring to the Brazilian scenario, the ABTN (Brazilian Association of Technical Norms) defines the solid wastes as “solid or semi-solid wastes originated by industrial,
household, hospital, commercial, agricultural, service and cleaning activities; besides
solid wastes include sludge from water treatment systems and pollution control facilities and equipment as well as some liquids whose characteristics make unfeasible their launch in the public network sewer, or that require for it solutions technically and economically unfeasible in face of the best available technique” [1].
The Federal Law nº 12.305/10 [2], which established the National Policy of Solid Waste in Brazil in 2010, remarks the distinction between:
- solid waste: rejected material, substance, object or item originated by anthropic activities in the society, that exists in a solid or semi-solid state, for which final disposal is arranged or is required to be arranged; besides the definition includes gases in containers as well as liquids whose characteristics make unfeasible their launch in the public network sewer, or that require for it solutions technically and economically
unfeasible in face of the best available technique;
- refuses: solid wastes that, after having exhausted all the possibilities for treatment and recovery by technological processes available and economically viable, have no other possibility than a final environmentally appropriate disposal.
The classification of the solid waste can be carried on the basis of the nature of waste (dry or wet), its chemical composition, its origin (household, industrial, commercial, hospital, agricultural, airport or port, road or rail terminal, civil constructions, nuclear),
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and its degree of dangerousness. In this regard the ABTN made the following classification [1].
o Class I – Dangerous: those which, due to their intrinsic characteristics of flammability, corrosiveness, reactivity, toxicity or pathogenicity, present risks to public health increasing the mortality or morbidity, or cause adverse effects to the environment when handled or disposed improperly.
o Class II - Not inert: the waste that can present characteristics of combustibility, biodegradability and solubility, with the possibility to pose risks to the health or the environment, not included in waste classifications.
o Class III – Inert: those which, by their intrinsic characteristics, offer no risk to the health and the environment, and that, when sampled in a representative way, according to NBR 10,007, and subjected to a static or dynamic contact with distilled water or deionized, at room temperature, in accordance with the solubility test founded in NBR 10,006, do not have any of its solubilized constituent at concentrations above the standards of drinkability of water as listing in nº 8 (Annex H of ISO 10,004), except for the patterns of appearance, color, turbidity and taste.
This work will focus on Municipal Solid Waste (MSW), also known as Urban Solid Waste (USW). According to [3], MSW is waste generated in urban area by households, public places, commercial and agricultural activities. The dangerous industrial, hospital, airport and port waste is not included in MSW.
The Federal Law nº 12.305/10 [2] includes in the definition of MSW the household waste, originated from domestic activities in urban residences, and the waste from urban cleaning, derived from sweeping, cleaning public parks and roads and other urban cleaning services.
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3.2 Composition and characteristics of Municipal Solid Waste
In accordance with the previous definitions of Municipal Solid Waste, its composition results particularly variegated. The gravimetric composition expresses the weight percentage of every stream material in the MSW. The components mainly used for determining the gravimetric composition are: organic matter, paper, paperboard, rigid plastic, malleable plastic, PET, ferrous and non-ferrous metal, aluminum, white and dark glass, wood, rubber, leather, clothes and rags, bones, ceramic, fine aggregated.
Table 1 reports the MSW gravimetric composition for different countries. It refers to the medium composition of each country and only the main components are considered.
Table 3.1 - Comparison between the MSW gravimetric compositions of various countries [4]
Brazil Germany Netherlands USA
Organic matter 65 61.2 50.3 35.6
Glass 3 10.4 14.5 8.2
Metal 4 3.8 6.7 8.7
Plastic 3 5.8 6 6.5
Paper 25 18.8 22.5 41
It can be observed that in Brazil a large part consists of organic matter and paper; the percentage of glass, metal and plastic is low in comparison with the European countries and the USA. This work will take as reference the gravimetric composition of the municipality of Sao Paulo which is reported in the following Figure 1.
Figure 3.1 – MSW gravimetric composition of the municipality of Sao Paulo, elaborated by SMA/CPLA (2013) [5]
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The percentage of plastics is greater than the average of the country, due to the incidence of packaging, as expected in a megalopolis.
In general, the composition, and then the characteristics of MSW, varies as a function of demographic, social-economic, climatic and cultural factors, as listed below [6].
o Population density: the per capita generation increases with the number of urban population.
o Educational level: it can be observed that the higher educational level, the wider is the presence of recyclable materials instead of organic matter.
o Purchasing power: the higher purchasing power, the wider is the presence of recyclable materials instead of organic matter; monthly and weekly variations of this factor can modify the amount of residues (for example at the beginning of the month, or in the weekends).
o Marketing: the introduction of new products in the market and the shop promotions increase the amount of packaging.
o Climatic and seasonal factors: they alter the moisture content of waste and its composition (for example huge presence of beverage packaging in summer). o Festivities: in general the amount of packaging increase, in particular in touristic
cities.
The fraction of each material stream influences the physical, chemical and biological characteristics of MSW [4].
• Physical characteristics
o Moisture content: water percentage in the MSW expressed on weight basis; it depends on the period of the year and the amount of rain, varying generally between 40-60%.
o Apparent specific weight: specific weight without compaction of waste, expressed in kg/m3; a typical value for household waste is 230 kg/m3. o Compressibility: it is the degree of compaction and volume reduction
that a mass of waste can reach when compressed; for example with a pressure of 4 kg/cm2, the waste volume can be reduced from one third to one fourth of its original value.
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• Chemical characteristics
o Heat value: it expresses the chemical energy contained in the solid waste, being the amount of heat released when it is burnt.
o PH: it indicates the acidity or the alkalinity of the waste; in general the value lies in the range 5-7.
o Chemical composition: the percentage of carbon, nitrogen, potassium, calcium, phosphorus, mineral fraction, fats and ash.
o Carbon/Nitrogen ratio: it indicates the degree of decomposition of the organic matter in the waste; in general the ratio is between 35/1 and 20/1.
• Biological characteristics: they are determined by the microbial population and pathogenic agents present in the waste. Knowing the biological characteristics allows selecting the most appropriate methods of treatment and final disposal and developing the odor inhibitors and the accelerators of the decomposition of the organic substance.
3.3 Generation of Mucipal Solid Waste
According to the analysis of the ABRELPE1 (Brazilian association of public cleaning companies and special waste), the total generation of MSW in Brazil in 2014 was approximately 78.6 million tons, representing an increase of 2.9% from the previous year. The rate of growth was higher than that of population in the country in the same period, which was 0.9%. Considering a population of 202,799,518 [7] people the per capita year generation in 2014 was about 387.63 kg/person/yr.
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ABRELPE is a civil non-profit Brazilian association which connects and represents companies operating in urban cleaning services and solid waste management. Since 2003 it has been drawing up the “Panorama of Solid Waste in Brazil”. The collection of data is carried on by questionnaires to municipalities; in 2014 a number of municipalities representing 45.2% of the total population were involved
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Table 2 shows the evolution in the MSW generation and population growth, considering the period 2007-2014.
Table 2 - MSW generation and population growth in the period 2007-2014, elaborated by the author from ABRELPE [8-13]
MSW generation (ton/year)
Per capita generation (kg/person/year) Population (people) 2007 61,558,385 403.9 152,404,672 2009 57,011,136 359.4 158,628,647 2010 60,868,080 378.4 160,856,448 2012 62,730,096 383.2 163,700,668 2013 76,387,200 379.96 201,040,109 2014 78,583,405 387.63 202,799,518
It can be observed that the general trend consists in a growth in the generation of MSW; the decrease in 2009 is probably due to the increase of inflation and the decrease in purchasing power after the economic crisis of 2008.
The non-uniform density population and the differences in territorial organization and life conditions lead to different generation in the various regions of the country, as reported in Table 3, with reference to the year 2014.
Table 3 - MSW generation according to Brazilian regions for the year 2014, elaborated by the author from ABRELPE [13]
Total population (people)
MSW generation (tons/day)
Per capita index of generation (kg/inhab/day) North 17,261,983 15,413 0.893 Northeast 56,186,190 55,177 0.982 Middle-East 15,219,608 16,948 1.114 Southeast 85,115,623 105,431 1.239 South 29,016,114 22,328 0.770 Brazil 202,799,518 215,297 1.062
It results that the higher generation per capita index is in the Southeast region, followed by the Middle-East; this is due to the high population density and the presence of some of the biggest cities of the country (Sao Paulo, Rio de Janeiro, Belo Horizonte, and Brasilia). The lowest index is found in the South region, while the North has the
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smallest generation of waste (this is the widest region of the country, but the presence of the Amazon rainforest limits the population).
3.4 Collection and final disposal of Municipal Solid Waste
• Collection
The recent data on the collection of MSW in Brazil show an increase of 3.2% in 2014 respect of the total amount of 2013, reaching 71,260,045 tons/year. This value
corresponds to a coverage collection index of 90.6%; it means that over 7 million of
waste have an inadequate final disposal [].
Tables 4 and 5 present the trend of the collection capacity in the period 2007-2014 and the regional differences.
Table 4 - Collection of MSW in the period 2007-2014. Elaborated by the author from ABRELPE[8-13]
Collection of MSW (ton/year)
Per capita collection (kg/inh/year) Coverage collection (%) 2007 51,432,515 337.4 83.5 2009 50,258,208 316.7 88.15 2010 54,157,896 336.6 88.97 2012 56,861,856 348.5 90.64 2013 69,064,935 343.46 90.4 2014 71,260,045 351.49 90.68
Table 5 - Collection of MSW according to Brazilian regions for the year 2014, elaborated by the author from ABRELPE [13]
Total population (people) Collection of MSW (tons/day)
Per capita index of collection (kg/inhab/day) Coverage collection (%) North 17,261,983 12,458 0.722 80.8 Northeast 56,186,190 43,330 0.771 78.5 Middle-East 15,219,608 15,826 1.040 93.3 Southeast 85,115,623 102,572 1.205 97.2 South 29,016,114 21,047 0.725 94.2 Brazil 202,799,518 195,233 0.963 90.68
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The Southeast and Middle-east regions present high percentage of coverage collection, as well as the South region; the North and Northeast show a value lower than the national mean one.
In most Brazilian cities, the collection is made by private enterprises, in a form of concession or permission contracted with the municipality. Only few small-medium urban centers demand the service for waste collectors cooperatives or micro-companies. The consortium solutions are not very common in the field of the waste collection [4].
With reference to the selective collection, according to the Federal Law nº 12.305/2010 [2], it is defined as the collection of solid waste previously separated on the basis of its
composition and main constituents; its implementation is recommended for the
municipalities as a way to reach the principle of hierarchy in solid waste management. Approximately 63% of 5,654 Brazilian municipalities have developed initiatives in the field of selective collection [13]. However, in most cases, they consist in an implementation of urban spaces for selective collection and in a formalization of the agreements with the waste collector cooperatives for the execution of services [14]. According to [15] about 37.8% of the total selective separated MSW is collected by private enterprises, 18.7% directly by the municipalities and the other 43.5% by waste collectors, which often work individually.
• Final disposal
After the collection, the next step is the final disposal of MSW. In Brazil, the most of MSW is disposed in landfill, due to the low cost of this alternative and the large availability of open spaces [16]. Depending on the degree of the technology and the environmental impact, this type of final disposition can be classified as adequate or inadequate, as listed below [4, 6, 17].
o Sanitary landfill: this technique consists in an adequate disposal of waste in the soil, minimizing the environmental impact and the injuries for public health and safety. This goal is reached using engineering solutions as soil waterproofing, fencing and draining of gases, rain water and leachate; at regular intervals of
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time the waste is covered with a dirt layer. In this way the area, the volume and the emissions of residues are reduced.
o Controlled landfill: this is an inadequate alternative, generally located near a recovered dump, which was covered with clay and grass. The only safe expedients adopted are a waterproof blanket, a daily coverage with a dirt layer and a system of capture for gases and leachate (that often is recirculate on the waste pile).
o Uncontrolled landfill (dump): inadequate disposal, that consists in dumping residues and refuses in the soil, without any type of control, previous preparation of the ground or adoption of safety measures. It is the solution with the highest environmental impact: emissions of noxious and greenhouse gases, in particular methane; degradation of soil; pollution of aquifers, caused by leachate generation and penetration into the soil; proliferation of dangerous vectors (bacteria, insects, rats).
According to [13], of the 71,260,045 ton/year of waste yearly collected in Brazil, only 58.4% received an adequate disposal in 2014, with an increase of only 0.03% with
respect to the previous year; considering that, in the same period, the generation of
waste increase of 2.9%, it is evident that an huge part continues to be disposed in an unsafe way. In fact the remaining 41.6%, corresponding to 81,258 ton/day is destined to controlled landfills and dumps.
The evolution in the final disposition considering the period 2007-2014 can be seen in Table 6, which reported the percentage distribution for the different alternatives.
Table 6 - Final disposal of Brazilian MSW in the period 2007-2014, values in %, elaborated by the author from ABRELPE [8-13]
2007 2009 2010 2012 2013 2014 Sanitary landfill 38.6 56.8 57.6 58.0 58.3 58.4
Controlled landfill 31.8 23.9 24.3 24.2 24.3 24.2
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After a first reduction in the amount of waste disposed in uncontrolled and controlled landfill, the percentage distribution is almost the same. Considering the constant growth in the generation, it means that the quantity of residues disposed in dumps increased significantly every year, reaching 33,986 ton/day in 2014.
Table 7 shows the regional partition, according to the final destination.
Table 7 - Regional partition of the different final disposal for Brazilian MSW for the year 2014, adapted from ABRELPE [13]
North Northeast Middle-east Southeast South Brazil
Sanitary landfill 93 455 164 820 704 2,236
Controlled landfill 112 505 147 644 367 1,775
Uncontrolled landfill 245 834 156 204 120 1,559
Total 450 1,794 467 1,668 1,191 5,570
Inappropriate alternatives are present in 3,334 municipalities (59.8% of total), in all regions of the country. It is interesting to notice that the Northeast has the highest number of disposing places of which 834 are uncontrolled landfills, corresponding to
53.5% of the total amount in Brazil; dumps are the most common option also in the
North. This is due to the huge number of small municipalities that create their own, often open-air, dumps. In the other regions sanitary landfills are quite diffused, even with differences comparing to the number of disposing points: 33% in the Middle-east, 49% in the Southeast and 59% in the South.
According to [16], in most cases the Brazilian landfills are operated by the private sector, whose services are contracted by the municipalities (or municipal companies), in the form of outsourcing. In this case the company responsible for the management has a revenue related with the weight amount disposed in landfill (R$/ton). The fee for the disposition in sanitary landfill varies from 58 R$/ton to 116 R$/ton [18].
Besides, the creation of consortia is common: the municipality with the widest disposing area forms a consortium with the other near ones for allocating their waste, negotiating some economic advantages.
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• Recycle
As defined in the Federal Law 12.305/2010 [2], the recycle is the waste treatment process that involves the alteration of physical, physic-chemical or biological characteristics of MSW, transforming it into raw materials or new products. This activity was inserted in the Law, as one of the priorities according to the principle of hierarchy in solid waste management.
The available data on the recycle in Brazil come largely from representative associations of the aluminum, paper and plastics sector, which have a considerable participation in the recycling activities of the country.
In the aluminum sector, the 2012 scenario shows that the percentage of recycle was 35.2% (508,000 tons of aluminum) of the domestic consumption in the same period. It is a good result, considering that the global mean was 30.4%. Besides, the Brazil detains the world primate in the recycle of cans, reaching a recycling index of 97.9% in 2012 [19].
In the paper sector, the index of recuperation is calculated considering the potentially recyclable paper. In 2012 in Brazil it was 45.7%, with no variation with respect to the previous year, as declared from the Brazilian Association of Paper and Cellulose [13].
In the plastic sector, the index of mechanical recycle of plastic (IRmP) is defined as the ratio between the sum of the amount of recycled plastic and the exported plastic for recycle and the total generated plastic waste. According to the data of the mechanical recycle industry [13], in 2012 the IRmP was 20.9%, when the mean value of European countries was 25.4%. The PET recycle is growing and in 2012 it reached 58.9% of the total PET consumed.
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3.5 Public policy and sources of funding for MSW management
• Legislation
The Brazilian legislation in the field of Solid Waste is relatively recent. In the 2010 the Federal Law 12.305/2010 [2] established for the first time the National Policy of Solid Residues (PNSR). The Decree 7.404/2010 of the law determines the guidelines for the compilation of the National Plan of Solid Residues [20]. This document, accomplished in 2012, contains actions and procedures required to put into effect the solid waste policy in the country; it was drown up through national and regional public consultations and hearings, with the participation of specialized sectors (private providers, academy, private companies operating in the area), public sector and society in general.
The PNSR points out the responsibility of the municipalities in hiring services for collection, storage, transport, transfer, processing, and final disposal of waste; they are also compelled to pay for any damage to the environment or third part caused by the allocation or waste generated in its territory. Anyway, the role of municipalities was already established before of the creation of the PNRS. The Federal Constitution identifies the national, state and municipal institutions responsible for Municipal and Dangerous Solid Waste, as reported in the following determinations [4]:
o Items VI and IX of section 23 establish as common competence of the Federal Union, Estates and Districts to protect the environment and fight any form of pollution and to promote housing construction programs and the improvement of sanitation;
o Items I and V of section 30 establish as a municipal assignment to legislate on matters of local interest, especially the organization of its services, as in the case of urban cleaning, as well as the regulation of pollution activities and environmental control.
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One of the main focus of the PNRS is the dismissal of the uncontrolled landfill since, according to the FL 12.305/2010, sec. 54 “The environmentally correct final disposal of residues [...] must be accomplished within four years from the date of publication of this Law”, which means up to 2014. Other goals listed in the PNRS were the creation of state plans until 2013, and municipal, inter-municipal and micro-regional plans until 2014. In order to achieve those results, the qualified participation of society is fundamental; for this reason, the social control of the implementation and operation of PNRS was established by FL n° 12,305 / 2010 (item XI of section 15).
The PNRS identifies two principal types of instruments for reaching the actuation of the Policy: educational and economical. The environmental education is essential for spread into the citizenry the idea of sustainability. In this field the Federal Law 9,795 was already compiled in 1999, establishing the National Policy of Environmental Education, under the coordination of the Ministry of the Environment (MMA) and the Education (MEC). Some strategies reported in the PNRS for the accomplishment of the educational goals are the following.
o Implementing the educational initiatives for sustainable consumption, through guides, manuals, campaigns, researches diffused by commercial and cultural means.
o Including the environmental education in the Pedagogical Political Project for Brazilian schools, also in the higher education institutions.
o Promoting actions and elaborating promotional material for underline the importance of the selective separation and collection of waste, as well as the creation of associations, cooperatives and networks of collectors among the population involved (companies, consumers, and public sectors). The aim is also to strengthen the image of the waste collector and the appreciation of his work in the community, with actions to protect his/her health and physical integrity, observing the regional specificity.
o Creating criteria for support sustainable procurement in the public administration (in the three spheres of government), encouraging fair industries,
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companies, economic enterprises and including cooperatives and associations of waste collectors to expand the portfolio of sustainable services and products.
In Brazil, the utilization of economical instruments (IE) in solid waste management is at an early stage. Nevertheless, IEs are fundamental for finance the services, guide the actions of the stakeholders (public administration, population and productive sector) in the accomplishment of the local, state and federal goals and internalize the impact generated by the volume of produced waste. The Decree 7.404 of the FL 12.305, in its section 80, determines the employable IEs, already discussed in the chapter VI, section 29 of the Law 11.445/2007, which established the guidelines of the National Policy of Sanitation.
According to the PNSB (2008) [21], 61.4% of the Brazilian municipalities do not charge for solid waste management. Other 35.7% apply fees linked to the property tax (calculated on the basis of the property area), which corresponds to the Urban Cleaning Tax. In this way, regardless of the waste volume generated by households, there is a simple distribution of costs between the applicants of services, making zero the management marginal cost and thus dispersing the responsibility of economic agents to reduce at source the waste volume generated. Besides, this does not encourage the producers of solid waste to change their behavior, with the reduction at source, not allowing the implementation of the polluter-payer principle. In the country, very few cities apply taxes PAYT (Pay-As-You-Throw), which are proportional to the volume or the weight of collected waste [20]. The PAYT method is generally efficient when associated with an efficient selective collection, since the fee applied to it is lower or null. Basing on the characteristics of the residues generated (i.e. the gravimetric composition and the foreseeable production) and on the expected goals, the National Plan recommends a combination of the following IEs.
o Collection tax for unit of waste generated, in particular in big cities, for improving the efficiency of the entire system.
o Application of different tax depending on the final destination, paid by the municipality to the federal or state agency (or, in some cases, paid by the population), which aims to reduce the amount of waste disposed in dumps and controlled landfill.
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o Support for the implementation of CDM (Cleaning Development Mechanism) projects on landfills and agricultural waste treatment units.
o Financial incentives to the composting treatment units.
o Introduction of solid waste management criteria for develop the ecological ICMS (Goods and Services Movement Tax), in the estates that already have legislation about it.
o Adoption of tariffs on packaging and products included in the reverse logistics scheme. This is an instrument of shared responsibility for the product life cycle, that supposes a set of actions aiming to the collection and the recovery of products and remaining waste to the productive sector, in order to reuse them in their own cycle, or in other productive cycles or for their final adequate disposal. The products and residues for which the reverse logistics recovery is mandatory are: pesticides, their waste and packaging; batteries; tires; lubricating oils, their waste and packaging; fluorescent lamps, sodium vapor and mercury and mixed light; electronic products and components. In addition, medicines and packaging in general are also identified as priority. The reverse logistics collection points, such as the Voluntary Delivery Sites (LEVs) and Points (PEVs), can be created prioritizing the hiring of cooperatives and recyclable material collectors associations, since they are already responsible for much of the volume of recycled material in the country. In the European Union countries, the Green Dot, created by the European Directive 94/62 /EC, is the reverse logistics system for collection of recyclable materials and non-recyclable packaging. The founding for the management comes from the fees per type of recyclable product paid by the productive sector (distributors, packaging producers etc.). These revenues shall be invested efficiently in collection, sorting and recycling programs, in order to raise awareness and encourage the various stakeholders, especially consumers.
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• Sources of funding
In order to implement the actions for an adequate solid waste management and to achieve the results settled by the PNRS, high financial resources are necessary. In Brazil, the main possible sources of funding for private and public subjects can be classified as below [17].
o Financial agencies or funding programs such as: the National Bank of Economic and Social Development (BNDES), an authority connected with the Ministry of Development, Industry and foreign Trade; the National Fund for the Environment (FNMA), created in 1989 by the Ministry of the Environment (MMA); the Ministry of Cities, which manages the federal financial funds; the World Bank.
o Consortia of municipalities: as established in the section 45 of the FL 12.305/2010 [2], the public consortia of municipalities, created with the aim of facilitating the decentralization and the provision of services involving solid waste, have the priority in obtaining incentives provided by the Federal Government. Thus, public consortia for the management of solid waste can be a way of solving the problem of the municipalities that still have dumps as a form of final disposal.
o International sources for the Technical, Scientific and Financial Cooperation: for example the United Nation Development Program (UNDP), the United Nation Environment Program (UNEP), the Global Environmental Facility (GEF).
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3.6 Final considerations
The general described scenario shows that the evolution of the Solid Waste management in Brazil is a low process, presenting stagnation points that delay the full application of the directives of the National Policy of Solid Residues.
The yearly growth of the waste generation leads to increase the demand of services, logistics, infrastructures and human and financial resources. Considering only the period 2010-2014, the generation increased by 29%, the coverage of collection services passed from 88.98% to 90.86%, and the amount of employments in the solid waste sector grown of 18%. Anyway, the environmentally correct final disposal of residues, established by the FL 12.305/2010 to be accomplished until 2014, did not happen. Considering that the percentage of waste allocated in sanitary landfill is almost the same (57.6% in 2010 and 58.4% in 2014), it is evident that the amount of inadequately disposed residues increased, reaching about 30 million of ton per year in 2014.
The implementation of the selective collection system would be an efficient instrument
for the achievement of the proposed results; the lower taxation associated with it would
contribute to make economically unfavorable the high generation of waste, leading to a wider awareness of the population also in matter of recycle. In that sense, the use of reverse logistics methods, already implemented in Brazil, could be a positive key for developing the recycling culture. Anyway the social scenario has also to be considered since the selective collection is often accomplished by waste collectors working individually or in cooperatives.
In any case the right way to face the solid waste issue should be developing an Integrated Management of MSW, which includes not only the collection, treatment and final disposal of the waste, but also the research of sources of funding and the participation of the private, social and political part. The solid waste management is currently depending on the financial situation of municipalities, whose resources are often legally committed to other budgetary items. Generally from 7 to 15% of the municipal budget is employed for the urban cleaning system [4]. According to [13], in 2014 the financial resources invested in the collection of MSW and urban cleaning services were, in mean value, R$ 119.76 for inhabitants in one year. It seems evident
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that the charging of services could be an adequate way to assure a continuous financial incoming for municipalities. The cities that already charge the urban cleaning services apply fees linked to the property tax, calculated on the basis of the property area. The revenues are deposited in the Municipal Treasury, with no assurance that they will be invested in urban cleaning sector.
The economic reason seems to be the main justification for the delays in the accomplishment of the deadlines and in the development of an integrated management system. The main difficulty arises from the demand for investments for installation of waste treatment systems, under the direct responsibility of the municipality. Besides, the municipalities still have to pay for any damage to the environment or to third parties by the waste disposal process generated on its territory, although the new regulations introduce the concept of shared responsibility between all members of the production and consumption chain.
A recent research of ABRELPE [22] calculated the amount of financial resources required for developing a solid waste management as provided for in the PNRS: the investments in infrastructures shall be approximately R$ 11.6 million until 2031, while the operational costs of the implemented systems is about R$ 15 million for year.
The development of an efficient system of taxation, transparent and correctly dimensioned (based on social fair principles) supposes also a political commitment, which often the mayors are not intentioned to assume. In this way the situation goes to stagnation: the flaws in the urban cleaning sector, due to lack of funding, lead to the discontent of the population, which is not encouraged to pay for an inadequate service. Besides the requirement of a social function to the waste, as imposed by the Law, creates a situation of conflict in some cases, especially when the waste collector associations are not yet established. Alternatively, a conflict situation may appear when such associations already exist and, then, local authorities decide to give an energy allocation to the MSW, spreading the fear of losing jobs.
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Finally, it has to be noticed that the human and financial capital involved in the urban cleaning market is considerable: in 2014 it moved R$ 26 billion and generated more than 350,000 working places [13].
4.
Energy recovery from Municipal Solid Waste
4.1 Alternatives for the energy recovery from MSW
The energy recovery from MSW contributes to the enhancement of the waste, generally considered only rejected material with no more value or utility.
Depending on the type of transformation and the characteristics of the process, the different alternative of conversion can be divided as reported below [17].
o Thermochemical conversion. It includes processes in which the release of a huge
amount of heat and the presence of high temperature exothermic and endothermic reactions change the chemical and physical composition of the residues. The thermochemical processes used for the energy conversion of MSW are incineration, gasification and plasma gasification. The possible products are generally synthesis gases (composed by hydrogen, carbon monoxide, carbon dioxide), inert or vitrified (in the case of plasma) solid residues, organic oil (from pyrolysis) and organic vapors (or tars).
o Biochemical conversion. The transformation of residues involves the
decomposition by means of micro-organisms; this process is known as anaerobic digestion, since it does not involve oxygen. It can occur in landfill, provided with a gas capture system, or in apposite biodigestors. The product is the biogas,
mainly composed by methane and carbon dioxide; other by-products are liquids
and organic compost.
Another classification is between the technologies that suppose the direct combustion of waste (incineration, plasma) and those that provides for gaseous or liquid fuel for combustion or direct use (gasification, pyrolysis, anaerobic digestion).
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For a deeper comprehension, a brief description of the main energy conversion technology is presented.
• Incineration
According to [23], the objective of waste incineration is to treat wastes so as to reduce their volume and hazard, whilst capturing (and thus concentrating) or destroying potentially harmful substances that are, or may be, released during incineration. Incineration processes can also provide a means to enable recovery of the energy, mineral and/or chemical content from waste.
Waste incineration is the oxidation of the combustible materials contained in the waste. This process can be divided in different stages [23]: drying of feed and degassing of the volatile matter, at temperature between 100 and 300 °C; pyrolysis, that is the further decomposition of organic substances in absence of oxidizing agent, occurring at 250-700°C, and gasification of the carbonaceous residues through reactions with water vapor and CO2 at temperatures between 500-1000 °C; oxidation of the
combustible gases created in the previous stages, in excess air conditions. The ratio between the supplied incineration air to the stoichiometric required incineration air usually ranges from 1.2 to 2.5, depending on both the fuel state (gas, liquid or solid) and the furnace system. Generally the individual described stages overlap spatially and temporally during the incineration process, influencing each other. Nevertheless it is possible, using in-furnace technical measures, to influence these processes so as to reduce polluting emissions. The main measures include furnace design, air distribution and control engineering, in order to regulate time of residence, temperature and turbulence. In many cases, waste incinerators may have only limited control over the precise content of the wastes they receive. This results in the need to design installations sufficiently flexible to cope with the wide range of waste inputs they could receive. This applies to both the combustion stage and the subsequent flue-gas cleaning stages. The main types of waste to which incineration is applied as a treatment are: MSW not pretreated; pretreated MSW (e.g. selected fractions or RDF); non-hazardous industrial wastes and packaging; hazardous wastes; sewage sludge; clinical and hospital
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wastes. The variegated composition of the incoming waste feed, with different heat value, can lead to variations in the operational conditions of the incineration kiln, requiring the combustion of an auxiliary fuel for keeping the project thermal load. For example, in the case of MSW, paper, plastics and rubber increase the heat value, while the presence of organic matter, with high water content, reduces it.
Depending on the characteristics of waste, on its amount and its state (solid, semi-solid or liquid), different types of incinerators are available on the market. The incineration plant used for treating MSW is moving grate [6], in which an inclined moving grate is installed in a furnace-boiler; part of the preheated combustion air is introduced below the grid, while another part is added on the top of the furnace at high speed to create a region of high turbulence and promoting mixing with the gases and vapors generated during combustion. Fluidized bed and liquid injection plants are recommended for liquid wastes, while a rotary-kiln configuration is more versatile. The energy recovery from waste through the incineration process consists in the production of steam for industrial utilization and heat and power production.
In fully oxidative incineration the main constituents of the flue-gas are: water vapor, nitrogen, carbon dioxide and oxygen. The flue gas temperature is generally between 800 and 1450 °C. Depending on the composition of the material incinerated and the operating conditions, the flue gases can contain variable amounts of CO, HCl, HF, HBr, HI, NOX SO2, particulate matters, organic compounds (VOCs, PCDD/F, PCBs) and
heavy metal compounds. These substances are transferred from the input waste to both the flue-gas and the fly ash. A mineral residue fly ash (dust) and heavier solid ash (bottom ash) are created. In MSW incinerators, bottom ash, which includes any non-combustible material, is approximately 10 % by volume and approximately 20 to 30 % by weight of the solid waste input. Fly ash quantities are much lower, generally only a few per cent of input [23].
Both emitted gases and generated ash should receive special attention, within the legislation recommendations, requiring special equipment and facilities for their treatment. A special attention should be paid to the emission of dioxins, which are proven carcinogenic substances. Their formation occurs by a complex mechanism
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involving organic matter, oxygen and chlorine. These are destroyed at temperatures above 600 ° C, but synthesized again between 500 ° C and 250 ° C in the presence of chlorine and carbon. Most of the dioxins retain in the fly ash. For this reason the flue-gas cleaning system is often a significant contributor to overall incineration costs (approximately 15-35% of the total capital investment) [23].
• Gasification
Gasification is a thermochemical process that consists in a partial oxidation of the feed in presence of an oxidant amount lower than that required for the stoichiometric combustion. The oxidant can be air, oxygen or oxygen-enriched air; a moderator, such as steam, should be present. The result is not a hot flue gas as in the conventional direct combustion of wastes but a hot fuel gas (‘‘producer gas’’ or ‘‘syngas’’), containing large amounts of not completely oxidized products that have a calorific value and which can be utilized in a separate process equipment, even at different times or sites. A more detailed overview of the gasification technology is presented in Chapter 5.
• Plasma
Plasma technology is a high temperature process that consists in chemical decomposition of the waste. The solid residues are fed into the furnace through a lock hopper feed system. Preheated air is injected at the base of the furnace to support combustion of part of the material; the air can be enriched or not with oxygen. The combustion gases are directed to a reactor for thermal plasma decomposition; here an electric arc, generated by the passage of a DC or AC electric current between electrodes, induces the ionization of the gases (plasma conditions) at temperatures between 5,000 and 15,000 °C. The final constituents are basically hydrogen and carbon monoxide: the final flue gas has at 1200-1400 ºC. All the inorganic material (ash) is vitrified and collected at the bottom of the furnace, where it exits at a temperature of 1450 C [6].
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The advantages of this technology are related with [6, 24]: the high energy density, which allows the construction of reactors with smaller dimensions; the lower volume of emission gases and presence of pollutants, since they are destroyed into their atomic constituents (although not excluding the need for gas scrubbers); the presence of vitrified ashes, which can be used in the construction industry. Even if plasma is an established commercial technology, its installation and operation can be very complex and expensive; one of the main cost is the related with the use of a high amount of electricity, for generating the arc. Thus, in order to produce and sell an excess of electricity, a considerable quantity of waste is required.
• Anaerobic digestion
Anaerobic digestion is a microbial fermentation process occurring in absence of oxygen [17, 25]. The process can be divided in two stages: in the first stage, the complex organic compounds with carbohydrates, proteins and lipids are hydrolyzed, fermented and biologically converted into simple organic compounds (mainly volatile acids) by means of a group of anaerobic bacteria, known as acidogenic and fermentative; in the second phase, the organic acids are converted mainly into methane (CH4) and carbon
dioxide (CO2) through the action of strictly anaerobic bacteria, called methanogens.
Depending on the feed system, the digestors can be classified into batch and continuous. In the first type the biomass is added at the start of the process, reaching the maximum load capacity and it is replaced only after the complete digestion of the organic matter; batch processing needs inoculation with already processed material to start the anaerobic digestion. In the continuous type, the organic matter is constantly added (continuous complete mixing) or added in stages to the reactor (continuous plug flow); the end products are constantly or periodically removed, resulting in constant production of biogas.
The final products are biogas and organic compost. The biogas can be burnt in ICE or boilers for heat and power production, used as a vehicle fuel or injected in the natural
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gas grid after an upgrading to biomethane. The generated biogas includes a fraction of sulfur compounds, which are corrosive and should be removed after the use in equipment. The organic compost, or digestate, should be used as fertilizer [6].
An important control measure is the soil waterproofing for avoiding the penetration of the leachate. Besides to the presence of microorganisms necessitates a strict control of environmental conditions for proper operation. The variables to be considered for the monitoring and evaluation of the process are temperature, pH, presence of toxic substances and water content.
4.2
Advantages of energy recovery from MSW in the Brazilian
scenario
4.2.1 The Brazilian energy overview
The implementation of a system in which the MSW is considered as a source for energy production could improve the integrated solid waste management, helping to make feasible the adequate collection and disposal of waste. Besides, the integration of solid residues in the Brazilian energy mix, shown in Figure 2, could bring benefits to the energy scenario of the country.
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The main energy source in Brazil is water, utilized in hydro-power plants, due to the huge presence of river basins; it means that 407.2 TWh of 624.3 TWh produced in 2014 come from hydroelectric generation. The natural gas is in second place, followed in order by biomass, oil and its derivatives, carbon and its derivatives, nuclear and wind. For its nature, MSW would be included in the biomass part.
Considering only the electric production, the system presents some weak points [6]. The recent hydrological adverse situation led to low level of storage for hydro-electric power plant. Besides, many of the projected hydro-power plants, which are completed or under
construction, work with low flow rate of water; this reduces the environmental impact
but, on the other hand, limit the presence of storage. This context, combined with an increase in the energy demand, a lack of integrated energy planning and delays in infrastructure construction, lead to a wider use of thermal power plants, which generally worked only in peak or emergency situations. According to the National Energy Balance 2014 [26], in 2013 the use of thermal plants increased by 31%. This implies higher operational costs, since a free source (water) has to be replaced with a paid fuel (natural gas or oil derivatives). In this sense, the MSW would be a zero cost fuel for energy
producers; in fact its supply represents budgetary revenue.
In the meanwhile, the energy demand increases every year. According to [26], in 2013 the energy consumption has grown by 6.2% in the residential sector, by 0.2% in the industrial sector, and by 4.8% in the other sectors (public, commercial, transport). As reported in the forecasts of the Brazilian Energy Research Company (EPE) showed in Table 8, the energy consumption in destined to increase in particular in the range of consumers over 100 kW.
Table 8 - Forecast of increase in energy consumption according to consumption range (2013-2050) [27] Consumption range (kWh) 2013 2050 0-30 6,889 985 30-100 18,044 19,693 100-200 18,581 31,509 200-300 13,574 21,170 300-400 2,783 12,308 400-500 1,265 6,154 500-1000 1,474 4,923 >1000 336 1,723 TOTAL 62,947 98,466
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4.2.2 The problem of energy access
The energy access represents a problematic issue in the Brazilian scenario. The definition of energy access usually includes both accesses to electricity and to modern fuels for cooking and heating instead of traditional biomass (wood). In order to fix a reference target for define conditions of lack of energy access, the IEA (International Energy Agency) proposed the lowest threshold, namely 100 kWh of electricity and 100 kgoe (about 1200 kWh) per person per year [28]. The amount increases when we consider also the productive uses of energy (i.e., water pumping, irrigation, agricultural processes), which are necessary for social and economic development, contributing to poverty alleviation and increase the families revenues.
The Brazilian electric system is divided into the Interlinked (ILS) and the Isolated Systems (IS). In the ILS, all electric power plants are connected through long transmission lines from Southern to Northern Brazil, mainly along the coast; the IS, in North region (Brazilian Amazonia), is composed mostly by small thermoelectric power plants (diesel engines with difficulties on logistic for diesel supply through rivers in the rain forest). This region covers an area corresponding to 45% of Brazilian territory and 3% of the population (around 1.2 million consumers). Figure 3 shows the Brazilian electricity grid, with the main bowls and charge stations.
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As regards the ILS systems, in 2003 the Federal Government established the LPT (Luz
para Todos - Lighting for All) program, with the aim of providing free electricity access
for 10 million of people in rural areas in Brazil by 2008 (the LPT was supposed to end in 2014). According to the program, utilities were obliged to provide electricity access for free when required by the consumers (otherwise they have to inform deadlines to provide the access) [30]. The Household Census of 2010 accomplished by the Federal Government [7] shows that Brazil has achieved the level of 98.73% of universalization of electric power access in urban and rural areas (in the interlinked system), compared to 74.90% in 1981 and 94.54% in year 2000. Nevertheless, more than 280,000 households still did not have electric lighting, according to the National Household Survey 2013 [31], reflecting the position of the end of 2012.
The big challenge is the universal access of the North. It received less that 20% of the new connections, despite accounting for over 40% of not electrified households [28].The main difficulties are related with the dispersion of the built-up areas and the situation of regional power concessionaires, which are owned by federal government that are living dire financial straits. Besides the operation and maintenance costs of new electrified areas would impose burdens on locale fare. An alternative that could certainly contribute to minimizing the impact on the level of needed resources is the use of individual generation systems (SIGFIs), already regulated since 2004, or isolated micro-generation and distribution systems (MIGFIs), regulated in 2012, which can use diesel, biomass residues or hybrid systems. In this field some technologies are already developed and installed, such as Solar Home Systems (SHS), engines fed with Straight Vegetable Oil (SVO), Liquefied Petroleum Gas (LPG) for cooking and heating and small-scale biomass gasifier [28].
The use of MSW can be related with the last solution, considering also that the most of residues in the rural regions are constituted by organic matter. The main advantages of the biomass residues energy conversion are related to the emissions avoided from diesel engines, the presence of a locally produced and more easily supplied fuel, as well as the adequate disposal of waste, generally allocated in dumps or burned in open air. In any case, the power supply can be used for local activities only; in fact, in most cases, there
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is no possibility of selling surplus because of the absence of distribution grid linking one municipality to another or to the national grid. In addition, municipalities located in Amazon region of Brazil have no cost-effective solution able to build distribution lines through the forest (not even considering the environmental issue) [28].
Considering the fact that the most are low-income municipalities, the challenge is to make this energy conversion economically feasible and a source of income revenue. At the moment, there is no adequate legislation for the local utilities, mainly in the Amazon region, to change from diesel oil engines to other renewable systems. The existing CCC (Conta Consumo de Combustiveis – Fuel Consumption Account) policy gives incentives for power production from diesel engines but not from other renewable options [32].
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5.
Gasification of Municipal Solid Waste
Gasification technology has been developed firstly in the coal sector, from the beginning of the XX century. Then, it started to found applications for processing other type of solid, liquid and gaseous feed. Gasification is a relative recent alternative for the energy recovery of MSW; it allows an efficient treatment of different types of solid waste, in particular of unsorted residual waste i.e. the waste left downstream of selective collection, which cannot be conveniently recycled from an environmental and economic point of view [33].
5.1 Process description
Gasification is a thermochemical process that consists in a partial oxidation of the feed in presence of an oxidant amount lower than that required for the stoichiometric combustion. The oxidant can be air, oxygen or oxygen-enriched air; a moderator, such as steam, should be present. The result is not a hot flue gas as in the conventional direct combustion of wastes but a hot fuel gas (‘‘producer gas’’ or ‘‘syngas’’), containing large amounts of not completely oxidized products that have a calorific value and which can be utilized in a separate process equipment, even at different times or sites [33].
The process can be divided in the following steps [34, 35].
o Heating and drying of the feedstock, that occurs at temperatures up to about
160°C; it is a combination of events that involve liquid water, steam and porous solid phase through which liquid and steam migrate.
o Devolatilization (or pyrolisys, or thermal decomposition), which takes place at
low temperatures 350-800°C (often in parallel with heating) and involves thermal cracking reactions and heat and mass transfers, determining the release of light permanents gases (such as H2, CO, CO2, CH4, H2O, NH3), tar
(condensable hydrocarbon vapors) and char (the remaining devolatilized solid waste residue). Part of the produced vapors undergoes thermal cracking into gas and char. The rate of devolatilization, and so the quantity and chemical
35
composition of the species released, depends of several factors, including the rate of heating (that depends on the reactor type), the composition and particle size distribution of feed, the rate of gasification by water gas reaction, as well as the temperature and pressure of the reactor. For MSW, volatile matter represents a significant portion of the carbonaceous fuel.
o Volatile reactions, occuring in a reducing environment between the chemical
species released from the pyrolisys and the oxidant, whose amount is lower than the stoichiometric. The extent to which the oxidant is completely or only partially depleted depends on the amount of volatiles produced. In an auto-thermal gasification process, the partial oxidation provides the heat necessary for the thermal cracking of tars and hydrocarbons and for sustaining the endothermic gasification reactions of char by steam or carbon dioxide, and then to keep fixed the temperature (or the profile temperature) of the reactor. On the contrary, in the allo-thermal gasifiers, the heat is provided by an external source, by using heated bed materials, by burning some of the char or gases separately or by utilizing a plasma torch.
o Char gasification, which involves the heterogeneous reactions with carbon,
namely the water-gas, Boudouard and hydrogenation reactions. They take place between the devolatilized solid waste (char) and the gases excluding oxygen.
Those steps are not spatially and temporally separated, but they could occur in parallel, influencing the overall kinetic of the gasification process. If the heating up is slow, then the pyrolysis reactions set in from about 350°C. The concentration of volatiles increases rapidly, and gasification only starts after devolatilization is complete. In fact, the gasification reaction of both volatiles and char with steam is very slow at this temperature. If, however, the rate of heating is high, then both pyrolysis and gasification take place simultaneously, so that a high concentration of volatiles is never allowed to build up [35].
The partial oxidation of the volatile matter, being a reaction between gases, is much more rapid than the heterogeneous char gasification where mass transport limitations play a more important role. These reactions in fact are the slowest in the gasification
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process, and therefore they govern the overall conversion rate. The rates of reaction for the water gas and Boudouard reactions with char are comparable and are several orders of magnitude faster than for the hydrogenation reactions.
Table 9 shows the main reactions in homogenous and heterogeneous phase occurring in the solid waste gasification, with the relative heat of reaction.
Table 9 - Main reactions in homogenous and heterogeneous phase occurring in the solid waste gasification, adapted from [34] (a CxHy represents tars and CnHm represents hydrocarbons with a smaller number of carbon atoms and/or a larger degree of unsaturation than CxHy)
Oxidation reactions
R1 C+½O2→CO -111 MJ/kmol Carbon partial oxidation
R2 CO+½O2→CO2 -283 MJ/kmol Carbon monoxide oxidation
R3 C+O2→CO2 -394 MJ/kmol Carbon oxidation
R4 H2+½O2→H2O -242 MJ/kmol Hydrogen oxidation
Gasification reaction involving steam
R5 C+H2O↔CO+H2 +131 MJ/kmol Water-gas reaction R6 CO+H2O↔CO2+H2 - 41 MJ/kmol Water-gas shift reaction R7 CH4+H2O↔CO+3H2 +206 MJ/kmol Steam methane reforming Gasification reaction involving hydrogen
R8 C+2H2↔CH4 - 75 MJ/kmol Hydrogasification
R9 CO+3H2↔CH4+ H2O - 227 MJ/kmol Methanation Gasification reaction involving carbon dioxide
R10 C+CO2↔2CO +172 MJ/kmol Boudouard reaction
Decomposition of tars and hydrocarbonsa
R11 pCxHy→qCnHm +rH2 Endothermic Dehydrogenation R12 CnHm→nC+ m 2H2 Endothermic Carbonization
5.2 Operating process parameters
A review of the operating parameters that influence the gasification process, and so the syngas composition and heating value, is reported below.
o Reactor temperature, or better the temperature profile along the different reactor
