Washing  Gas. — A  large  number  of  different  forms  of
apparatus  have  been  devised  for  the  removal  of  dust  and
tar  from  gas  which  is  to  be  used  for  gas  engines.  The
methods  consist  in  passing  the  gas  up  towers  packed
with  some  suitable  material,  where  it  meets  a  descending
rain  of  water,  and  thus  the  dust  and  tar  are  washed  out,
or  in  bubbling  the  gas  through  water,  this  being,  as  a
rule,  much  less  efficient,  or,  as  in  many  modern  types  of
plant,  agitating  the  gas  with  water  by  means  of  dashers
or  other  mechanical  appliances.  This  is  perhaps  the  most
efficient,  and  at  the  same  time  the  cheapest,  method  of
bringing  about  the  desired  cleansing.
Ammonia  Recovery.  —  The  recovery  of  ammonia  has
already  been  discussed  in  connection  with  the  Mond
plant.  No  attempt  has  been  made  to  recover  the  ammonia
from  gas  producers  worked  in  the  ordinary  way,  as  the
quantity  would  be  too  small  to  be  profitable.  The  method
of  Dr.  Mond,  i.e.,  the  use  of  a  large  excess  of  steam,
could  no  doubt  be  applied  to  producers  of  almost  any
type,  provided  efficient  condensing  plant  be  provided.
It  has  been  quite  successfully  carried  out  with  Duff  pro-
ducers.
Initial  Cost. — The  initial  cost  of  the  simple  types  of
producer  is  small,  as  they  are  very  easily  built.  The
actual  cost  will,  of  course,  vary  with  circumstances.
126


DUFF-WHITFIELD    PRODUCER.
FIG.  33.— SECTIONAL  ELEVATION  AND  PLAN  OF  THE  DUFF-WHITFIELD
GAS  PRODUCES.
COST   OF    PRODUCER   GAS.  127

Cost  of  Labour. — The  cost  of  labour  is  small,  as  one  man
can  attend  to  two  or  more  producers,  and  additional
labour  will  only  be  required  for  removal  of  the  ashes.
