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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Gasification PDF Print E-mail

 

1. General
2. Feedstock
3. Gasifier
4. Oxygen plant
5. Gas Clean-Up
6. Mass and energy balances
7. Market potential for products
8. Environmental impacts
9. Economic data
10. Applicability in the target area
References

1. General

Gasification is a process of incomplete combustion of solid waste (under stoichiometric conditions regulating the supply of oxidant). A variety of processes take place, while the gas is formed at temperatures above 700°C and is rich in H2, CH4, H2O, N2, CO, CO2 and small amounts of high H/Cs. The purpose of this method is the maximum release of CO and H2. The mixture of CO and H2 is known as synthesis gas (or syngas).

 

It is theoretically the next stage of pyrolysis. At this stage the residual coke is oxidized at high temperatures (> 800oC). As a gasification agent steam, CO2, O2 or air are used.

 

The main reactions taking place during the process of gasification are:

(1) Oxidation (exothermic)

C + O2 → CO2

(2) Reaction of water evaporation (endothermic)

C + H2O → CO + H2

(3) CO + H2O → CO2 + H2 (exothermic)

(4) Boudouard Reaction (endothermic)

C + CO2 → 2CO

(5) Reaction of formation of CH4 (exothermic)

C + 2H2 → CH4

The heat to keep the process going derives from the exothermic reactions, while the combustion products are mainly produced by the endothermic reactions.

 

It is likely that other reactions take place at low temperatures where with the addition of H2O CO2 is formed and at higher temperatures CO.

 

The gasification reaction rate depends on temperature, porosity, internal structure of the fuel source and diameter of the pores. Specifically, untreated waste is harder to gasify than that from cracking. Similarly, the loose material is brittle compared to the coherent solid material. Solid materials readily allow the passage of air through the plating reactor.

 

The difference in the gasification of pyrolysis is that during gasification additional fuel gas is fed for further conversion of organic residues into gaseous products (Figure 2) (Gidarakos 2006, Girods et al. 2009, Klein 2002).

 

Syngas generally has a heating value of 250-300 Btu/scf, compared to natural gas at approximately 1,000 BTU/scf. Typically, 70–85% of the carbon in the feedstock is converted into the syngas. The ratio of carbon monoxide to hydrogen depends in part upon the hydrogen and carbon content of the feedstock and the type of gasifier used.

Typical gasification plant

The main types of gasification facilities are:

  • Vertical fixed bed
  • Horizontal fixed bed
  • Fluidized bed
  • Multiple foci
  • Rotating furnace.

 

From all five of these types of facilities (Rezaiyan & Cheremisinoff 2000), the vertical fixed bed facilities (Figure 3), the horizontal fixed bed (Figure 4) (Dalai et al. 2009) and the fluidized bed (Groi et al. 2008) are more widespread.

 

The vertical fixed bed plants have advantages, such as simplicity and low investment costs, but are directly affected by fluctuations in the composition of incoming waste (it is preferred to be homogeneous, e.g. the RDF in concentrated form - pellets). The gas product of the plant is of low calorific value and simultaneously small quantities of liquid and important quantities of solid products are produced.

 

Based on the results of pilot applications for units operating at 650 - 820oC, it has been proved that:

  • The resulting solids have great adsorptive capacity and can be used in tertiary treatment facilities and sewage water.
  • The gas product can be used as fuel in engines burning oil at a ratio of 4:1, the performance of the machine can reach 76% of the performance that it would have if there was exclusive use of oil.

 

The gases resulting from the treatment of the gaseous product (high performance cyclones) are comparable in composition to the gases produced by incineration and in some cases contain less polluting load (Bebar et al. 2005).

Regarding facilities of horizontal fixed bed, they are the type widely used in commerce. The facility consists of two parts: (a) the main gasification chamber and (b) the combustion chamber. The first stage carries the process of gasification and gas produced is burned completely in the second chamber with excess air at 650 - 900°C. The exhaust gases of the combustion chamber are products through complete combustion that have temperatures ranging from 650oC - 900oC and can be exploited through the recovery of energy contained in them. The exhaust gases are driven through heat recovery to produce steam or hot H2O. The low velocity and turbulence in the first chamber minimize the entry of particles into the gas stream and lead to lower particulate emissions than conventional combustion chambers. Such units are commercially available from different manufacturers in standard sizes capacity 200 – 1,700 kg / h.

Finally, the fluidized bed plants are still at pilot level. With minor modifications, the fluidized bed combustion with excess air can act as a gasification plant fluidized bed with air flow below the stoichiometric ratio.

 

But other than the horizontal bed units, the other systems have not been developed at full-scale and additional research is required towards this direction.

 

The produced gas can be utilized in various ways, including:

  • Combustion to produce steam. The advantages compared to incineration, is that the gases are cleaned before combustion, thus enabling operation at higher pressure boiler and superheater of steam at higher temperatures, to achieve improved performance and electricity, which can approach 30%.
  • Power internal combustion engine which drives electrical generator. The electrical energy can exceed 40% but requires a very thorough cleaning of gas before feeding the machine.
  • Movement of Steam turbine and combined cycle. And this method, which also requires a good cleaning before the gas supply, can result in yields of 40% in electricity.
  • Feeding into the city gas network. Requires good cleaning and stable quality.
  • Provision of gas to the industry, such as cement for direct combustion in burner. In this case a significantly reduced cleaning is required.
  • Supply of the gas to an industry where it is used for Steam generation. The cleaning requirements are a function of boiler operating conditions (Ahmed & Gupta 2009, Belgiorno et al. 2003, Bjorklunda et al. 2001, Brothier et al. 2007, Ganana et al. 2006, He et al. 2009).

 

An indicative waste gasification plant is shown in Picture 1.

Emissions from gasification units

The end products of gasification are:

  • gas rich in CO, H2, CO2 and saturated H / C (mainly CH4) that can be used as fuel (Scheidl et al. 1991)
  • Solid waste material consisting of C and aggregates.

 

Table 1 summarizes all types of solid waste, wastewater and off-gases generated during the operation of a gasification unit.

Summary of solid waste, wastewater and air emissions generated during the operation of a gasification unit

Solids

pure C embedded in various inert materials

Gases CO, H2, saturated H / C

The gasification plant can operate with either supply of air or supply of pure O2. In the case that there is supply of air, because of the presence of atmospheric nitrogen, the calorific value of the gas product is relatively low and is around 5.6 MJ/m3. A typical composition is: 10% CO2, 20% CO, 15% H2, 2% CH4, 53% N2.

 

If the supply is pure O2, the standard composition is: 14% CO2, 50% CO, 30% H2, 4% CH4, 1% CxHy, 1% N2 and energy content between 10 and 11.2 MJ / m3.

 

Based on the principle of the gasification processes (this also accounts for the case of pyrolysis) there are limited air emissions comparing with the implementation of the incineration process due to the less air used (US Department of Energy 2000, Radian International LLC 2000). In each case, regarding the permissible levels of emissions generated during gasification, they are identical with all techniques of thermal processing of solid waste and what has already been described about the limits of the combustion – incineration process.

 

 

In the following figure, a schematic flow diagram of the Gasification Plant in Caribbean of the ITI Energy Limited is presented.


2. Feedstock

Gasification enables the capture — in an environmentally beneficial manner — of the remaining “value” present in a variety of low-grade hydrocarbon materials (“feedstocks”) that would otherwise have minimal or even negative economic value. Gasifiers can be designed to run on a single material or a blend of feedstocks:

  • Solids: All types of coal and petroleum coke (a low value byproduct of refining) and biomass, such as wood waste, agricultural waste, and household waste.
  • Liquids: Liquid refinery residuals (including asphalts, bitumen and other oil sands residues) and wastewater from chemical plants and refineries.
  • Gas: Natural gas or refinery/chemical off-gas.

3. Gasifier

The core of the gasification system is the gasifier, a pressurized vessel where the feed material reacts with oxygen (or air) and steam at high temperatures. There are several basic gasifier designs, distinguished by the use of wet or dry feed, the use of air or oxygen, the reactor’s flow direction (up-flow, downflow, or circulating), and the gas cooling process. Currently, gasifiers are capable of handling up to 3,000 tonnes/day of feedstock throughput and this will increase in the near future. After being ground into very small particles — or fed directly (if a gas or liquid) — the feedstock is injected into the gasifier, along with a controlled amount of air or oxygen and steam. Temperatures in a gasifier range from 1,400-2,800 degrees Fahrenheit. The heat and pressure inside the gasifier break apart the chemical bonds of the feedstock, forming syngas. The syngas consists primarily of H2 and CO and, depending upon the specific gasification technology, smaller quantities of CH4, CO2, H2S and water vapor.

4. Oxygen plant

Most gasification systems use almost pure oxygen (as opposed to air) to help facilitate the reaction in the gasifier. This oxygen (95–99% purity) is generated in a plant using proven cryogenic technology. The oxygen is then fed into the gasifier through separate co-feed ports in the feed injector.

5. Gas Clean-Up

The raw syngas produced in the gasifier contains trace levels of impurities that must be removed prior to its ultimate use. After the gas is cooled, the trace minerals, particulates, sulfur, mercury, and unconverted carbon are removed to very low levels using commercially proven cleaning processes common to the chemical and refining industries.

For feeds (such as coal) containing mercury, more than 95% of the mercury can be removed from the syngas using relatively small and commercially available activated carbon beds (WASTESUM, 2006).

6. Mass and energy balances

A typical mass balance of the gasification process is shown in Figure 6. On the basis of the diagram it can be stated that 1 tonne of treated feedstock leads to 680-810 kg produced syngas, 170-300 kg carbon char and ash that can be recycled or disposed of at a landfill, while the remaining 20 kg is the residue from the flue gas treatment that must be sent to a hazardous waste landfill.

 

In the following figure (Figure 7), the energy and mass balances of the Gasification Plant in Caribbean of the ITI Energy Limited are presented.


 


Assuming 100% chemical energy in the fuel feedstock, the typical gasifier converts this fuel to 70-80% chemical energy in the gaseous phase, 15-20% heat and some heat loss and unconverted fuel, which depends on the type of gasifier and the fuel.

7. Market potential for products

Syngas can be combusted to produce electric power and steam or used as a building block for a variety of chemicals and fuels. Most solid and liquid feed gasifiers produce a glass-like byproduct called slag, which is non-hazardous and can be used in roadbed construction or in roofing materials. Also, in most gasification plants, more than 99% of the sulfur is removed and recovered either as elemental sulfur or sulfuric acid.

Hydrogen and carbon monoxide, the major components of syngas, are the basic building blocks of a number of other products, such as chemicals and fertilizers. In addition, a gasification plant can be designed to produce more than one product at a time (co-production or “polygeneration”), such as the production of electricity, steam, and chemicals (e.g. methanol or ammonia). This polygeneration flexibility allows a facility to increase its efficiency and improve the economics of its operations (Rezaiyan & Cheremisinoff, 2005; Klein, 2002; Radian International LLC, 2000; Belgiorno et al., 2003).

8. Environmental impacts


The environmental impacts of the use of gasification systems are generally much milder than incineration. They focus on air emissions and solid residues, as in all thermal technologies. At high temperatures used in gasification, toxic metals including cadmium and mercury, acid gases including hydrochloric acid and ozone-forming nitrogen oxides could be released. Also, dioxins and furans may be generated in the cooling process following the burning of ordinary paper and plastic in case that the operation of the unit is not made and controlled properly. Using municipal solid waste for fuel releases into the atmosphere the carbon which is in the paper, cardboard, food waste, yard waste and other biological materials, plus the carbon in plastic products and containers made from petroleum. The gasification of petroleum-based plastics adds to greenhouse gases in the same way as burning fossil fuels, such as coal, oil or natural gas.

Gasification may reduce solid waste volume by 85 to 92%. In addition, the use of gasification processes reduce methane emissions produced from the disposal to landfill sites, while being a waste to energy treatment method, it enables the displacement of CO2 that would have been emitted if the electricity had been generated from fossil fuels.

9. Economic data

According to the Carbon Finance Unit of the World Bank in 2008, the capital cost of a gasification system with a capacity of 900 tonnes per day is 10-115 /tonne, while the operation and maintenance cost is 55-100 /tonne. The relevant cost is approximately 130 Euro/t on the basis of the estimations of Neamt Master Plan.

10. Applicability in the target area

The application potential of gasification and plasma gasification is also considered high, since these methods have recently proved that they are effective and flexible, since they can also be used for the treatment of other waste streams (e.g. sludge, hospital waste, etc.) apart from municipal waste. That is why the gasification practices are considered as suitable alternative especially in the case of isolated areas, such as islands. The relevant cost is similar to that of other thermal management practices, higher than that of biological options, the relevant land demand is limited and the energy yield is also considered of vital importance. The experience from the operation of such plants is less than that from incineration units. Without doubt, the existing economic crisis in the whole Balkan Region is a significant obstacle for such attempts today. An example on the application of this technology is Slovenia, a country with less economic problems comparing to Romania, Greece and Bulgaria, is provided below.

EfW Gasification CHP Plant in Celje, Slovenia

The Energy from Waste (EfW) Gasification plant in Celje, Slovenia constructed by KIV constitutes part of an integrated waste management scheme for the city of Celje and the surrounding districts.

The waste treatment facilities comprise kerbside recycling, Eco Islands, a picking station with baling machinery, MBT and the EfW. The remaining MSW goes through a MBT facility, where loose RDF output from the residual waste becomes the fuel supplying the KIV EfW plant. Additionally belt pressed sewage sludge is mixed into the RDF just prior to being fed into the KIV capacity gasifier.

 

The waste treatment plants (MBT + EfW) are designed to cope with the waste from up to 240,000 people across 24 Municipalities. The plant has been designed to divert the waste away from landfill. It is a town / city sized solution, only requiring short waste shipments thereby minimising carbon footprint.

 

The mixed RDF (80%) and Sewage Sludge (20%) has a combined net calorific value (CV) of 13.6MJ/kg. With this CV the 18MWth capacity gasifier plant is capable of handling up to 37,000 tpa, based on guaranteed operational hours of 7,800. 15MWth of high pressure superheated steam produces 2.1MWe of power as it passes through a steam turbo alternator. The plant is ‘heat led’ and feeds the recovered energy of up to 13MWth into the existing District Heating scheme as hot water at 110⁰C. If the scheme was ‘electricity led’, it would produce 3.8MWe of gross power.

 

 

REFERENCES

Ahmed, I. & Gupta, A.K., Evolution of syngas from cardboard gasification, Applied Energy 86 (2009) 1732–1740.

Bebar, L., Stehlik, P., Havlen, L. & Oral, J., Analysis of using gasification and incineration for thermal processing of wastes, Applied Thermal Engineering 25 (2005) 1045-1055.

Belgiorno, V., De Feo, G., Rocca, C. D. & Napoli, R.M.A. (2003). Energy from gasification of solid wastes, Waste Management 23, 1-15

Bjorklunda, A., Melainab, M. & Keoleianb, G., Hydrogen as a transportation fuel produced from thermal gasification of municipal solid waste: an examination of two integrated technologies, International Journal of Hydrogen Energy 26 (2001) 1209–1221.

Brothier, M., Gramondi, P., Poletiko, C., Michon, U., Labrot, M. & Hacala, A., Biofuel and hydrogen production from biomass gasification by use of thermal plasma, High Temperature Material Processes 11 (2007) (2) 231-244.

Dalai, A.K., Batta N., Eswaramoorthi I. & Schoenau G.J., Gasification of refuse derived fuel in a fixed bed reactor for syngas production, Waste Management 29 (2009) 252–258.

Ganana, J., Turegano, J.P., Calama, G, Roman S. & Al-Kassir, A., Plant for the production of activated carbon and electric power from the gases originated in gasification processes, Fuel Processing Technology 87 (2006) 117 – 122.

Gidarakos E. (2006). Hazardous Waste, Management, Treatment, Disposal, Zigos Editions, Thessaloniki

Girods, P., Dufour, A., Rogaume, Y., Rogaume, C. & Zoulalian, A., Comparison of gasification and pyrolysis of thermal pre-treated wood board waste, Journal of Analytical and Applied Pyrolysis, 85 (2009) 171–183.

Groί, B., Eder, C., Grziwa P., Horst, J. & Kimmerle, K., Energy recovery from sewage sludge by means of fluidised bed gasification, Waste Management 28 (2008) 1819–1826.

He, M., Xiao, B., Liu, S., Guo, X., Luo, S., Xu, Z., Feng,Y., & Hu, Z., Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of steam to MSW ratios and weight hourly space velocity on gas production and composition, International Journal of hydrogen energy 34 (2009) 2174-2183.

Klein, A., Gasification: An alternative process for energy recovery and disposal of Municipal Solid Wastes, MS Thesis, Columbia University, 2002.

Radian International LLC (2000). A Comparison of gasification and incineration of hazardous wastes, DVN 99.803931.02, Austin, Texas

Rezaiyan, J. & Cheremisinoff N. (2005). Gasification Technologies, A Primer for Engineers and Scientists, Taylor & Francis Group, LLC

Scheidl, K., Boos, R., Prey T. & Wurst, F., High temperature gasification (HTG) pilot plant studies with different waste materials: formation of PCDD/F and older organic pollutants, Chemosphere, 23, (8-10) (1991) 1507-1514.

U.S. Department of Energy, National Energy Technology Laboratory, A Comparison of Gasification and Incineration of Hazardous Wastes, Final Report (2000).

Vasudevan, R. & Mathew, G., Waste Management, Indian Scenario, ASSOCHAM Environment Summit, 2007Waste to Wealth, Innovations and Emerging opportunities, 24 October, 2007 Hotel Le Meridien, New Delhi, INDIA.

Wastesum (2006). Management and Valorisation of Solid Domestic Waste for the Small Urban Communities in Morocco, LIFE-3rd Countries, 2007-2009, European Commission

World Bank Carbon Finance Unit (2008), Municipal Solid Waste Treatment Technologies and Carbon Finance, Thailand.