SMALL-SCALE GASIFICATION OF BIOMASS AND SOLID WASTE

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					SMALL-SCALE GASIFICATION OF BIOMASS AND SOLID WASTE
Mr. Geir Johannessen Technical Director

Organic Power ASA, Klingenberggt. 7a P.b. 1237 Vika, 0110 Oslo, Norway

ABSTRACT: Organic Power ASA has developed small-scale combined gasification and combustion modules in collaboration with scientists at the Norwegian Agricultural University. The core technology builds on more than 100 years joint operational time in plants incinerating municipal solid waste. The gasification technology has now been further developed based on the past experience, and Organic Power ASA have now contracted deliveries of multi-fuel waste-to-energy plants handling from 6000 – 36,000 tons of solid fuel per year. Emission values are at levels well below the maximum limits set for waste-to-energy plants by the Norwegian Pollution Control Agency and the new European Union 2000 regulations for large-scale waste incineration plants1. INTRODUCTION Established energy carriers such as coal, oil, gas and electricity have well organised distribution networks. Their utilisation equipment such as burners, turbines, motors and engines are sufficiently reliable, they work well enough, we are accustomed to their uses and most of us accept their disadvantages. If bio-fuels and solid waste are to compete with such well established energy carriers, their comparative advantages have to be utilised: namely that waste and biomass are widely distributed resources often found close to an energy consumer and that they are cheap or even have negative prices.
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The plants satisfy emissions and other requirements in the Directive EU/76/EC of the European Parliament and of the Council of 4 December 2000 on the incineration of waste.

Our approach to this challenge was to develop a combined gasification and combustion system meeting at least the following demands:  The gasifier should require little pre-processing of the fuel.  The gasifier should handle all accepted fuels in any mixture and with varying water content.  It may be costly to guarantee that the fuel is clean and free from items, which can destroy or obstruct the gasifier. It should therefore, to a limited extent accept stones, glass, nails, iron pieces etc.  The size of the gasifier should fit the local energy needs and/or the supply of waste-derived fuel or biomass. If not, costly transport of fuel or heat and logistics problems may occur.  The need for labour during operation and maintenance should be low.  The gasifiers must meet the current emission requirements. FUEL SPECIFICATIONS

The gasifier accepts bio fuels, such as chip-wood, pellets, bark, sawdust, biogas and refuse derived fuels (RDF) from municipal and industrial solid waste. Most RDF can be mixed in a wide range of proportions. Generally the requirements are;  Fuel composition
- Ash - Maximum size of inert material - Energy content (effective) - Hazardous waste - Maximum content by weight N & Cl: 10 –15 % 7 x 10 x 20 cm 1800 -> 5000 kWh/t none (ref.: European Hazardous Waste List) 1%

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Fuel delivery
- To be delivered as block/bales, max size: - Bale strapping material: - Bale wrapping material: 1,1 x 1,2 x 0,8 m Plastic straps (not steel wires) Chlorine-free plastics

A simple system for processing waste into RDF suitable for the gasifier is needed. This is not part of the delivery from Organic Power but the responsibility of the RDF supplier, i.e., generally the waste management company. Typically such a system will consist of: classification/sorting, course grinding, mixing and compacting of waste to produce 300-500 kg bales, or “fuel blocks”, strapping and wrapping. The plastic wrapped fuel bales are subsequently batch fed directly into the gasifier. An illustration of such a RDF preparation setup is shown in the figure below.

Figur 1.: Example of a RDF processing plant for making plastic wrapped RDF bales

GASIFIER DESIGN Gasifier silo and fuel filling
The gasifier is a parallel down draft fixed bed shaft type gasifier that operates at atmospheric pressure.The plastic wrapped fuel bales are batch fed via a load chamber at the top of the shaft.

The fuel level in the gasifier getting below a set value triggers automatic refilling of the gasifier. By a specially designed feeding system the plastic wrapped compacted bales are fed into the load chamber, the chamber is closed and inert flue gases are led in to expel residual air. The horizontal gate to the gasification chamber are then opened and the fuels blocks fall by gravity down onto the top of the remaining fuel in the gasification chamber. Thus the gasification process «sees» an endless column of fuel.

Figure 2: Standard Module for Gasification and Combustion

The gasification process
The whole column of fuel in the shaft falls by gravity downward into the primary combustion zone. In this way, the primary combustion zone is constantly supplied with new fuel even though the gasification shaft is refilled in batches. On its way down the fuel goes through drying and degassing of volatile components in the upper part of the shaft. Further down where the fuel is exposed to primary (blast) air and hot gases from underneath, it is decomposed in a coking process. In the final stage partial combustion of the coke residue take place at substoichiometric conditions generating a burned-out ash in the bottom zone of the gasifier. The gasification rate is controlled by injection of blast air through specially designed nozzles on the tapered sidewalls at the bottom of the gasifier shaft. In the ash forming bottom zone of the gasifier the temperature is kept at 500 – 600 C or at a level where the ash will remain solid, i.e. not sinter or melt.
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The generated hot (typically 700 C) low calorie (typically 1->1,5 kWh/Nm ) syn-gas is taken out alongside of the shaft and into the secondary combustion chamber. There secondary blast air is added for final combustion. To limit the secondary combustion temperature (generally set to be from 850°C to 1100°C.) and at the same time maintain low oxygen surplus in the flue gas (typically 3%), the secondary blast air is mixed with cooled (inert) flue gas from the chimney. In this way the NOx concentration is kept well within the required limit and typically well below 100 mg/Nm .
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Following the secondary combustion chamber the flue gas is led into an insulated refractory lined cyclone for complete combustion The volume of the cyclone is designed so as to achieve the required retention time of min. 2 sec. at minimum 850 C for the flue gas. In the cyclone particles are also separated out (cut-off point is around 15/20 micron), collected and discharged at the bottom. The flue gas then flows to a heat exchanger for
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energy recovery and generation of hot water or steam. The driving force of the gas flow through the gasifier and the heat exchanger is a slight underpressure created by the flue gas fan. The heat exchanger is designed to cool down the flue gas to typically 130 C
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INERT FLUE GAS FUEL

BY-PASS VALVE

RECYCLING OF INERT FLUE GAS

Figure 3: Schematic flow diagram of the Organic Power basic energy module.

Ash handling
The ash discharge system, through which the ash is discharged in a dry nonfused state is an integral part of the gasifier. The main component is a rotating spring-suspended cylinder mounted underneath the gasifier. It has the same length as the rectangular shaft. When the rotor rotates , in either direction, ash and other material like metal fragments, stone, sand etc. will be discharged. The ash is discharged at a temperature around 500
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C. The

largest foreign object that can pass the ash discharge system has a cross section of around 5x10 cm with a maximum length of around 30 cm.

From underneath the rotor a wet chain conveyor brings the ash to a closed ash container. The discharge rotor and the chain conveyor is housed in a compartment sealed from the surroundings with a water seal. This prevents suction of false air into the gasifier via the ash discharge opening. The enclosed design of the ash handling system also prevents dust from entering the surrounding area.

The entire ash discharge system is activated by the ash temperature at the bottom of the shaft or by a timer, and it runs therefore, intermittently.

Gas Cleaning
Flue gas cleaning is carried out in a dry bag-house filter. In the system a number of integrated reactions take place. Upstream of the bag house filter hydrated lime and active coal is injected into the flue gas, hydrated lime to remove HCl and SO2, and active coal to remove heavy metals, dioxins and HF. The filter bag separates out the fly ash. To maximize the chemical reactions and at the same time avoid acid condensation and corrosion problems in the cooled flue gas area the flue gas temperature is kept at around 130 C.
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The flue gas is drawn through the bag-house filter by the flue gas fan and blown further on to the chimney for discharge into the ambient air and for recirculation to the combustion camber.
The separated fly ash, reaction products and used reagents are automatically discharged into a closed container (big-bag) for subsequent disposal as hazardous waste.

The filter design conditions and reactant dosing system are set so that the effluent quality meets or is better than the EC 2000 directive on emission to air for waste incineration plants.
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Figure 4: Illustration of Organic Power’s 2 MWth standard RDF energy module
The plastic-wrapped compacted fuel blocks (1) supplied from waste rec ycling plants are automatically batch-fed into the waste-to-energy plan t. The bales are fed through an airtight gate arrangement (2), and are gravity-fed into the gasification and combustion zone at the bottom of t he primary chamber (3). The waste-to-energy plant uses a gasification process, where the waste feedstock is heated to a temperature betwe en 400 and 700°C in the primary chamber. At this stage, the volatile fr actions of the waste feedstock are gasified into low calorific carbon mo noxide (CO) gas. The gasification temperature is maintained at a stable level by the carbonization of the remaining heavy fuel fractions at the bottom of the primary chamber.

The CO gas is drawn from the gasification stage and is led into the se cond stage (4) where secondary air is injected and the gas combustion completed. The temperature in the secondary stage increases to betwe en 850 and 1100°C. The hot flue gas flows through the heat exchanger

EMISSION TESTS OF VARIOUS WASTE-DERIVED FUELS

Table 1: Emission tests carried out on four different fuels
Fuel no. Fuel content A Comments

Mixed wood from demolished Briquettes; 30 x 100 mm. Fuel did buildings, glued wood, painted not contain impregnated wood wood fractions/waste. Mixed 80/20 due to calorific

B

Recycled paper + plastics

content, coarsely grinded. C

Fuel A + recycled paper + plastics
Municipal solid waste

Mixed 20/65/15 due to calorific content, briquettes, 30 x 100mm + Mixed 60/40 due to weight,

D

industrial solid waste

coarsely grinded. Did not contain wet organic fractions

E

Municipal solid waste + wet MSW with 19% wet organic waste organic fractions compacted in 500 kg bales and plastic wrapped.

The emission tests A, B, and C were conducted by Det Norske Veritas (DnV), an independent, autonomous foundation conducting certified emission tests. Test E was carried out by Kjelforeningen Norsk Energi, a non-profit institute conducting certified emission tests.

Recent in-house tests have also been successfully carried out burning straw without experiencing any problem of sintering/clinker formation of the ash..

Results

Table 2: Test burning results compared to current and new EU-Directives
Parameter Current EU limit on waste incineration EU draft Directive on waste incineration Fuel number A B C D E

Temp.2nd chamber (°C) Time 2nd chamber (s) Dust NOx NH3 CO SO2 HF HCl TOC Heavy Met. Hg Cd + Tl Dioxsin(WHO) O2 level (%)
B

> 850

> 850

870 - 950

>2 100 100 300 4 100 20 0,5 0,05 0,05 2

>2 10 200 50 50 1 10 10 0,5 0,05 0,05 0,1

2-3 2,0 0,2 1.0 <1,4 <0,2 0,9 * 0,021 0,0008

2-3 6 210 0,15 5 16 0,1 3 1,6 0,052

2-3 2,3 200 0,25 20 12 <0,08 6 * 0,01

2-3 3,1 160 1,7 1.5 1.0 <0,1 5 * 0,03 0,001 0,004 NA 6,8 1,0 130 NA 38 23 0,1 4 4 0,067 0,0016 0,028 0,08 3,9

0,0004 NA

0,00035 0,0016 0,002 0,07 2-4 NA 3,5 NA 6-8

All values are in mg/nm and referred to 11% O2 concentration. Values in ng/nm .
3 B

3

A

Sb+As+Pb+Cr+Co+Cu+Mn+Ni+V+Sn.

Each test burning lasted for about one day. The flue gas analysis was based on mean values for 1-hour steady burning.

As may be seen for the above two tables emission values for a fairly broad variety of RDF has been tested and found to be within the EU requirements. No re-adjustment of the gasifier process was done when changing from one fuel test to the next. The RDF bales were fed into the gasifier as delivered from the supplier. Spot checks of the bales did show fairly significant variations in waste fraction content between bales without this affecting the emission quality or gasifier operation.

PLANTS IN OPERATION OR UNDER CONSTRUCTION
Following the development and testing phase national and international marketing has been initiated. Organic Power is presently engaged in

activities in Norway, Denmark, Sweden, Germany, UK, Spain, Netherlands and South Korea. More countries are added continuously. So far it appears that our small scale, decentralized and modular approach for energy recovery from waste is met with a substantial interest in the market. Solving both energy need and a waste problem in an environmentally acceptable and cost effective way is clearly a win-win concept and the idea of small distributed systems as supplements to large centralized systems is gaining acceptance. A brief description of ongoing contracts is given below.

Plant Nordmøre BioEl Boseong NTE Brenneri Sundes

Location Norway Korea Norway

Capacity 0,8 MW, 10 ton/day 2 MW, 20 ton/day 2 MW, 20 ton/da y

remarks SK-500 SK-1000 SK-1000

Elverum Fjernvarme Lier Fjernvarme Naskov Fjernvarme

Norway

4 MW, 40 ton/day

SK-1000 * 2

Norway Denmark

4 MW, 40 ton/day 12 MW

SK-1000 * 2 SK-1000 * 6

CONCLUSIONS
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The gasifier base module acts as a properly functioning waste-to-energy plant at output power levels from 500 to 2000 kWth.. This corresponds roughly to a capacity of 2000 to 6000 tons of waste per year assuming a heat value of about 3000 kWh/ton. The plant meets the emission regulations of the new EU-2000 requirements.

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The modular concept has been met with considerable interest in the mark et. Present contracts illustrate how the installation size can be economica lly fitted to energy needs or fuel supply of the customer. This increases t he competitiveness of biomass and solid waste in local energy production.

About The Author
Mr. Geir Johannessen is the Technical Director of Organic Power ASA in Norway. He has an engineering degree in automation and electrical engineering as well as thermodynamics. He has been working both in the Offshore Industry as well as in the shipping Industry. Mr. Johannessen held previous positions as Director in ABB and later as Director in the INC group. He is currently responsible for the gasification technology in Organic Power ASA.


				
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