Use of Heat from Biogas Cogeneration for Sludge Drying

Supply of hot water from cogeneration to our KULT® BT+ belt dryer
Schematic process diagram of a KULT® BT+ belt dryer
Schematic process diagram of a biogas plant

Biogas is preferably used for cogeneration of power and heat. Generating power from renewable energy is state-aided in some countries. In Germany power utilities are obliged by a law to purchase power generated from biomass for a fixed price of 0.115 €/kWh over a period of 20 years. But the co-generated heat is sometimes not effectively used, in spite of the fact that this would improve the cost-effectiveness of cogeneration. This is the reason why the power price in Germany is further increased by 0.02 €/kWh if co-generated is continuously and externally used. And selling heat generates additional revenues.

Sludge drying is an excellent option for heat use. Sewage sludge is continuously produced. A great portion is still applied on land, either in liquid form or after being dewatered. Sewage sludge disposal on landfills is no longer permitted in Europe, but is still common in other parts of the world. An increasing portion of sludge is incinerated. Whatever the final disposal way, it is increasingly expensive to haul liquid or dewatered sludge that still contains 70 to 85 % water. Sludge hauling consumes much fuel and contributes to global warming and climate change by emission of carbon dioxide. The longer the distance, the higher are the costs and the worse the environmental damage. By drying of sewage sludge a granular product of substantial caloric value is produced; its caloric value is equal to that of brown coal. Depending on the solids content of the dewatered sludge, drying reduces its mass by a further 60 to 90 %.

Selling co-generated heat from biogas

During cogeneration of biogas 30 to 35 % of its energy content is transformed into electrical power and 55 to 60 % into useable heat. Some of the generated heat is needed to heat the digesters or fermenters for biogas production, but the remainder is available for other uses. Cogeneration of power and heat is the most efficient use of any fuel, whether fossil or renewable. Cogeneration is an easy way to reduce energy consumption and emission of greenhouse gases. If the owner of a biogas plant in Germany sells the generated power to a power utility and the generated heat for sludge drying, he receives a power price that is increased by a further 0.02 €/kWh above the 0.115 €/kWh he would receive anyway for selling power that is generated from biomass. If he generates 100 kW power and 200 kW heat, he earns over 100,000 € per year from power sales. And assuming that he needs 50 % of his generated heat for fermenter heating and sells the other 50 % for a price of 0.05 €/kWh, his revenues from selling heat are over 40,000 € per year. This is good business for the owner of a biogas plant of this size.

Buying heat for sludge drying

Mechanical dewatering is the most cost-effective step to reduce the mass of sewage sludge, and it was for a long time the final step of sludge treatment. Land application of sludge, though it remains the best option to reuse its nutrient and organic carbon content, is becoming more restricted or even impossible. It is no longer permitted in Switzerland and virtually made impossible in the Netherlands. Administrations of several German States are also discouraging land application. The new EU Directive for sewage sludge has lowered the limits for heavy metals, including copper and zinc, and set new limits for several organic pollutants. Other countries will most likely follow. Consequently, ever more sludge will need to be incinerated worldwide. Since incineration is very expensive, it forces the operators of wastewater treatment plants to further reduce the water content of their sludge. Conventional drum or disk dryers are operated with fossil fuels, but our low and medium-temperature belt dryers can effectively use waste heat from cogeneration for sludge drying. Heat from cogeneration has generally a temperature between 80 and 90 °C. Our medium-temperature belt dryers KULT® BT+ need 800 to 850 kWh of heat per metric ton of water evaporation. A wastewater treatment plant for a total population of 10,000 generates about 200 tons of sludge solids per year. If the dewatered sludge has a solids content of 20 %, the sludge mass is 1,000 tons per year. For drying of the sludge to a solids content of 90 %, our belt dryer needs around 700 MWh of heat per year and has an average heat demand of around 80 kW. Yearly heating costs of 35,000 € mean that energy costs € 44 per metric to of water evaporation. This is only a small fraction of landfill or incineration costs. Warm water with a temperature of 80 to 90 °C is pumped through heat exchangers in our belt dryer to heat the dryer air. The dryer air is drawn through sludge layers on the belts, whereby it evaporates water and its temperature drops. The humid air is further cooled and dried in a condenser and then re-circulated to the heat exchangers and heated up again. This air re-circulation leads to very efficient use of the supplied heat. A small portion of the re-circulated dryer air is removed and deodorized in a bio-filter. It is replaced with incoming ambient air. In this way the dryer is kept at a slight under-pressure to prevent odour emission.

Conclusions

Our medium-temperature belt dryer KULT® BT+ is specifically well suited to be operated with heat from cogeneration with digester gas or biogas. Owners of biogas plants receive over a period of 20 years an extra bonus price for selling power that is not only generated from renewable biomass, but where co-generated heat is also used. Our automatically operating sludge dryers have continuous heat demand, 24 hours per day and all year round. Long-term external use of co-generated heat is precondition for the bonus power price. Sludge drying reduces the sludge mass to about 20 %. Sludge hauling costs, fuel consumption and CO2-emission are reduced accordingly. Dried sewage sludge is a renewable fuel with a caloric value like brown coal and can replace fossil fuel in power stations.