9 Drying An Essential Step in Microwave Gasification of Human Faecal Matter Introduction
Drying and dewatering
Drying; a complex process
Drying of faecal matter preceded by dewatering is an integral part of the integrated processing plant aimed at thermal decomposing of dried human wastes through gasification of dried wastes in a microwave-generated plasma. Depending on the source, the original faecal wastes may contain 80-90% w.b.(wet basis) of liquid. In our TU Delft concept we have a combined flow of faeces and urine with some flush water, which after initial dewatering, need to be dried to 20-40% w.b. before it can be used as fuel for the plasma gasification.
Although dewatering is not a part of the research in this phase of the project, based on literature and sewage sludge experiences, a low energy consuming screw press shows a good prospect for integration with the drying part.
Different drying processes are known and many types of dryer are commercially available. The selection of the appropriate drying technique and type of dryer requires expertise, knowledge and experience. TU Delft has in-depth knowledge and experience in energy integration and optimization of energy conversion systems via thermodynamic system studies and experimental hands-on expertise concerning thermal biomass pre-treatment techniques.
Quality change
Input continuous / intermittent Various heat modes: convection, conduction, radiation, microwave
Phase change liquid to vapour Change of physical structure
Drying is a complex process
Coupled with mass transfer
Chemical/biochemical reactions Shrinkage
Multicomponent moisture transport
Selection
Selection Criteria Besides typical selection criteria for a drying technique such as feed characteristics and product / output requirements (see table) there were two more critical selection criteria particularly related to the RTTC concept of TU Delft: Firstly the drying unit has to operate in an intensified way in a non-industrial, slum-environment. The equipment should be robust in such a way that maintenance should be simple and limited to a minimum.
Secondly, challenges exist in the development and management of the integration of different available energy sources: electrical power, sensible heat and low calorific value (LCV) gas. These different energy sources should be used efficiently in the process scheme of the drying unit. The LCV gas is the off-gas originating from the fuel cell anode part. Sensible heat originates from the plasma gasification as well as the fuel cell. Electrical power is generated by the fuel cell. Using the sensible heat and LCV gas efficiently are the main energy challenges.
Typical checklist for selecting dryers
Current options studied
Physical form of feed
Sludge, liquid , paste, suspension
Average throughput
kg/h (wet/dry), continuous or kg/batch
Inlet/outlet moisture content
Dry / wet basis
Fuel choice
Oil, gas, electricity
Product requirements
Power, flakes, pellets
Special requirements
Flamability limits, toxicity, fire hazard
Theoretical (design) study of combustor design for LCV gasification product gas in pulse combustion and drying. Collaboration with PCS, USA. Experimental study of (artificial) faeces drying using modified (energy saving) LaDePA intensified belt dryer. Collaboration with Kwazulu Natal University, South Africa.
Based on the criteria the LaDePa (Latrine Dehydration Pasteurisation) drying technique, a combination of a belt and a Medium Wave Infrared Radiation drying section, and the Pulse Combustion Dryer, a drying technique based on spraying wet sludge in a combustion chamber with high-frequency pulsating combustion, were selected. The research of the LaDePa drier is focused on energy input minimization via adjustment of the belt length, the recirculation of flue gas, and optimal control of the infrared part of the drier. The focus in research of the Pulse Combustion Drier is directed towards managing flame stability of LCV gas in the upfront part of the drier.
Detritus shredders with variable speed Medium wave infrared emitters (MWIR) with variable intensity Existing PSS patent Air flow
Air flow Pasteurised sludge pulse combustion
Spray drying
Inlet
Outlet Porous steel belt approximately 16m between the pulleys Normal time under MWIR emitters 8 minutes
Feed Inlet Tube
http://www.pulsedry.com/tech.php
ModiďŹ ed LaDePa dryer (KwaZulu Natal University, South Africa).
Pulse Combustion Drying (PCS, Arizona, USA)
The focus of the original LaDePa dryer is to obtain a dried and disinfected product that can be used as fertilizer. However, as the gasification process produces a disinfected low-carbon (low calorific value) product, the LaDePa dryer for the TU Delft concept has been modified to use medium wave infrared radiation for intensification of the drying instead of disinfection. Modifications are also related to the adjustment of the belt length, the introduction of recycle flows of dried material (back-mixing) and heated air to improve the energy efficiency of this dryer. By the end of March 2014, the first tests on drying using the LaDePa drier will have been performed in close cooperation with KwaZulu Natal University (South Africa).
Pulse combustion drying is an attractive drying technology as it is a very intensified process. Foot print is relatively small and maintenance is low. New is the aspect of stable combustion of low calorific value gas (off gas derived from the Fuel Cell). During this project phase at first instance it is studied theoretically via CFD modelling. Calibration and validation of the modelling is carried out by using information and conventional fuel based data sets provided by PCS, USA. The first CFD computational results are available for the pulse combustion drying technology based on LCV gas.
Next steps:
Further simulation and experiments with pulse combustion drying of (faecal) sludge using low calorific value gas.
Integration of LADEPA drier with fuel cell system; utilization of low calorific value gas.
Selecting most appropriate drying technique for full-scale integration.
www.tudelft.nl/reinventthetoilet