Gasification as a new thermal processes of sewage

sludge utilization

Gasification can be defined as a set of chemical reactions carried out at a temperature of

800-900°C, which convert the organic matter of a solid fuel into a gaseous form. The

components in the product gas depend upon the composition of the feed sludge and the

type of gasification process. Gasification is endothermic, i.e. it requires heat input, and this

heat can be provided by partial combustion of the feed with air or oxygen, or even enriched

air.

Gasification is the thermal process during which carbonaceous content of sewage sludge is

converted to combustible gas and ash in a net reducing atmosphere. Moreover one could

state that the optimum targets of sewage sludge gasification are the production of clean

combustible gas at high efficiency. Compared to incineration, gasification, due to fact that it

is a net chemically reductive process, can prevent problems from occurring, including the

need for supplemental fuel, emissions of sulfur oxides, nitrogen oxides, heavy metals and fly

ash and the potential production of chlorinated dibezodioxins and dibenzofurans.

The gasification process is constituted of several repeated cycles of mixing char residue,

from previous gasification, with moist sludge, drying and gasification. This results to almost

total conversion of all of the organic carbon in sludge to combustible gas and mineral residue

Heavy metals presented in sewage sludge feed-stream areaccumulated to the final residue,

rendering its disposal problematic. Therefore, it is crucial to ascertain any mobility of heavy

metals and where they finally end. Potentially, heavy metals can be met in the following

phases of the gasification process: (a) the char residue in the gasifier; (b) the condensate; (c)

the char filter.

Although gasification (in its various forms) has been applied to the conversion of solid and

liquid sources of energy for many decades, its benefit to the disposal of sewage sludge has

only been realised recently, and a number of such projects are being executed.

Maximisation of Revenue from Biomass Waste to

Energy Conversion Systems on Rural Farms

Dairy farms have installed biomass waste to energy conversion systems to manage the

manure disposal problems. This was in the face of increasing legislature on manure storage

and disposal. As a result of the success of these systems, dairy farmers started considering

them as sources of revenue. The revenue is obtained from the sale of excess electricity to

the grid and from avoided costs of electricity from the grid, and propane. There are no clear

guidelines on sizing and operation of these systems, that would maximise revenue. Such

guidelines would enable existing farms to assess their currently installed capacity in a bid to

maximise their revenue.

A biomass waste to energy conversion system comprises of a digester, a lagoon, an engine

generator set, a heat exchanger, a boiler and a propane tank (Figure 1). The biomass waste

from rural farms is manure. Manure from cows is stored in a lagoon before being fed to a

digester. In the digester, the manure undergoes anaerobic digestion to produce biogas. The

biogas is fed to the boiler and the engine-generator set. The engine-generator set comprises

of an induction machine coupled to an internal combustion engine. Biogas is combusted in

the internal combustion engine to generate torque. The induction machine produces

electricity from the applied torque. There is a connection to the electricity grid since excess

electricity can be sold to the grid, and can also be obtained from the grid if required. Exhaust

heat from the internal combustion engine is captured through a heat exchanger and

aggregated to the heat output of the boiler. The latter generates heat from the combustion

of biogas. A propane tank is included in the system model to act as a backup fuel supply for

the boiler. This is required if insufficient biogas is produced by the digester.

Three existing biomass waste to energy conversion systems were optimised with the

objective of maximising revenue. The maximum revenue that could be obtained from each

of the farms was specified. Two of the systems had the right installed generation capacity

required to maximise revenue. These were Emerling farm and Sunny Knoll farm. Emerling

farm’s system was however not being operated in a way that would maximise revenue. With

an improved operational strategy, Emerling farm can increase its cost savings by 18%. Sunny

Knoll farm was operating its system close to optimum, with the difference between

predicted cost savings and current cost savings being 3%. The third farm A.A. Dairy, has a

higher installed generation capacity than what is required to maximise revenue. This farm

could however increase its cost savings by 46% with an improved operational strategy, using

the currently installed generation capacity. In conclusion, through analysis of existing

systems, this paper has shown the importance of optimisation of a biomass waste to energy

conversion system in order to maximise revenue