Energy From Solid And Liquid Wastes - Vi

Lecture No.12 Urban Waste to Energy from Landfill Biogas Projects and by Pyrolysis Plants 12.1. Introduction The urban waste is usually dumped in so called municipal land fills or municipal refuse-dumps. These land fills are usually away from the city and occupy substantial land areas. Urban waste is transported by road trucks and is dumped into the land fills. Landfill waste gets fermented by natural bacterial decay (by anaerobic fermentation) and releases methane rich fuel gas. This gas is called land-fill gas or refuse-tip-gas. Obtaining the methane rich fuel gas from land-fills is the most economical and environmentally attractive method of obtaining energy from urban waste. Land-fill Gas is being used as a renewable energy source in several countries in the world (Table 12.1). Table 12.1 Landfill Gas Project Sites (1998) Country

United states West Germany UK Sweden Italy Holland Denmark Canada France Norway Switzerland Australia Brazil India Chile

Total

Outlet Electricity Purification Other Total Trials/ (pipeline) known schemes scheme generation/ * vehicle fuel application planned chp 22* 10 14 54 13

Boilers/ heating

Kilns furnaces

7

1

14+

5

17

---

7

43+

1

5

7

7**

---

2

19

6

2

---

4++

---

1

7

---

---

---

2

1

3

6

---

1

---

---

---

3

4

8

---

---

3

---

---

3

---

1

2

---

---

---

3

---

1

---

---

---

---

1

---

---

---

----

----

1

1

1

1

---

----

----

---

1

---

---

---

1

---

---

1

1

---

---

---

1

---

1

---

---

---

1

---

---

1

1

---

---

---

1

---

1

1

33

15

57

13

31

146

32

* one scheme generates electricity and sells gas. + includes one research project. ** two schemes generates electricity and also gas for heating. ++ Four schemes are recorded as “ Boiler, CHP”. •

CHP = combined heat and power. Pyrolysis was tried for converting biomass from urban waste to energy. However, the

pyrolysis is used mainly for making wood-charcoal. 12.2. Applications of Landfill Gas Landfill gas contains predominantly methane (54% by volume). The landfill gas is used in following applications directly: --- As a fuel for burning in boilers (without purification) --- As a fuel for Kilns, Furnaces. The purified methane obtained from landfill gas is used in following applications ---- As a vehicle fuel. URBAN WASTE

RAW LANDFILL GAS

LANDFILL GAS

FOR

FILTERS & PURIFIERS

FURNACES KILNS DOMESTI C FUEL

COLLECTIO FUEL FOR IC ENGINE ELECTRIC POWER

ENERGY FOR URBAN CONSUMERS

FROM

Fig 12.1 Application of Landfill gas (LFG).

---As a fuel for diesel engines.

--- As a fuel for Diesel Engine, to produce electrical energy ---After upgrading, supplied as fuel gas to domestic consumers. 12.3. Composition of Landfill Gas The land-fill gas is generated by the fermentation of organic matter dumped in the landfill. The process sis called anaerobic fermentation i.e. decomposition caused by (anaerobe, the microorganisms) without need of oxygen. This process is suitable for municipal manure. The process takes place at low temperatures up to 60°C and requires moisture. The gases produced vary in composition with time taken by the process (Fig.12.2). After a period of 2 months from starting, the landfill gas has mainly methane (52%) and carbon dioxide (46%). During initial periods other gases like oxygen, hydrogen, nitrogen etc. are released in different proportions.

Fig.12.2 Composition of the landfill gas changing with time

During the decomposition, the temperature of the upper portion of the land-fill rises to about 60°C. Landfill gas is not a pure methane carbon dioxide mix. It has several other gases including some corrosive gases. For simple burning applications such as furnaces and kilns, the landfill gas is used without separation of methane and other constituents. For domestic cooking gas, the landfill gas is converted to compressed Natural GAS (CNG) or Liquid Natural Gas (LNG) by intermediate process. For use in vehicles as a fuel, the methane gas is separated form the total landfill gas and is purified to pure methane.

12.4 Landfill Gas Collection System Landfill sit is usually a void, valley or a former quarry in which the urban waste is dumped. For example, a landfill site near Bedford, UK is a former brick quarry having original void volume of 10 million m3 and covering 100 km2 land area. The site receives about 1000 t of urban waste per day by rail, road from London. The gas collection system consist of Wells comprising vertical pipes of 80 to 120 mm diameter with holes in the cylindrical body. The wells are driven in the landfill. The wellpipes and collection pipes are of polythetene. Knockout drums are installed in the pipelines for removal of water. A typical landfill site has 20 to 40 wells and the collection pipe system. The wells are connected to manifolds and the gas is collected from the manifolds and piping system.

Fig 12.3. Schematic of a Landfill Gas Energy Supply System

12.4.1. Gas Compression Equipment. The gas if filtered before and after compressor. The compressor increases the pressure required by the consumer device (e.g. diesel engine or a furnace). A single vane type gas compressor may be provided. A typical rating is 690m3/h of gas at discharge pressure of 1.3 bar. After compression the gas is passed through after cooler, baffle water separator, fine filter etc, before feeding to the gas consumer device. 12.4.2. Gas Purification. The landfill gas contains methane, carbon dioxide and many other impurities. Purification of this gas is complex and expensive. The cheaper and effective

methods of purification have been developed recently (1990). These methods employ semipermeable membrane and molecular sieves. The purified methane can be used as a fuel for transport vehicles. 12.4.3. Energy Conversion Equipment. The landfill gas can be purified to pure methane and then used as a fuel for internal combustion engine. The Internal Combustion Engine can drive pumping sets. Alternatively, the Internal Combustion Engine can drive generator to produce electrical energy. For examples, a typical land-fill gas site may have one to four spark ignition type four stroke internal combustion engines. The engine drives a 350 kVA, 415 V generator. The generator electric power is supplied to the distribution system at 6.6 kV via a step-up transformer in the substation. Technically, producing electricity from landfill gas project is more complex and demanding than use of gas directly as a fuel. Before admitting into the IC Engine, the gas should be cleaned, filtered and purified. The plant demands --- More operational controls. --- Greater security at outlet of Landfill Gas System. --- Better operation and management of the total plant. For producing electricity sufficient gas should be available continuously to operate the plant throughout the year with a good plant load factor. 12.5. Pyrolysis of Urban Waste to obtain Methane Pyrolysis is an old process used commonly for making charcoal form wood. This process can be used for obtaining methane gas (CH4) and other hydrocarbon gases and oil from wood and urban waste. Solid urban waste is carefully prepared by receiving, storing, shredding, passing through air classifier, drier, magnetic separators etc. and the combustible matter is separated form non-combustible metals, glass, etc. For better pyrolysis, the matter is shredded to small pieces and fed to the pyrolysis reactor.

In the pyrolysis process, the waste biomass is heated to about 500°C to 600°C to obtain methane (CH4), The gas obtained has various other constituents like CO 2, H2, CO etc. depending upon the composition of fuel and the temperatures of pyrolysis. (Fig. 13.4). The main problems in the pyrolysis process are --- The urban waste has a wide mixture of constituents, high moisture content. --- Careless dumping of volatile matter and metal sparks during processing leads

to explosions in the pyrolytic reactor. --- The gas has several constituents, some are corrosive and toxic. --- The gas has low heat value compared with natural gas. For urban waste to energy conversion the pyrolysis process has not proved to be successful due to following difficulties. 1. Danger of explosion. 2. Corrosive gases released by the process, 3. Lesser efficiency due to thermal losses. 4. Low heat value of the gas. For urban waste to gas conversion, the anaerobic digestion process at low temperature (60°) without air is most popular. The gas produced is a mixture of methane and carbon dioxide. This process is simpler, safer and economical. The plan for such process is called biogas plant. Pyrolysis Process for Waste Dry Biomass. Several processes have been developed. The choice of process depends upon the following: ---In feed biomass ---Requirement of end products ---Available technology ---Rating of plant and purpose ---Capital cost allocated Fig. 12.4 illustrates a schematic of a typical process. The waste dry biomass is received and stored (1). The Shredder (2) cuts the dry infeed to small pieces (app.2.5 cm dia). The air classifier moves the lighter dry Shredded biomass pieces with its flow and separates the heavy non-combustible metal pieces/glass pieces from the fuel. The fuel is dried in drier (4). The dried fuel is fed into the Pyrolytic Reactor (Gasifier). The pyrolytic reactor converts the dry infeed to char and fuel gas. The cyclone (7) separates the Char from the fuel gas plus organic oil. The wet scrubber (8) separates (1) the organic oil (2) fuel gas and (3) sludge. The gas purifier (9) filter the gas and delivers it to gas outlet pipe. Residual gas from (9) is reprocessed in the pyrolytic reactor.

Fig 12.4. Pyrolysis of urban waste

Lecture No: 13 13.1. Wood Gasification (Pyrolysis of wood ) Wood is a source of energy having following composition.  Volalite matter 70 75%  Fixed carbon 25%  Ash 0.5 to 5% The heat value varies with, type, density and moisture content. The chemical composition of dry wood is given in Table 13.1 Table 13.1 Chemical Composition of Dry Wood Carbon Hydrogen Nitrogen Oxygen

C H N O

50% 0.6% 0.5% 43%

The organic compounds in wood are cellulose, lignin, sap and some minerals. Natural fresh wood has 30 to 50% moisture which reduces to about 20% by natural drying. Average heat value of dry wood is between 19 MJ/kg and 21 MJ/ kg Wood Gasification is a type of pyrolysis of wood. The pyrolysis of wood (wood gasification) gives mainly the following, depending upon the process. 1. charcoal, or 2. charcoal, methane gas, and organic oils For obtaining charcoal the medium-energy wood gas generated during pyrolysis of wood is consumed in the process itself. For obtaining charcoal plus methane gas from wood, the heat is supplied by auxiliary fuel source and the gas released from the pyrolysis is collected into a separate gas collector. In the wood gasification process, the main infeed is dry wood. In addition dry agricultural biomass may also be added.

Charcoal production is carried out by pyrolysis of wood at high temperature (1000 to 1200°C) in absence of air. Charcoal is used as a solid fuel having a heat value of 25 to 32

MJ/kg. Charcoal is also used in metallurgical processes as a absorber of gases and vapours; production of carbon, decolorizing agent etc. For production of wood gas, lower temperature between 350°C to 500°C is preferred. The gas obtained by low temperature calcinations has a composition. H2

= 20%

CO

= 30%

CO2

= 30%

CH4

= 14%

N2

= 7%

Wood gas is used as a fuel furnaces and ovens. The process of producing wood gas is similar to that of producing producer gas.The plant is called Wood Gasifier. Wood pieces are used as the infeed. The ignition temperature is produced by auxiliary burners. Air is injected into the wood gasifier from the bottom side and is pumped out from the other side. The wood gas is released from the top.When air comes in contact with solid dry wood above its ignition temperature (above 500°C), the following reactions occurred.

Organic Matter in Wood + Oxygen + Nitrogen = Wood Gas + Tar, Charcoal, Oils, Acids

Wood gasification is an economical process for cheap wood and wood waste. For achieving total gasification, it is necessary to circulate air or oxygen or hydrogen. The processes are named accordingly. Commercial wood gas producer gives the following byproducts. Tar

---12%

Acetic Acid and other volatile acids

--- 3%

Wood gas

---1.8 to 1.9 Nm3/kg

Crude liquor containing methyle alcohol

---0.5%

Typical composition of wood gas is as follows: H2

= 14.5%

O2

= 0.3%

H2

= 14.3%

CH4

= 2.5%

C

=

0.7%

N2

= 47%

CO

= 29%

13.2. Wood to Oil Processes About 1000 kg of wood waste containing cellulose contains about 50% of organic matter which can yield about 1 barrel of oil. Such oil has low sulphur content and is suitable fuel for power plants. The process of obtaining oil from wood has following steps: ---- Drying of wood to 5% of moisture. ---- Shredding and then pulverizing (making fine powder). ----Reaction of powdered wood with carbon monoxide (CO) and Sodium Carbonate, at temperature of 375°C and pressure of 275 bar. ----Sodium format is formed, this reacts with Carbon monoxide to give oil and Sodium carbonate. The reactions in the above process are as follows: Na2CO3 + H2O + 2CO → CO2 + 2HCOONa 2(C6H10O5)n + 2nHCOONa → 2NC6H10O4 + nH2O + nCo2 + nNa2CO3 The reactor flue gases contain CO which may be burnt along with synthetic gas (H 2 + CO) obtain from wood waste to provide about 90% heat required in the above process. The Wood Oil liquid obtained from the above reaction has following compositions: C

= 76.6%

H

= 7.05%

N

= 0.13%

S

= 0.15%

O

= 20.05%

The Wood Oil has following characteristics:

Specific gravity

=

110 kg/m3

Heating value

=

35 MJ/kg

Viscosity

=

515 cp at 60°C

Summary Urban Waste (Municipal Waste) Landfill Gas is a source of energy. The organic portion in the landfill gets fermented by Anaerobic Digestion. The gas is mostly methane. In a landfill gas project, several Pipes are driven in the organic rubbish. These pipes have holes on the surface. The heads are connected to piping system (Gas Collection System). The plant has Gas Collection System, Compressor, Filter, Monitoring System and Supply System. Methane a obtained from Landfill Projects is supplied to urban consumers. It is used as a fuel for domestic use, driving diesel engine generator sets. Wood Gasification gives Wood Gas, Char Coal, Wood Alcohol.

Lecture No: 14 14.1. Land filling, Principles and hydrology Landfilling of solid wastes is in most developed countries regarded as a last option for disposition of solid wastes. In Denmark it is now a requirement that wastes that can be recycled or used for energy production (incineration) cannot be deposited at landfills. Landfilling has several negative environmental effects and it is therefore becoming increasingly difficult to find suitable locations for landfills. A landfill will affect the surrounding environment during a long time period and construction and operation of landfills is often more expensive compared to other types of waste treatment. 14.2. Landfill Gas 14.2.1 Extraction of Landfill Gas (i) General Landfill gas extraction, in contrast to other forms of waste energy conversion, may be required for health and safety or environmental reasons, even if the product is simply flared without heat recovery. There have been many cases reported in the last decade of high gas levels within property boundaries surrounding landfill sites, within buildings constructed on reclaimed land and potentially dangerous incidents have occurred in Europe, whilst some in the USA have been fatal. In addition, gas migration has caused vegetation kill in surrounding agricultural fields and domestic gardens essentially by displacement of soil oxygen as opposed to direct toxicity. Landfill gas can also create a small nuisance due to the trace mercaptans. Gas extraction has therefore evolved from migration prevention schemes. Landfill can be viewed as a simple form of anaerobic digestion system and an inefficient means of composting. No pre-treatment of the wastes is required before deposition, although much research is currently concerned with improving the efficiency of methane production by optimizing the rate of waste decomposition in the landfill. This could be achieved through control of moisture and nutrient regimes and adoption of waste emplacement systems amenable to landfill gas generation, in particular higher compaction levels to reduce airspaces.Relating to the objectives of and approach to the study, it was determined on submission of an interim report that this study should not examine landfill gas utilization in depth. The coverage which follows is therefore limited. (ii) The Process

After land filling, wastes pass through a period of aerobic decomposition at high temperatures of up to 70oC. As conditions become anaerobic, carbon dioxide levels continue to be high, an early peak of hydrogen occurs (often up to 20% v/v) and, providing there is no ingress of air, nitrogen levels will decline to zero. Within a period of one to several years, equilibrium is attained of about 50-60% methane and 50-40% carbon dioxide. This represents the period of maximum rate of methane generation. However, experts suggest that methane extraction could continue for a further 2-10 years at suitable levels, possibly longer following control of limiting factors such as C:N ratio of moisture content. If landfills are purpose managed for methane generation then the bulk of extraction may well be complete in as little as five years. Figure 14.1 represents the composition of gases produced by decomposing wastes with time in a landfill.

Figure 14.1 Composition of gases during decomposition of municipal refuse Several of the landfill variables are potentially manageable to enhance both the rate and quantity of methane produced. These include •

Composition of refuse materials;

•

Exclusion or treatment of toxic or inhibitory materials;

•

Time/space of refuse placement within a landfill volume;

•

Moisture content and moisture recycling

•

Nutrient addition;

•

pH control (ideally in the range 7.0 to 7.2);

•

particle size of refuse (impacts on degree of compaction);

•

permeability and porosity;

•

microbial population (seeding).

Temperature is probably the one parameter which is not readily amenable to management. (iii) Landfill Gas Systems – Characteristics and Patterns of Use There are many variations in the pattern of use of extracted landfill gas. These are generally:  Feed to natural gas main after purification and upgrading;  Feed to local user in an unpurified or relatively unpurified state either for: a. direct firing to produce heat, or; b. fuelling internal combustion engines to produce electricity by connection to a generator and recovery of waste heat. Figure 14.2 summarises in diagrammatic form the various systems in use or postulated showing the options available for each process stage.

STAGE METHODOLOGY I. Boreholes Ground probe Vertical /horizontal gas vents Capped trench systems (permeable material, e.g. rubble) Buried channel (permeable material ) II. Water removal CO2 removal

: cooling/drinking or cyclone separation : scrubbing, e.g. MEA (monethanolamine), or molecular sieving (SOLEXOL Process)

Gas processing

: upgrading, e.g. adding propane/butane

SO2 removal

: iron wool/fillings on impregnated material methanol production

III. Gasometer Pressure tank Liquid gas tank IV. Combustion with/without energy recovery i.e. hot water, hot water/electricity, electricity via conventional boiler or combination internal combustion engine/heat exchanger, or gas turbines. V. Hot water tank Electric grid system or batteries. VI. Heating – industrial process, commercial or domestic heating Mechanical Energy Electric Motors Vehicles (liquefied gas )

Further possibilities for landfill gas utilization include:  liquefaction of gas and use as a fuel for refuse trucks and plant equipment;  collection, purification and storage/sale via pressure bottles;  production of methanol. All three are the subject of continuing research.

A list of known projects is included in Table 14.1. The main projects are in the USA, Germany, Holland, UK and France but developments in other countries as diverse as Brazil and Switzerland are following rapidly. (iv) Equipment A simplified representation of a typical landfill gas recovery scheme is presented as Figure 14.3.

Fig 14.3. Typical landfill gas recovery scheme

Typically, systems are kept simple but with considerable emphasis on safety. They generally comprise a series of 100mm PVC pipes sunk to a depth of 6-10m connected via a flexible header to 150mm HDPE feeder pipes. The gas is generally drawn from the landfill using a blower and/or pump and the gases are passed through a cooler, to cool the compressed gases. Condensate is removed to reduce the moisture content, landfill gas being generally saturated. The gases then pass through a filter to remove any debris and also any sulphide content (if the gas is to be used in metal manufacture) by passing through sawdust or a similar material impregnated with iron oxide. Safety equipment usually comprises:  oxygen analysers;  low-pressure warning systems;  anti-flashback apparatus/flame arrestors;  ventilators or ventilated buildings;  vandal-proof or buried pipework systems on the landfills;  explosion/flame-proof pumps and electrical equipment.

14.2.2 Energy Production by Landfill Gas Utilisation Calculation of expected net energy production and overall energy conversion efficiency from landfills is complex since it needs to consider:  energy involved in landfill disposal operations;  energy content of the refuse input;  gas yield, including production and losses;  the balance of gas supply and demand and need for flaring off excess gas;  efficiency of conversion to useful energy as heat or electricity. Based on detailed calculations optimal conditions would result in a net energy production of 1.28 to 1.40 GJ/tonne of refuse deposited and an energy conversion efficiency of waste to gas of 12 to 22%. Upgrading of the gas to remove acid gases and water vapor would require additional energy and reduce the overall efficiency by up to one third. Conversion to useful energy would depend on the equipment used. The use of a ‘total energy installation’ based on an internal combustion engine with heat recovery from the exhaust gas, as commonly used, has an efficiency of 90%, reducing the overall conversion efficiency from refuse to usable energy to 11 to 19%. However, if the gas is used in a conventional boiler, with efficiency of 50 to 60%, the overall conversion efficiency would be correspondingly lower. 14.2.3 Constraints on Development of Landfill Gas Utilisation Projects Major constraints on the development of further gas utilization projects are likely to be:  Size of landfill voids utilized;  Ownership of gas, e.g. gas belongs to site owner, to site lessee, waste disposer or waste generator?  Availability of potential user or gas-pipeline (for upgraded gas) in close proximity to the site;  cost of upgrading gas;  feasibility and economic viability of liquefaction following purification;  moisture content and presence of other incombustible contamination in unpurified gas;  corrosion problems on collection and purification equipment;  site specific factors: ✔ organic content of deposited materials;

✔ C:N ratio of deposited materials; ✔ moisture content and density of deposited materials; ✔ depth of landfill; ✔ permeability of surrounding geology and capping materials to landfill gas; ✔ depth of unsaturated zone within waste materials. Table 14.1 Landfill Gas Systems

Location

Country

Status

Under Trial/Development Hersin Compigny (Pas de Calais)

France

Constructed, test flarings

Beuvry (Nord), Fretin (Nord

France

Experimental, under construction

Arnouville-les-Mantes (Yvelines)

France

Under

development/construction

Montaubert, Vert-le-Grand (Essone)

France

Under

development/construction

Villeparis

France

Under

development/construction

Jeandelaincourt

France

Under

development/construction

Seraf (Fosse Marmitains )

France

Under

development/construction

Hannover

W.Germany

Under Construction

Neuss

W.Germany

Trials in progress

Birkenhead

U.K.

Trials in progress

Cambridge

U.K.

Trials in progress

Plymstock (Plymouth)

U.K.

Trials in progress

France

Operational, system optimization

Barneveld

Holland

Commissioning

Bavel

Holland

Operational

Pilot Plant Gas treated & Sold Roche-la-Moliere (Loire)

Fuel to Boiler/Generator Engine

Deldem

Holland

Operational

Markelo

Holland

Commissioning

Nunen

Holland

Commissioning

Oss

Holland

Commissioning

Veendam

Holland

Commissioning

Westwoud

Holland

Commissioning

Vam-wijster

Holland

Operational

Winterswijk

Holland

Commissioning

Zutphen

Holland

Commissioning

Turin

Italy

Operational

Modena

Italy

Operational

Emilia Romagna

Italy

Operational

Am Lemberg

W.Germany

Operational

Pforzheim

W.Germany

Operational

Braunschweig

W.Germany

Operational

Giessen/Abendstern

W.Germany

Operational

Moschheim

W.Germany

Operational

Datteln

W.Germany

Operational

Breinermoor

W.Germany

Operational

Bensberg

W.Germany

Operational

Ahrenshoft

W.Germany

Operational

Gerolsheim

W.Germany

Operational

Zentraldeponie Emshersbruch

W.Germany

Operational

Croglio (Tessin)

Switzerland

Operational

Aveley

U.K.

Operational, July 1983

Barnsley

U.K.

Commissioning

Bletchley

U.K.

Commissioning

Calvert

U.K.

Commissioning

Normanton

U.K.

Constructed/Operational

Stewarthy

U.K.

Operational

Azusa, California

USA

Operational

Kenilworth, Washington

USA

Operational

Ascon, California

USA

Operational

City of Industry, California

USA

Operational

Montery Park, California

USA

Operational

Cinamminson, New Jersey

USA

Part operational,(user shutdown)

Sheldon Arleta, California

USA

Operational

Bradley, California

USA

Operational

Freshkills, New York

USA

Operational

School Canyon, California

USA

Operational

Davis St., San Leandro, CA

USA

Operational

Puente Hills, California

USA

Operational

Hewitt, California

USA

Operational

North Valley, California

USA

Operational

Deer Valley, Arizona

USA

Operational

Brattlesborough, Vermont

USA

Operational

Signal hill, California

USA

Operational

Tampa, Florida

USA

Operational

Mission, California

USA

Operational

1) Sao Paulo

Brazil

Operational

2) Rio de Janiero

Brazil

Operational

Palos Verdes, California

USA

Operational

Mountain View, California

USA

Operational

CID, Chicago, Illinois

USA

Operational

Full scale commercial Gas to pipeline Central Landfills at