Sludge Co-combustion:
The cost effective solution for sewage sludge management

The future and economics of biosolids in agriculture is becoming uncertain. Whilst incineration is viewed in a negative light, sludge co-combustion is seen throughout Europe as a green alternative. With this in mind, DIRK now provides sludge co-combustion in coal-fired power stations. With co-combustion, DIRK provides a complete solution at a fixed gate price for either dewatered or undewatered sewage sludge. DIRK can offer its expertise based on experiences gained from its state of the art full-scale installation at the EnBW power station in Heilbronn/Germany.

In this plant, energy is recovered from dewatered sewage sludge cake with a dry substance of 25%. The advantages of sludge co-firing at a power station include:

•	Low cost
•	Security
•	No drying equipment required (since drying occurs in the coal mills) - simplified operations
•	Works with either undigested or digested sludge
•	Safe, reliable and proven technology with a low cost solution
•	Sludge combustion is only a small part of the capacity of the power station; therefore process is not adversely affected
•	Availability of well trained and experienced personnel from power station
•	In-built flue gas cleaning facilities

The Heilbronn power station (below) provides 760 MW power and operates at a mid load factor. The plant is equipped with coal dust combustion with dry ash extraction. Tangential burners are directed to the centre of the furnace creating a vortex of fire with a temperature of 1200°C at the middle of the combustion chamber.

Four roller pot mills consume the coal at a rate of 240 t/hr at full load. The coal arrives from all over Germany and Four roller pot mills consume the coal at a rate of 240 t/hr at full load. The coal arrives from all over Germany and bitumous coal is also imported. The station is also equipped with a state of the art SCR-DeNox plant using electric precipitation technology and a new desulphurisation plant based on the limestone/gypsum method for the treatment of flue gases.

Sewage sludge co-combustion

During 1996, tests with 3700 tonnes of dried and dewatered sludge which made up 4% of the boiler capacity were conducted at the plant. The plant received approval to co-combust dried sludge on the 16 February 1998 and dewatered sludge on the 3 July of the same year. This permitted the sludge co-combustion of 40,000 dry tonnes of sludge/year (i.e. 20,000 t/dry sludge and 60,000 t/sludge cake). Since 1998, 15,000 tonnes of dried sludge and 20,000 of sludge cake have been successfully co-combusted. The co-combustion of dewatered sludge cake proved to be the preferred method causing the least operational and environmental impact.

Impact on operation

Dewatered cake is added to the mill coal supply. The boiler/mill airing system is designed to dry the coal as it is pulverised. The only thermodynamic limit on the mill’s evaporative capacity is the design of the air heaters. Hence the co-combustion percentage is dependant on the moisture content of the coal and sludge. Additional moisture from the co-combustion of 2% (boiler capacity) of dewatered sludge reduced the temperature in the coal mills from 100°C to 85°C. Co-combustion has to be abandoned at approximately 14% moisture content in the coal.

Sludge co-combustion made no environmental impact on the quality of emissions from the power station. Importantly, there was no increase in the concentration of problem pollutants during co-combustion especially, SO2, NOX, heavy metals (esp. Hg), acid gases and dioxins. The emissions were all compliant with German standards and the average results are presented in Table 1.

	Permitted emission for coal fired power stations (mg/m3)	Average emissions for Heilbronn plant (mg/m3) 100% coal fired	Average emissions for Heilbronn plant (mg/m3) 96% coal with 4% co-combusted sewage sludge
CO	150	3 - 10	4.7 - 8.5
SO2	400	80 - 270	175 - 270
NOx	200	150 - 190	170 - 180
Dust	50	5 - 20	4.6 - 6.1
HF	10	1 - 3.4	1.5 - 2.5
HCI	90	0.6 - 7	0.7 - 2.3
org.C	-	1.0	0.3 - 1.3
CD, Ti	-	<0.005	<0.005
Hg	-	0.3 - 12 (µg/m3)	0.1 - 13 (µg/m3)
Heavy Metals	-	<0.075	<0.075
Dioxine/Furane	-	<5 (pg/m3)	<5 (pg/m3)

Table 1. Effect of sludge co-combustion on pollutant emission levels

The benefits of sludge co-combustion are:

•	Environmentally acceptable - renewable energy
•	Secure recycling route
•	Low cost solution
•	Short planning and project timescales
•	Sludge producers are free of technological risks

We keep our customers green, but we don't cost them the earth.