Proceedings

Abstract:

Traditionally, sludge pyrolysis has been investigated on semi dry biosolids such as sludge cake. This paper presents the results and conclusions drawn from a pyrolysis trial performed on dried sludge granules. As a first of its kind on this scale, the trial presented new challenges to the pyrolysis operation. The results are very encouraging when analysing the quality and quantity of the products generated by the process. The pyrolysis achieved a very high efficiency, turning the full amount of volatile solids to gas and leaving a char made of carbon, metals and inert minerals. A calorific value of 17 MJ.m-3 was measured. The second part of the paper analyses the attractiveness of these results in terms of feasibility and energy balance. The conclusions are that the technology is guaranteeing a high quality output and considerable additional energy recovery from sludge solids. The business model is yet to be elaborated to bring a sustainable solution to sludge processing technology. It will need to reflect the differing strategies and cost structures of the various water companies.

Introduction:

Southern Water processes sludge from wastewater works to produce bio-solids in the form of cake and granules. Approximately 25,000 TDS of granules are produced every year through five dryers. The sludge granules have a high calorific value which is not exploited via the land spreading recycling route. Pyrolysis is amongst the new technologies presented to water companies; it provides opportunity for full energy recovery with the ROCs incentive being one of the highest. According to Palmer (2010), a conventional wastewater treatment (including a sludge dryer) leaves up to 70% of the raw sludge calorific value in the land spread product. This available energy explains the interest of adding a final thermal treatment process. Digested sludge contains a large amount of energy, in the region of 15,361 MJ/TDS (Barber, W.P.F.,2009).

A trial was agreed with Environmental Power International UK(EPi) to pyrolyse sludge pellets using their facilities in Mitcham South West London. The waste was conveyed through the process at a regular rate and was exposed to temperature changing between 800 and 900oC. The residency at this high temperature varies with the waste quality. This trial was a first for EPi whose main process feedstock is general community waste. The trial’s aim was to answer the questions below:

• Can sludge granules be pyrolysed and what initial specifications are required?
• Does it present unforeseen difficulties compared to the usual running of the pyrolysis plant?
• What is the quantity and quality of process outputs?
• What can be gained from adding pyrolysis to a sludge treatment process?
• Is the pyrolysis process technically viable and which business model should be used?

This paper describes the trial with (Environmental Power International UK (EPi). It looks at the results in terms of value of the pyrolysis process achieved from sludge granules supplied. The treatment generates by-products of which three have a calorific or potential retail value and one is continuously recycled. The first and primary product is the gas, cleaned and useable as an energy source (a methane and hydrogen rich mixture). The second product is the char which is the result of the degradation of the granules (high carbon content) in absence of oxygen . The third product is a filter press cake composed of fine Carbon particles and oil residues, resulting from the gas clean up systems. The other two by-products are (i) the mineral oil recovered from the gas cleaning and (ii) the aqueous media. The aqueous media contains mainly water from the starting material and also from the chemical reaction occurring in the pyrolysis process. The mineral oil needs to be changed three times a year. This oil can then be mixed with organic waste and fed back to the pyrolysis plant or alternatively, it can be sent to a licensed oil recycling facility.